| 0/0 |
2026/09/02 01:29 |
flow |
assessment-security |
40m
Results: map[DenialOfService:true Explanation:**Bug Analysis:**
The bug is a NULL pointer dereference in `gfs2_quota_lock()` when trying to
access `ip->i_qadata->qa_qd_num`.
In the GFS2 filesystem, the `ip->i_qadata` structure is allocated on-demand by
calling `gfs2_qa_get()` before performing operations that might require quota
locks (such as `truncate`, `fallocate`, `chown`, etc.). However, `gfs2_qa_get()`
has an explicit check to skip allocation and return success (`0`) if the inode
has the `GFS2_DIF_SYSTEM` flag set:
```c
if (ip->i_diskflags & GFS2_DIF_SYSTEM)
return 0;
```
If an operation like `truncate` is performed on a file with the
`GFS2_DIF_SYSTEM` flag, `gfs2_setattr_size()` will call `gfs2_qa_get()` (which
does nothing and returns 0), and then proceed to call `do_grow()` ->
`gfs2_quota_lock_check()`.
If the user does not have the `CAP_SYS_RESOURCE` capability,
`gfs2_quota_lock_check()` will call `gfs2_quota_lock()`. Unlike `gfs2_qa_get()`,
`gfs2_quota_lock()` does not check for the `GFS2_DIF_SYSTEM` flag and
unconditionally dereferences `ip->i_qadata`, which is `NULL`, leading to the
crash.
**Trigger and Exploitability:**
The `GFS2_DIF_SYSTEM` flag is an internal filesystem flag. It cannot be set by
userspace on normal files (it is explicitly filtered out in
`gfs2_fileattr_set()`). Therefore, this bug can only be triggered by mounting a
maliciously crafted GFS2 filesystem image where a normal, accessible file has
the `GFS2_DIF_SYSTEM` flag set.
The crash occurs when reading from offset `0x100` (the offset of `qa_qd_num` in
`struct gfs2_qadata`). Because this address falls within the unmapped zero page
(protected by `mmap_min_addr`), it will reliably cause a page fault and a kernel
oops/panic (Denial of Service). It is not exploitable for privilege escalation
or arbitrary code execution.
Because GFS2 does not support unprivileged user namespace mounts
(`FS_USERNS_MOUNT` is not set), an unprivileged user cannot mount the malicious
image themselves. The attack requires either root privileges to mount the image,
or physical access to a system that auto-mounts external media (e.g., a USB
drive). Once the image is mounted, an unprivileged user can trigger the crash by
simply calling `truncate` on the crafted file.
**Conclusions:**
* **Exploitable:** No. It is a pure NULL pointer dereference (read from `0x100`)
which cannot be mapped by an attacker.
* **Denial of Service:** Yes. It reliably crashes the kernel.
* **Accessible From Unprivileged Processes:** No. While the final trigger
(`truncate`) is done by an unprivileged user, the required setup (mounting the
GFS2 image) requires `CAP_SYS_ADMIN` in the initial namespace.
* **Accessible From User Namespaces:** No. GFS2 cannot be mounted inside a user
namespace.
* **Malicious Filesystem Trigger:** Yes. The bug relies on a crafted inode with
the `GFS2_DIF_SYSTEM` flag.
* **Peripheral Trigger:** Yes. Can be triggered by plugging in a malicious USB
drive on systems with auto-mount enabled. Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/09/02 01:29 |
action |
syz-repro-to-c-repro |
0m
Results: map[SimplifiedCRepro:// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <setjmp.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <linux/loop.h>
#ifndef __NR_memfd_create
#define __NR_memfd_create 319
#endif
static unsigned long long procid;
//% This code is derived from puff.{c,h}, found in the zlib development. The
//% original files come with the following copyright notice:
//% Copyright (C) 2002-2013 Mark Adler, all rights reserved
//% version 2.3, 21 Jan 2013
//% This software is provided 'as-is', without any express or implied
//% warranty. In no event will the author be held liable for any damages
//% arising from the use of this software.
//% Permission is granted to anyone to use this software for any purpose,
//% including commercial applications, and to alter it and redistribute it
//% freely, subject to the following restrictions:
//% 1. The origin of this software must not be misrepresented; you must not
//% claim that you wrote the original software. If you use this software
//% in a product, an acknowledgment in the product documentation would be
//% appreciated but is not required.
//% 2. Altered source versions must be plainly marked as such, and must not be
//% misrepresented as being the original software.
//% 3. This notice may not be removed or altered from any source distribution.
//% Mark Adler madler@alumni.caltech.edu
//% BEGIN CODE DERIVED FROM puff.{c,h}
#define MAXBITS 15
#define MAXLCODES 286
#define MAXDCODES 30
#define MAXCODES (MAXLCODES + MAXDCODES)
#define FIXLCODES 288
struct puff_state {
unsigned char* out;
unsigned long outlen;
unsigned long outcnt;
const unsigned char* in;
unsigned long inlen;
unsigned long incnt;
int bitbuf;
int bitcnt;
jmp_buf env;
};
static int puff_bits(struct puff_state* s, int need)
{
long val = s->bitbuf;
while (s->bitcnt < need) {
if (s->incnt == s->inlen)
longjmp(s->env, 1);
val |= (long)(s->in[s->incnt++]) << s->bitcnt;
s->bitcnt += 8;
}
s->bitbuf = (int)(val >> need);
s->bitcnt -= need;
return (int)(val & ((1L << need) - 1));
}
static int puff_stored(struct puff_state* s)
{
s->bitbuf = 0;
s->bitcnt = 0;
if (s->incnt + 4 > s->inlen)
return 2;
unsigned len = s->in[s->incnt++];
len |= s->in[s->incnt++] << 8;
if (s->in[s->incnt++] != (~len & 0xff) ||
s->in[s->incnt++] != ((~len >> 8) & 0xff))
return -2;
if (s->incnt + len > s->inlen)
return 2;
if (s->outcnt + len > s->outlen)
return 1;
for (; len--; s->outcnt++, s->incnt++) {
if (s->in[s->incnt])
s->out[s->outcnt] = s->in[s->incnt];
}
return 0;
}
struct puff_huffman {
short* count;
short* symbol;
};
static int puff_decode(struct puff_state* s, const struct puff_huffman* h)
{
int first = 0;
int index = 0;
int bitbuf = s->bitbuf;
int left = s->bitcnt;
int code = first = index = 0;
int len = 1;
short* next = h->count + 1;
while (1) {
while (left--) {
code |= bitbuf & 1;
bitbuf >>= 1;
int count = *next++;
if (code - count < first) {
s->bitbuf = bitbuf;
s->bitcnt = (s->bitcnt - len) & 7;
return h->symbol[index + (code - first)];
}
index += count;
first += count;
first <<= 1;
code <<= 1;
len++;
}
left = (MAXBITS + 1) - len;
if (left == 0)
break;
if (s->incnt == s->inlen)
longjmp(s->env, 1);
bitbuf = s->in[s->incnt++];
if (left > 8)
left = 8;
}
return -10;
}
static int puff_construct(struct puff_huffman* h, const short* length, int n)
{
int len;
for (len = 0; len <= MAXBITS; len++)
h->count[len] = 0;
int symbol;
for (symbol = 0; symbol < n; symbol++)
(h->count[length[symbol]])++;
if (h->count[0] == n)
return 0;
int left = 1;
for (len = 1; len <= MAXBITS; len++) {
left <<= 1;
left -= h->count[len];
if (left < 0)
return left;
}
short offs[MAXBITS + 1];
offs[1] = 0;
for (len = 1; len < MAXBITS; len++)
offs[len + 1] = offs[len] + h->count[len];
for (symbol = 0; symbol < n; symbol++)
if (length[symbol] != 0)
h->symbol[offs[length[symbol]]++] = symbol;
return left;
}
static int puff_codes(struct puff_state* s,
const struct puff_huffman* lencode,
const struct puff_huffman* distcode)
{
static const short lens[29] = {
3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258};
static const short lext[29] = {
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2,
3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 0};
static const short dists[30] = {
1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
8193, 12289, 16385, 24577};
static const short dext[30] = {
0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6,
7, 7, 8, 8, 9, 9, 10, 10, 11, 11,
12, 12, 13, 13};
int symbol;
do {
symbol = puff_decode(s, lencode);
if (symbol < 0)
return symbol;
if (symbol < 256) {
if (s->outcnt == s->outlen)
return 1;
if (symbol)
s->out[s->outcnt] = symbol;
s->outcnt++;
} else if (symbol > 256) {
symbol -= 257;
if (symbol >= 29)
return -10;
int len = lens[symbol] + puff_bits(s, lext[symbol]);
symbol = puff_decode(s, distcode);
if (symbol < 0)
return symbol;
unsigned dist = dists[symbol] + puff_bits(s, dext[symbol]);
if (dist > s->outcnt)
return -11;
if (s->outcnt + len > s->outlen)
return 1;
while (len--) {
if (dist <= s->outcnt && s->out[s->outcnt - dist])
s->out[s->outcnt] = s->out[s->outcnt - dist];
s->outcnt++;
}
}
} while (symbol != 256);
return 0;
}
static int puff_fixed(struct puff_state* s)
{
static int virgin = 1;
static short lencnt[MAXBITS + 1], lensym[FIXLCODES];
static short distcnt[MAXBITS + 1], distsym[MAXDCODES];
static struct puff_huffman lencode, distcode;
if (virgin) {
lencode.count = lencnt;
lencode.symbol = lensym;
distcode.count = distcnt;
distcode.symbol = distsym;
short lengths[FIXLCODES];
int symbol;
for (symbol = 0; symbol < 144; symbol++)
lengths[symbol] = 8;
for (; symbol < 256; symbol++)
lengths[symbol] = 9;
for (; symbol < 280; symbol++)
lengths[symbol] = 7;
for (; symbol < FIXLCODES; symbol++)
lengths[symbol] = 8;
puff_construct(&lencode, lengths, FIXLCODES);
for (symbol = 0; symbol < MAXDCODES; symbol++)
lengths[symbol] = 5;
puff_construct(&distcode, lengths, MAXDCODES);
virgin = 0;
}
return puff_codes(s, &lencode, &distcode);
}
static int puff_dynamic(struct puff_state* s)
{
static const short order[19] =
{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15};
int nlen = puff_bits(s, 5) + 257;
int ndist = puff_bits(s, 5) + 1;
int ncode = puff_bits(s, 4) + 4;
if (nlen > MAXLCODES || ndist > MAXDCODES)
return -3;
short lengths[MAXCODES];
int index;
for (index = 0; index < ncode; index++)
lengths[order[index]] = puff_bits(s, 3);
for (; index < 19; index++)
lengths[order[index]] = 0;
short lencnt[MAXBITS + 1], lensym[MAXLCODES];
struct puff_huffman lencode = {lencnt, lensym};
int err = puff_construct(&lencode, lengths, 19);
if (err != 0)
return -4;
index = 0;
while (index < nlen + ndist) {
int symbol;
int len;
symbol = puff_decode(s, &lencode);
if (symbol < 0)
return symbol;
if (symbol < 16)
lengths[index++] = symbol;
else {
len = 0;
if (symbol == 16) {
if (index == 0)
return -5;
len = lengths[index - 1];
symbol = 3 + puff_bits(s, 2);
} else if (symbol == 17)
symbol = 3 + puff_bits(s, 3);
else
symbol = 11 + puff_bits(s, 7);
if (index + symbol > nlen + ndist)
return -6;
while (symbol--)
lengths[index++] = len;
}
}
if (lengths[256] == 0)
return -9;
err = puff_construct(&lencode, lengths, nlen);
if (err && (err < 0 || nlen != lencode.count[0] + lencode.count[1]))
return -7;
short distcnt[MAXBITS + 1], distsym[MAXDCODES];
struct puff_huffman distcode = {distcnt, distsym};
err = puff_construct(&distcode, lengths + nlen, ndist);
if (err && (err < 0 || ndist != distcode.count[0] + distcode.count[1]))
return -8;
return puff_codes(s, &lencode, &distcode);
}
static int puff(
unsigned char* dest,
unsigned long* destlen,
const unsigned char* source,
unsigned long sourcelen)
{
struct puff_state s = {
.out = dest,
.outlen = *destlen,
.outcnt = 0,
.in = source,
.inlen = sourcelen,
.incnt = 0,
.bitbuf = 0,
.bitcnt = 0,
};
int err;
if (setjmp(s.env) != 0)
err = 2;
else {
int last;
do {
last = puff_bits(&s, 1);
int type = puff_bits(&s, 2);
err = type == 0 ? puff_stored(&s) : (type == 1 ? puff_fixed(&s) : (type == 2 ? puff_dynamic(&s) : -1));
if (err != 0)
break;
} while (!last);
}
*destlen = s.outcnt;
return err;
}
//% END CODE DERIVED FROM puff.{c,h}
#define ZLIB_HEADER_WIDTH 2
static int puff_zlib_to_file(const unsigned char* source, unsigned long sourcelen, int dest_fd)
{
if (sourcelen < ZLIB_HEADER_WIDTH)
return 0;
source += ZLIB_HEADER_WIDTH;
sourcelen -= ZLIB_HEADER_WIDTH;
const unsigned long max_destlen = 132 << 20;
void* ret = mmap(0, max_destlen, PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANON, -1, 0);
if (ret == MAP_FAILED)
return -1;
unsigned char* dest = (unsigned char*)ret;
unsigned long destlen = max_destlen;
int err = puff(dest, &destlen, source, sourcelen);
if (err) {
munmap(dest, max_destlen);
errno = -err;
return -1;
}
if (write(dest_fd, dest, destlen) != (ssize_t)destlen) {
munmap(dest, max_destlen);
return -1;
}
return munmap(dest, max_destlen);
}
static int setup_loop_device(unsigned char* data, unsigned long size, const char* loopname, int* loopfd_p)
{
int err = 0, loopfd = -1;
int memfd = syscall(__NR_memfd_create, "syzkaller", 0);
if (memfd == -1) {
err = errno;
goto error;
}
if (puff_zlib_to_file(data, size, memfd)) {
err = errno;
goto error_close_memfd;
}
loopfd = open(loopname, O_RDWR);
if (loopfd == -1) {
err = errno;
goto error_close_memfd;
}
if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
if (errno != EBUSY) {
err = errno;
goto error_close_loop;
}
ioctl(loopfd, LOOP_CLR_FD, 0);
usleep(1000);
if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
err = errno;
goto error_close_loop;
}
}
close(memfd);
*loopfd_p = loopfd;
return 0;
error_close_loop:
close(loopfd);
error_close_memfd:
close(memfd);
error:
errno = err;
return -1;
}
static void reset_loop_device(const char* loopname)
{
int loopfd = open(loopname, O_RDWR);
if (loopfd == -1) {
return;
}
if (ioctl(loopfd, LOOP_CLR_FD, 0)) {
}
close(loopfd);
}
static long syz_mount_image(
volatile long fsarg,
volatile long dir,
volatile long flags,
volatile long optsarg,
volatile long change_dir,
volatile unsigned long size,
volatile long image)
{
unsigned char* data = (unsigned char*)image;
int res = -1, err = 0, need_loop_device = !!size;
char* mount_opts = (char*)optsarg;
char* target = (char*)dir;
char* fs = (char*)fsarg;
char* source = NULL;
char loopname[64];
if (need_loop_device) {
int loopfd;
memset(loopname, 0, sizeof(loopname));
snprintf(loopname, sizeof(loopname), "/dev/loop%llu", procid);
if (setup_loop_device(data, size, loopname, &loopfd) == -1)
return -1;
close(loopfd);
source = loopname;
}
mkdir(target, 0777);
char opts[256];
memset(opts, 0, sizeof(opts));
if (strlen(mount_opts) > (sizeof(opts) - 32)) {
}
strncpy(opts, mount_opts, sizeof(opts) - 32);
if (strcmp(fs, "iso9660") == 0) {
flags |= MS_RDONLY;
} else if (strncmp(fs, "ext", 3) == 0) {
bool has_remount_ro = false;
char* remount_ro_start = strstr(opts, "errors=remount-ro");
if (remount_ro_start != NULL) {
char after = *(remount_ro_start + strlen("errors=remount-ro"));
char before = remount_ro_start == opts ? '\0' : *(remount_ro_start - 1);
has_remount_ro = ((before == '\0' || before == ',') && (after == '\0' || after == ','));
}
if (strstr(opts, "errors=panic") || !has_remount_ro)
strcat(opts, ",errors=continue");
} else if (strcmp(fs, "xfs") == 0) {
strcat(opts, ",nouuid");
} else if (strncmp(fs, "gfs2", 4) == 0 && (strstr(opts, "errors=panic") || strstr(opts, "debug"))) {
strcat(opts, ",errors=withdraw");
}
res = mount(source, target, fs, flags, opts);
if (res == -1) {
err = errno;
goto error_clear_loop;
}
res = open(target, O_RDONLY | O_DIRECTORY);
if (res == -1) {
err = errno;
goto error_clear_loop;
}
if (change_dir) {
res = chdir(target);
if (res == -1) {
err = errno;
}
}
error_clear_loop:
if (need_loop_device)
reset_loop_device(loopname);
errno = err;
return res;
}
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// prlimit64 arguments: [
// pid: pid (resource)
// res: rlimit_type = 0xe (8 bytes)
// new: ptr[in, rlimit] {
// rlimit {
// soft: intptr = 0x8 (8 bytes)
// hard: intptr = 0x88 (8 bytes)
// }
// }
// old: nil
// ]
*(uint64_t*)0x200000000000 = 8;
*(uint64_t*)0x200000000008 = 0x88;
syscall(__NR_prlimit64, /*pid=*/0, /*res=RLIMIT_RTPRIO*/0xeul, /*new=*/0x200000000000ul, /*old=*/0ul);
// sched_setscheduler arguments: [
// pid: pid (resource)
// policy: sched_policy = 0x2 (8 bytes)
// prio: ptr[in, int32] {
// int32 = 0x7 (4 bytes)
// }
// ]
*(uint32_t*)0x200000000240 = 7;
syscall(__NR_sched_setscheduler, /*pid=*/0, /*policy=SCHED_RR*/2ul, /*prio=*/0x200000000240ul);
// syz_mount_image$gfs2 arguments: [
// fs: ptr[in, buffer] {
// buffer: {67 66 73 32 00} (length 0x5)
// }
// dir: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 00} (length 0xfa)
// }
// flags: mount_flags = 0x1010084 (8 bytes)
// opts: ptr[inout, array[ANYUNION]] {
// array[ANYUNION] {
// union ANYUNION {
// ANYBLOB: buffer: {73 75 69 64 64 69 72 2c 6d 65 74 61 2c 64 69 73 63 61 72 64 2c 6e 6f 6c 6f 63 63 6f 6f 6b 69 65 2c 6e 6f 64 69 73 63 61 72 64 2c 71 75 6f 74 61 3d 61 63 63 6f 75 6e 74 2c 00 35 60 96 c0 35 40 a3 b7 05 b3 a4 a9 46 3d 3e e3 98 71 dd b7 00 35 7d 42 3e 7b 19 33 37 79 ae d9 03 38 c8 c1 bc 2c 08 96 88 cf 78 10 98 f8 55 69 ee 00 6e 49 eb ed 67 85 8f ae 45 1e 0d b0 08 1d 3e 13 3a 37 b1 ed 1d 97 67 0a 40 91 3e b9 94 e5 b6 03 75 c1 d8 2a f9 30 42 1f ae b3 eb 45 db 0b e0 64 4e 6f 5f a8 d2 76 22 b7 c8 c7 f6 69 ab b3 a0 a1 37 85 4b 69 c3 cb 3f 7c de fe a0 98 37 30} (length 0xba)
// }
// union ANYUNION {
// ANYRES64: ANYRES64 (resource)
// }
// }
// }
// chdir: int8 = 0x9 (1 bytes)
// size: len = 0x37f73 (8 bytes)
// img: ptr[in, buffer] {
// buffer: (compressed buffer with length 0x37f73)
// }
// ]
// returns fd_dir
memcpy((void*)0x200000037f40, "gfs2\000", 5);
memcpy((void*)0x2000000008c0, "... [truncated large byte array] ...", 250);
memcpy((void*)0x200000000380, "... [truncated large byte array] ...", 186);
*(uint64_t*)0x20000000043a = -1;
memcpy((void*)0x20000006ff40, "... [truncated large byte array] ...", 229235);
syz_mount_image(/*fs=*/0x200000037f40, /*dir=*/0x2000000008c0, /*flags=MS_POSIXACL|MS_STRICTATIME|MS_NODEV|MS_DIRSYNC*/0x1010084, /*opts=*/0x200000000380, /*chdir=*/9, /*size=*/0x37f73, /*img=*/0x20000006ff40);
// capset arguments: [
// hdr: ptr[in, cap_header] {
// cap_header {
// var: cap_version = 0x20071026 (4 bytes)
// pid: pid (resource)
// }
// }
// data: ptr[in, cap_data] {
// cap_data {
// eff0: int32 = 0x200003 (4 bytes)
// perm0: int32 = 0x200003 (4 bytes)
// inher0: int32 = 0x801 (4 bytes)
// eff1: int32 = 0x4 (4 bytes)
// perm1: int32 = 0x7 (4 bytes)
// inher1: int32 = 0xc (4 bytes)
// }
// }
// ]
*(uint32_t*)0x200000000500 = 0x20071026;
*(uint32_t*)0x200000000504 = 0;
*(uint32_t*)0x200000000200 = 0x200003;
*(uint32_t*)0x200000000204 = 0x200003;
*(uint32_t*)0x200000000208 = 0x801;
*(uint32_t*)0x20000000020c = 4;
*(uint32_t*)0x200000000210 = 7;
*(uint32_t*)0x200000000214 = 0xc;
syscall(__NR_capset, /*hdr=*/0x200000000500ul, /*data=*/0x200000000200ul);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 00} (length 0x8)
// }
// flags: open_flags = 0x2c41 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000580, "./file0\000", 8);
syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000580ul, /*flags=O_NONBLOCK|O_CREAT|FASYNC|O_APPEND|0x1*/0x2c41, /*mode=*/0);
// truncate arguments: [
// file: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 00} (length 0x8)
// }
// len: intptr = 0x8fff5 (8 bytes)
// ]
memcpy((void*)0x200000000180, "./file0\000", 8);
syscall(__NR_truncate, /*file=*/0x200000000180ul, /*len=*/0x8fff5ul);
return 0;
}
]
|
| 2/1 |
2026/09/02 01:29 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/bafc31abfecdd56b10bb2005cdf2914031d9a451]
|
| 3/1 |
2026/09/02 01:30 |
action |
kernel-builder |
20m
Results: map[KernelObj:/app/workdir/cache/build/3e20023968f6d96c860012cc7e096769fbc3c333]
|
| 4/1 |
2026/09/02 01:50 |
action |
codesearch-prepare |
14m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/09/02 02:05 |
agent |
expert |
4m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
capability: warning: `syz-executor311' uses deprecated v2 capabilities in a way that may be insecure
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000020: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000100-0x0000000000000107]
CPU: 1 UID: 0 PID: 5636 Comm: syz-executor311 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:gfs2_quota_lock+0x103/0x7b0 fs/gfs2/quota.c:1105
Code: 00 00 4d 89 fd 49 c1 ed 03 41 80 7c 1d 00 00 74 08 4c 89 ff e8 1e fd e7 fd 49 8b 3f 48 8d 87 00 01 00 00 48 89 c1 48 c1 e9 03 <0f> b6 0c 19 84 c9 0f 85 c9 05 00 00 8b b7 00 01 00 00 ba 08 00 00
RSP: 0018:ffffc90003bcf8d0 EFLAGS: 00010202
RAX: 0000000000000100 RBX: dffffc0000000000 RCX: 0000000000000020
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff907a487f R09: 1ffffffff20f490f
R10: dffffc0000000000 R11: fffffbfff20f4910 R12: ffff888012189320
R13: 1ffff110024312f9 R14: 00000000ffffffff R15: ffff8880121897c8
FS: 000055558edf6400(0000) GS:ffff888124dcd000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000006a9ee0 CR3: 0000000073f60000 CR4: 00000000003526f0
Call Trace:
<TASK>
gfs2_quota_lock_check fs/gfs2/quota.h:50 [inline]
do_grow fs/gfs2/bmap.c:2117 [inline]
gfs2_setattr_size+0x9df/0x1270 fs/gfs2/bmap.c:2198
gfs2_setattr+0x269/0x430 fs/gfs2/inode.c:2138
notify_change+0xbba/0xea0 fs/attr.c:556
do_truncate+0x1c2/0x250 fs/open.c:68
vfs_truncate+0x4b4/0x540 fs/open.c:118
ksys_truncate+0xf3/0x1c0 fs/open.c:142
__do_sys_truncate fs/open.c:154 [inline]
__se_sys_truncate fs/open.c:152 [inline]
__x64_sys_truncate+0x5b/0x70 fs/open.c:152
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f49688f2409
Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd4cc3da68 EFLAGS: 00000246 ORIG_RAX: 000000000000004c
RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 00007f49688f2409
RDX: 00007f49688f2409 RSI: 000000000008fff5 RDI: 0000200000000180
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffd4cc3db98
R13: 0000000000000002 R14: 00007f49689b3c80 R15: 0000000000000002
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:gfs2_quota_lock+0x103/0x7b0 fs/gfs2/quota.c:1105
Code: 00 00 4d 89 fd 49 c1 ed 03 41 80 7c 1d 00 00 74 08 4c 89 ff e8 1e fd e7 fd 49 8b 3f 48 8d 87 00 01 00 00 48 89 c1 48 c1 e9 03 <0f> b6 0c 19 84 c9 0f 85 c9 05 00 00 8b b7 00 01 00 00 ba 08 00 00
RSP: 0018:ffffc90003bcf8d0 EFLAGS: 00010202
RAX: 0000000000000100 RBX: dffffc0000000000 RCX: 0000000000000020
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: ffffffff907a487f R09: 1ffffffff20f490f
R10: dffffc0000000000 R11: fffffbfff20f4910 R12: ffff888012189320
R13: 1ffff110024312f9 R14: 00000000ffffffff R15: ffff8880121897c8
FS: 000055558edf6400(0000) GS:ffff888124dcd000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000006a9ee0 CR3: 0000000073f60000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
0: 00 00 add %al,(%rax)
2: 4d 89 fd mov %r15,%r13
5: 49 c1 ed 03 shr $0x3,%r13
9: 41 80 7c 1d 00 00 cmpb $0x0,0x0(%r13,%rbx,1)
f: 74 08 je 0x19
11: 4c 89 ff mov %r15,%rdi
14: e8 1e fd e7 fd call 0xfde7fd37
19: 49 8b 3f mov (%r15),%rdi
1c: 48 8d 87 00 01 00 00 lea 0x100(%rdi),%rax
23: 48 89 c1 mov %rax,%rcx
26: 48 c1 e9 03 shr $0x3,%rcx
* 2a: 0f b6 0c 19 movzbl (%rcx,%rbx,1),%ecx <-- trapping instruction
2e: 84 c9 test %cl,%cl
30: 0f 85 c9 05 00 00 jne 0x5ff
36: 8b b7 00 01 00 00 mov 0x100(%rdi),%esi
3c: ba .byte 0xba
3d: 08 00 or %al,(%rax)
It is reproducible with the followint program.
Keep in mind that it may lack the precise threading, sandboxing, and some arguments of a working reproducer.
But it should give an idea of the involved syscalls.
// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <errno.h>
#include <fcntl.h>
#include <setjmp.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <linux/loop.h>
#ifndef __NR_memfd_create
#define __NR_memfd_create 319
#endif
static unsigned long long procid;
//% This code is derived from puff.{c,h}, found in the zlib development. The
//% original files come with the following copyright notice:
//% Copyright (C) 2002-2013 Mark Adler, all rights reserved
//% version 2.3, 21 Jan 2013
//% This software is provided 'as-is', without any express or implied
//% warranty. In no event will the author be held liable for any damages
//% arising from the use of this software.
//% Permission is granted to anyone to use this software for any purpose,
//% including commercial applications, and to alter it and redistribute it
//% freely, subject to the following restrictions:
//% 1. The origin of this software must not be misrepresented; you must not
//% claim that you wrote the original software. If you use this software
//% in a product, an acknowledgment in the product documentation would be
//% appreciated but is not required.
//% 2. Altered source versions must be plainly marked as such, and must not be
//% misrepresented as being the original software.
//% 3. This notice may not be removed or altered from any source distribution.
//% Mark Adler madler@alumni.caltech.edu
//% BEGIN CODE DERIVED FROM puff.{c,h}
#define MAXBITS 15
#define MAXLCODES 286
#define MAXDCODES 30
#define MAXCODES (MAXLCODES + MAXDCODES)
#define FIXLCODES 288
struct puff_state {
unsigned char* out;
unsigned long outlen;
unsigned long outcnt;
const unsigned char* in;
unsigned long inlen;
unsigned long incnt;
int bitbuf;
int bitcnt;
jmp_buf env;
};
static int puff_bits(struct puff_state* s, int need)
{
long val = s->bitbuf;
while (s->bitcnt < need) {
if (s->incnt == s->inlen)
longjmp(s->env, 1);
val |= (long)(s->in[s->incnt++]) << s->bitcnt;
s->bitcnt += 8;
}
s->bitbuf = (int)(val >> need);
s->bitcnt -= need;
return (int)(val & ((1L << need) - 1));
}
static int puff_stored(struct puff_state* s)
{
s->bitbuf = 0;
s->bitcnt = 0;
if (s->incnt + 4 > s->inlen)
return 2;
unsigned len = s->in[s->incnt++];
len |= s->in[s->incnt++] << 8;
if (s->in[s->incnt++] != (~len & 0xff) ||
s->in[s->incnt++] != ((~len >> 8) & 0xff))
return -2;
if (s->incnt + len > s->inlen)
return 2;
if (s->outcnt + len > s->outlen)
return 1;
for (; len--; s->outcnt++, s->incnt++) {
if (s->in[s->incnt])
s->out[s->outcnt] = s->in[s->incnt];
}
return 0;
}
struct puff_huffman {
short* count;
short* symbol;
};
static int puff_decode(struct puff_state* s, const struct puff_huffman* h)
{
int first = 0;
int index = 0;
int bitbuf = s->bitbuf;
int left = s->bitcnt;
int code = first = index = 0;
int len = 1;
short* next = h->count + 1;
while (1) {
while (left--) {
code |= bitbuf & 1;
bitbuf >>= 1;
int count = *next++;
if (code - count < first) {
s->bitbuf = bitbuf;
s->bitcnt = (s->bitcnt - len) & 7;
return h->symbol[index + (code - first)];
}
index += count;
first += count;
first <<= 1;
code <<= 1;
len++;
}
left = (MAXBITS + 1) - len;
if (left == 0)
break;
if (s->incnt == s->inlen)
longjmp(s->env, 1);
bitbuf = s->in[s->incnt++];
if (left > 8)
left = 8;
}
return -10;
}
static int puff_construct(struct puff_huffman* h, const short* length, int n)
{
int len;
for (len = 0; len <= MAXBITS; len++)
h->count[len] = 0;
int symbol;
for (symbol = 0; symbol < n; symbol++)
(h->count[length[symbol]])++;
if (h->count[0] == n)
return 0;
int left = 1;
for (len = 1; len <= MAXBITS; len++) {
left <<= 1;
left -= h->count[len];
if (left < 0)
return left;
}
short offs[MAXBITS + 1];
offs[1] = 0;
for (len = 1; len < MAXBITS; len++)
offs[len + 1] = offs[len] + h->count[len];
for (symbol = 0; symbol < n; symbol++)
if (length[symbol] != 0)
h->symbol[offs[length[symbol]]++] = symbol;
return left;
}
static int puff_codes(struct puff_state* s,
const struct puff_huffman* lencode,
const struct puff_huffman* distcode)
{
static const short lens[29] = {
3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 15, 17, 19, 23, 27, 31,
35, 43, 51, 59, 67, 83, 99, 115, 131, 163, 195, 227, 258};
static const short lext[29] = {
0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2,
3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 0};
static const short dists[30] = {
1, 2, 3, 4, 5, 7, 9, 13, 17, 25, 33, 49, 65, 97, 129, 193,
257, 385, 513, 769, 1025, 1537, 2049, 3073, 4097, 6145,
8193, 12289, 16385, 24577};
static const short dext[30] = {
0, 0, 0, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6,
7, 7, 8, 8, 9, 9, 10, 10, 11, 11,
12, 12, 13, 13};
int symbol;
do {
symbol = puff_decode(s, lencode);
if (symbol < 0)
return symbol;
if (symbol < 256) {
if (s->outcnt == s->outlen)
return 1;
if (symbol)
s->out[s->outcnt] = symbol;
s->outcnt++;
} else if (symbol > 256) {
symbol -= 257;
if (symbol >= 29)
return -10;
int len = lens[symbol] + puff_bits(s, lext[symbol]);
symbol = puff_decode(s, distcode);
if (symbol < 0)
return symbol;
unsigned dist = dists[symbol] + puff_bits(s, dext[symbol]);
if (dist > s->outcnt)
return -11;
if (s->outcnt + len > s->outlen)
return 1;
while (len--) {
if (dist <= s->outcnt && s->out[s->outcnt - dist])
s->out[s->outcnt] = s->out[s->outcnt - dist];
s->outcnt++;
}
}
} while (symbol != 256);
return 0;
}
static int puff_fixed(struct puff_state* s)
{
static int virgin = 1;
static short lencnt[MAXBITS + 1], lensym[FIXLCODES];
static short distcnt[MAXBITS + 1], distsym[MAXDCODES];
static struct puff_huffman lencode, distcode;
if (virgin) {
lencode.count = lencnt;
lencode.symbol = lensym;
distcode.count = distcnt;
distcode.symbol = distsym;
short lengths[FIXLCODES];
int symbol;
for (symbol = 0; symbol < 144; symbol++)
lengths[symbol] = 8;
for (; symbol < 256; symbol++)
lengths[symbol] = 9;
for (; symbol < 280; symbol++)
lengths[symbol] = 7;
for (; symbol < FIXLCODES; symbol++)
lengths[symbol] = 8;
puff_construct(&lencode, lengths, FIXLCODES);
for (symbol = 0; symbol < MAXDCODES; symbol++)
lengths[symbol] = 5;
puff_construct(&distcode, lengths, MAXDCODES);
virgin = 0;
}
return puff_codes(s, &lencode, &distcode);
}
static int puff_dynamic(struct puff_state* s)
{
static const short order[19] =
{16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15};
int nlen = puff_bits(s, 5) + 257;
int ndist = puff_bits(s, 5) + 1;
int ncode = puff_bits(s, 4) + 4;
if (nlen > MAXLCODES || ndist > MAXDCODES)
return -3;
short lengths[MAXCODES];
int index;
for (index = 0; index < ncode; index++)
lengths[order[index]] = puff_bits(s, 3);
for (; index < 19; index++)
lengths[order[index]] = 0;
short lencnt[MAXBITS + 1], lensym[MAXLCODES];
struct puff_huffman lencode = {lencnt, lensym};
int err = puff_construct(&lencode, lengths, 19);
if (err != 0)
return -4;
index = 0;
while (index < nlen + ndist) {
int symbol;
int len;
symbol = puff_decode(s, &lencode);
if (symbol < 0)
return symbol;
if (symbol < 16)
lengths[index++] = symbol;
else {
len = 0;
if (symbol == 16) {
if (index == 0)
return -5;
len = lengths[index - 1];
symbol = 3 + puff_bits(s, 2);
} else if (symbol == 17)
symbol = 3 + puff_bits(s, 3);
else
symbol = 11 + puff_bits(s, 7);
if (index + symbol > nlen + ndist)
return -6;
while (symbol--)
lengths[index++] = len;
}
}
if (lengths[256] == 0)
return -9;
err = puff_construct(&lencode, lengths, nlen);
if (err && (err < 0 || nlen != lencode.count[0] + lencode.count[1]))
return -7;
short distcnt[MAXBITS + 1], distsym[MAXDCODES];
struct puff_huffman distcode = {distcnt, distsym};
err = puff_construct(&distcode, lengths + nlen, ndist);
if (err && (err < 0 || ndist != distcode.count[0] + distcode.count[1]))
return -8;
return puff_codes(s, &lencode, &distcode);
}
static int puff(
unsigned char* dest,
unsigned long* destlen,
const unsigned char* source,
unsigned long sourcelen)
{
struct puff_state s = {
.out = dest,
.outlen = *destlen,
.outcnt = 0,
.in = source,
.inlen = sourcelen,
.incnt = 0,
.bitbuf = 0,
.bitcnt = 0,
};
int err;
if (setjmp(s.env) != 0)
err = 2;
else {
int last;
do {
last = puff_bits(&s, 1);
int type = puff_bits(&s, 2);
err = type == 0 ? puff_stored(&s) : (type == 1 ? puff_fixed(&s) : (type == 2 ? puff_dynamic(&s) : -1));
if (err != 0)
break;
} while (!last);
}
*destlen = s.outcnt;
return err;
}
//% END CODE DERIVED FROM puff.{c,h}
#define ZLIB_HEADER_WIDTH 2
static int puff_zlib_to_file(const unsigned char* source, unsigned long sourcelen, int dest_fd)
{
if (sourcelen < ZLIB_HEADER_WIDTH)
return 0;
source += ZLIB_HEADER_WIDTH;
sourcelen -= ZLIB_HEADER_WIDTH;
const unsigned long max_destlen = 132 << 20;
void* ret = mmap(0, max_destlen, PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANON, -1, 0);
if (ret == MAP_FAILED)
return -1;
unsigned char* dest = (unsigned char*)ret;
unsigned long destlen = max_destlen;
int err = puff(dest, &destlen, source, sourcelen);
if (err) {
munmap(dest, max_destlen);
errno = -err;
return -1;
}
if (write(dest_fd, dest, destlen) != (ssize_t)destlen) {
munmap(dest, max_destlen);
return -1;
}
return munmap(dest, max_destlen);
}
static int setup_loop_device(unsigned char* data, unsigned long size, const char* loopname, int* loopfd_p)
{
int err = 0, loopfd = -1;
int memfd = syscall(__NR_memfd_create, "syzkaller", 0);
if (memfd == -1) {
err = errno;
goto error;
}
if (puff_zlib_to_file(data, size, memfd)) {
err = errno;
goto error_close_memfd;
}
loopfd = open(loopname, O_RDWR);
if (loopfd == -1) {
err = errno;
goto error_close_memfd;
}
if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
if (errno != EBUSY) {
err = errno;
goto error_close_loop;
}
ioctl(loopfd, LOOP_CLR_FD, 0);
usleep(1000);
if (ioctl(loopfd, LOOP_SET_FD, memfd)) {
err = errno;
goto error_close_loop;
}
}
close(memfd);
*loopfd_p = loopfd;
return 0;
error_close_loop:
close(loopfd);
error_close_memfd:
close(memfd);
error:
errno = err;
return -1;
}
static void reset_loop_device(const char* loopname)
{
int loopfd = open(loopname, O_RDWR);
if (loopfd == -1) {
return;
}
if (ioctl(loopfd, LOOP_CLR_FD, 0)) {
}
close(loopfd);
}
static long syz_mount_image(
volatile long fsarg,
volatile long dir,
volatile long flags,
volatile long optsarg,
volatile long change_dir,
volatile unsigned long size,
volatile long image)
{
unsigned char* data = (unsigned char*)image;
int res = -1, err = 0, need_loop_device = !!size;
char* mount_opts = (char*)optsarg;
char* target = (char*)dir;
char* fs = (char*)fsarg;
char* source = NULL;
char loopname[64];
if (need_loop_device) {
int loopfd;
memset(loopname, 0, sizeof(loopname));
snprintf(loopname, sizeof(loopname), "/dev/loop%llu", procid);
if (setup_loop_device(data, size, loopname, &loopfd) == -1)
return -1;
close(loopfd);
source = loopname;
}
mkdir(target, 0777);
char opts[256];
memset(opts, 0, sizeof(opts));
if (strlen(mount_opts) > (sizeof(opts) - 32)) {
}
strncpy(opts, mount_opts, sizeof(opts) - 32);
if (strcmp(fs, "iso9660") == 0) {
flags |= MS_RDONLY;
} else if (strncmp(fs, "ext", 3) == 0) {
bool has_remount_ro = false;
char* remount_ro_start = strstr(opts, "errors=remount-ro");
if (remount_ro_start != NULL) {
char after = *(remount_ro_start + strlen("errors=remount-ro"));
char before = remount_ro_start == opts ? '\0' : *(remount_ro_start - 1);
has_remount_ro = ((before == '\0' || before == ',') && (after == '\0' || after == ','));
}
if (strstr(opts, "errors=panic") || !has_remount_ro)
strcat(opts, ",errors=continue");
} else if (strcmp(fs, "xfs") == 0) {
strcat(opts, ",nouuid");
} else if (strncmp(fs, "gfs2", 4) == 0 && (strstr(opts, "errors=panic") || strstr(opts, "debug"))) {
strcat(opts, ",errors=withdraw");
}
res = mount(source, target, fs, flags, opts);
if (res == -1) {
err = errno;
goto error_clear_loop;
}
res = open(target, O_RDONLY | O_DIRECTORY);
if (res == -1) {
err = errno;
goto error_clear_loop;
}
if (change_dir) {
res = chdir(target);
if (res == -1) {
err = errno;
}
}
error_clear_loop:
if (need_loop_device)
reset_loop_device(loopname);
errno = err;
return res;
}
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// prlimit64 arguments: [
// pid: pid (resource)
// res: rlimit_type = 0xe (8 bytes)
// new: ptr[in, rlimit] {
// rlimit {
// soft: intptr = 0x8 (8 bytes)
// hard: intptr = 0x88 (8 bytes)
// }
// }
// old: nil
// ]
*(uint64_t*)0x200000000000 = 8;
*(uint64_t*)0x200000000008 = 0x88;
syscall(__NR_prlimit64, /*pid=*/0, /*res=RLIMIT_RTPRIO*/0xeul, /*new=*/0x200000000000ul, /*old=*/0ul);
// sched_setscheduler arguments: [
// pid: pid (resource)
// policy: sched_policy = 0x2 (8 bytes)
// prio: ptr[in, int32] {
// int32 = 0x7 (4 bytes)
// }
// ]
*(uint32_t*)0x200000000240 = 7;
syscall(__NR_sched_setscheduler, /*pid=*/0, /*policy=SCHED_RR*/2ul, /*prio=*/0x200000000240ul);
// syz_mount_image$gfs2 arguments: [
// fs: ptr[in, buffer] {
// buffer: {67 66 73 32 00} (length 0x5)
// }
// dir: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 61 00} (length 0xfa)
// }
// flags: mount_flags = 0x1010084 (8 bytes)
// opts: ptr[inout, array[ANYUNION]] {
// array[ANYUNION] {
// union ANYUNION {
// ANYBLOB: buffer: {73 75 69 64 64 69 72 2c 6d 65 74 61 2c 64 69 73 63 61 72 64 2c 6e 6f 6c 6f 63 63 6f 6f 6b 69 65 2c 6e 6f 64 69 73 63 61 72 64 2c 71 75 6f 74 61 3d 61 63 63 6f 75 6e 74 2c 00 35 60 96 c0 35 40 a3 b7 05 b3 a4 a9 46 3d 3e e3 98 71 dd b7 00 35 7d 42 3e 7b 19 33 37 79 ae d9 03 38 c8 c1 bc 2c 08 96 88 cf 78 10 98 f8 55 69 ee 00 6e 49 eb ed 67 85 8f ae 45 1e 0d b0 08 1d 3e 13 3a 37 b1 ed 1d 97 67 0a 40 91 3e b9 94 e5 b6 03 75 c1 d8 2a f9 30 42 1f ae b3 eb 45 db 0b e0 64 4e 6f 5f a8 d2 76 22 b7 c8 c7 f6 69 ab b3 a0 a1 37 85 4b 69 c3 cb 3f 7c de fe a0 98 37 30} (length 0xba)
// }
// union ANYUNION {
// ANYRES64: ANYRES64 (resource)
// }
// }
// }
// chdir: int8 = 0x9 (1 bytes)
// size: len = 0x37f73 (8 bytes)
// img: ptr[in, buffer] {
// buffer: (compressed buffer with length 0x37f73)
// }
// ]
// returns fd_dir
memcpy((void*)0x200000037f40, "gfs2\000", 5);
memcpy((void*)0x2000000008c0, "... [truncated large byte array] ...", 250);
memcpy((void*)0x200000000380, "... [truncated large byte array] ...", 186);
*(uint64_t*)0x20000000043a = -1;
memcpy((void*)0x20000006ff40, "... [truncated large byte array] ...", 229235);
syz_mount_image(/*fs=*/0x200000037f40, /*dir=*/0x2000000008c0, /*flags=MS_POSIXACL|MS_STRICTATIME|MS_NODEV|MS_DIRSYNC*/0x1010084, /*opts=*/0x200000000380, /*chdir=*/9, /*size=*/0x37f73, /*img=*/0x20000006ff40);
// capset arguments: [
// hdr: ptr[in, cap_header] {
// cap_header {
// var: cap_version = 0x20071026 (4 bytes)
// pid: pid (resource)
// }
// }
// data: ptr[in, cap_data] {
// cap_data {
// eff0: int32 = 0x200003 (4 bytes)
// perm0: int32 = 0x200003 (4 bytes)
// inher0: int32 = 0x801 (4 bytes)
// eff1: int32 = 0x4 (4 bytes)
// perm1: int32 = 0x7 (4 bytes)
// inher1: int32 = 0xc (4 bytes)
// }
// }
// ]
*(uint32_t*)0x200000000500 = 0x20071026;
*(uint32_t*)0x200000000504 = 0;
*(uint32_t*)0x200000000200 = 0x200003;
*(uint32_t*)0x200000000204 = 0x200003;
*(uint32_t*)0x200000000208 = 0x801;
*(uint32_t*)0x20000000020c = 4;
*(uint32_t*)0x200000000210 = 7;
*(uint32_t*)0x200000000214 = 0xc;
syscall(__NR_capset, /*hdr=*/0x200000000500ul, /*data=*/0x200000000200ul);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 00} (length 0x8)
// }
// flags: open_flags = 0x2c41 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000580, "./file0\000", 8);
syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000580ul, /*flags=O_NONBLOCK|O_CREAT|FASYNC|O_APPEND|0x1*/0x2c41, /*mode=*/0);
// truncate arguments: [
// file: ptr[in, buffer] {
// buffer: {2e 2f 66 69 6c 65 30 00} (length 0x8)
// }
// len: intptr = 0x8fff5 (8 bytes)
// ]
memcpy((void*)0x200000000180, "./file0\000", 8);
syscall(__NR_truncate, /*file=*/0x200000000180ul, /*len=*/0x8fff5ul);
return 0;
}
Reply: **Bug Analysis:**
The bug is a NULL pointer dereference in `gfs2_quota_lock()` when trying to access `ip->i_qadata->qa_qd_num`.
In the GFS2 filesystem, the `ip->i_qadata` structure is allocated on-demand by calling `gfs2_qa_get()` before performing operations that might require quota locks (such as `truncate`, `fallocate`, `chown`, etc.). However, `gfs2_qa_get()` has an explicit check to skip allocation and return success (`0`) if the inode has the `GFS2_DIF_SYSTEM` flag set:
```c
if (ip->i_diskflags & GFS2_DIF_SYSTEM)
return 0;
```
If an operation like `truncate` is performed on a file with the `GFS2_DIF_SYSTEM` flag, `gfs2_setattr_size()` will call `gfs2_qa_get()` (which does nothing and returns 0), and then proceed to call `do_grow()` -> `gfs2_quota_lock_check()`.
If the user does not have the `CAP_SYS_RESOURCE` capability, `gfs2_quota_lock_check()` will call `gfs2_quota_lock()`. Unlike `gfs2_qa_get()`, `gfs2_quota_lock()` does not check for the `GFS2_DIF_SYSTEM` flag and unconditionally dereferences `ip->i_qadata`, which is `NULL`, leading to the crash.
**Trigger and Exploitability:**
The `GFS2_DIF_SYSTEM` flag is an internal filesystem flag. It cannot be set by userspace on normal files (it is explicitly filtered out in `gfs2_fileattr_set()`). Therefore, this bug can only be triggered by mounting a maliciously crafted GFS2 filesystem image where a normal, accessible file has the `GFS2_DIF_SYSTEM` flag set.
The crash occurs when reading from offset `0x100` (the offset of `qa_qd_num` in `struct gfs2_qadata`). Because this address falls within the unmapped zero page (protected by `mmap_min_addr`), it will reliably cause a page fault and a kernel oops/panic (Denial of Service). It is not exploitable for privilege escalation or arbitrary code execution.
Because GFS2 does not support unprivileged user namespace mounts (`FS_USERNS_MOUNT` is not set), an unprivileged user cannot mount the malicious image themselves. The attack requires either root privileges to mount the image, or physical access to a system that auto-mounts external media (e.g., a USB drive). Once the image is mounted, an unprivileged user can trigger the crash by simply calling `truncate` on the crafted file.
**Conclusions:**
* **Exploitable:** No. It is a pure NULL pointer dereference (read from `0x100`) which cannot be mapped by an attacker.
* **Denial of Service:** Yes. It reliably crashes the kernel.
* **Accessible From Unprivileged Processes:** No. While the final trigger (`truncate`) is done by an unprivileged user, the required setup (mounting the GFS2 image) requires `CAP_SYS_ADMIN` in the initial namespace.
* **Accessible From User Namespaces:** No. GFS2 cannot be mounted inside a user namespace.
* **Malicious Filesystem Trigger:** Yes. The bug relies on a crafted inode with the `GFS2_DIF_SYSTEM` flag.
* **Peripheral Trigger:** Yes. Can be triggered by plugging in a malicious USB drive on systems with auto-mount enabled.
|
| 6/2 |
2026/09/02 02:05 |
llm |
expert |
0m
Model:
Tokens:
input: 17744
output: 40
thoughts: 175
Thoughts: **Analyzing a Kernel Bug: A Deep Dive into Quota Locking**
Okay, so I've got a kernel bug report here. It's an "oops" – a general protection fault, likely stemming from an invalid address, specifically a non-canonical address, and it's KASAN-related, indicating a null-pointer dereference. The trace points to `gfs2_quota_lock` in `fs/gfs2/quota.c`, line 1105. This is where things get interesting. I need to understand what's happening within that function, at that specific line.
My immediate thoughts are, what data structure is being accessed that's possibly null at that point in the function? It looks like it is trying to lock something. Are there any race conditions potentially leading to a null pointer? Is it a context issue?
I need to dive into the code for `gfs2_quota_lock`. I need to understand its purpose within the GFS2 file system, how it interacts with quota management, and what locks it's attempting to acquire. This involves understanding the control flow leading up to line 1105 and looking at the function's arguments. I need to check to make sure the arguments that are being passed are valid at this point.
The KASAN report is particularly helpful since it points directly to a null-pointer dereference. My immediate instinct is to check for potential NULL checks that may be missing, or perhaps occurring too late. I need to review the code around that line, looking for potential areas where a pointer could be uninitialized, freed prematurely, or corrupted. I should also investigate if there are other similar code segments in the system which might be impacted by this flaw.
The non-canonical address suggests some sort of memory corruption or a problem with addressing modes. This also tells me that this is very likely a subtle bug, possibly triggered by specific workloads or system configurations. I'm going to need to reproduce this, if possible, to get more information, or at least understand the conditions under which it's triggered.
This is going to be a fun one.
|
| 7/2 |
2026/09/02 02:05 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/quota.c Name:gfs2_quota_lock]
Results: map[SourceCode:1091: int gfs2_quota_lock(struct gfs2_inode *ip, kuid_t uid, kgid_t gid)
1092: {
1093: struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
1094: struct gfs2_quota_data *qd;
1095: u32 x;
1096: int error;
1097:
1098: if (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF)
1099: return 0;
1100:
1101: error = gfs2_quota_hold(ip, uid, gid);
1102: if (error)
1103: return error;
1104:
1105: sort(ip->i_qadata->qa_qd, ip->i_qadata->qa_qd_num,
1106: sizeof(struct gfs2_quota_data *), sort_qd, NULL);
1107:
1108: for (x = 0; x < ip->i_qadata->qa_qd_num; x++) {
1109: qd = ip->i_qadata->qa_qd[x];
1110: error = do_glock(qd, NO_FORCE, &ip->i_qadata->qa_qd_ghs[x]);
1111: if (error)
1112: break;
1113: }
1114:
1115: if (!error)
1116: set_bit(GIF_QD_LOCKED, &ip->i_flags);
1117: else {
1118: while (x--)
1119: gfs2_glock_dq_uninit(&ip->i_qadata->qa_qd_ghs[x]);
1120: gfs2_quota_unhold(ip);
1121: }
1122:
1123: return error;
1124: }
SourceFile:fs/gfs2/quota.c]
|
| 8/2 |
2026/09/02 02:05 |
llm |
expert |
0m
Model:
Tokens:
input: 18993
output: 43
thoughts: 171
Thoughts: **Debugging a Null Pointer Dereference**
Okay, so I've got a crash, and it's pinpointed at line 1105. The `sort` function call seems to be the culprit: specifically, it's attempting to access `ip->i_qadata->qa_qd` and `ip->i_qadata->qa_qd_num`. The KASAN report is telling me it's a null pointer dereference in a very specific memory range, which strongly suggests that the problem lies with `ip->i_qadata` being NULL.
This means I need to dig into the `struct gfs2_inode` data structure. I need to understand under what circumstances `ip->i_qadata` might *be* NULL. Are there initialization paths where it's not properly set? Is there a race condition where it's being cleared out from under the `sort` function? Or is there an earlier error that's leading to this uninitialized state? I’ll need to trace back from this point to figure out how `ip` and the `i_qadata` member are handled, and how that could possibly become NULL here.
|
| 9/2 |
2026/09/02 02:05 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:fs/gfs2/quota.c FieldOffset:<nil> Name:gfs2_inode]
Results: map[Fields:[map[Name:i_inode OffsetBits:0 SizeBits:8704] map[Name:i_no_addr OffsetBits:8704 SizeBits:64] map[Name:i_no_formal_ino OffsetBits:8768 SizeBits:64] map[Name:i_generation OffsetBits:8832 SizeBits:64] map[Name:i_eattr OffsetBits:8896 SizeBits:64] map[Name:i_flags OffsetBits:8960 SizeBits:64] map[Name:i_gl OffsetBits:9024 SizeBits:64] map[Name:i_iopen_gh OffsetBits:9088 SizeBits:448] map[Name:i_qadata OffsetBits:9536 SizeBits:64] map[Name:i_rgd_gh OffsetBits:9600 SizeBits:448] map[Name:i_res OffsetBits:10048 SizeBits:384] map[Name:i_goal OffsetBits:10432 SizeBits:64] map[Name:i_sizehint OffsetBits:10496 SizeBits:32] map[Name:i_rw_mutex OffsetBits:10560 SizeBits:1152] map[Name:i_ordered OffsetBits:11712 SizeBits:128] map[Name:i_hash_cache OffsetBits:11840 SizeBits:64] map[Name:i_entries OffsetBits:11904 SizeBits:32] map[Name:i_diskflags OffsetBits:11936 SizeBits:32] map[Name:i_height OffsetBits:11968 SizeBits:8] map[Name:i_depth OffsetBits:11976 SizeBits:8] map[Name:i_rahead OffsetBits:11984 SizeBits:16]]]
|
| 10/2 |
2026/09/02 02:05 |
llm |
expert |
0m
Model:
Tokens:
input: 19964
output: 39
thoughts: 57
Thoughts: Here's my summary, as though I'm thinking through this:
**Analyzing the Memory Dump**
Okay, the crash is happening. I need to figure out where. The debugger says the crash occurred at address `0x100`, which is 256 bytes. My first thought is to correlate this to where relevant data structures are in memory. The log mentions `i_qadata` - and according to the memory map, that's located at an offset of 9536 bits, or 1192 bytes. That offset is considerably larger than where the crash occurred. This is suspicious. Now I want to take a closer look at the gfs2_qadata structure. It's time to investigate `struct gfs2_qadata` and see if the crash location sheds any light on corruption.
|
| 11/2 |
2026/09/02 02:05 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:fs/gfs2/quota.c Name:gfs2_qadata]
Results: map[Fields:[map[Name:qa_qd OffsetBits:0 SizeBits:256] map[Name:qa_qd_ghs OffsetBits:256 SizeBits:1792] map[Name:qa_qd_num OffsetBits:2048 SizeBits:32] map[Name:qa_ref OffsetBits:2080 SizeBits:32]]]
|
| 12/2 |
2026/09/02 02:05 |
llm |
expert |
0m
Model:
Tokens:
input: 20343
output: 42
thoughts: 168
Thoughts: **My Analysis of the KASAN Report**
Okay, so I'm looking at this KASAN report, and it's pretty clear what's happening. The report points to a null-pointer dereference, specifically, accessing memory at offset `0x100` (256 bytes) from an address that turns out to be zero. The report pinpoints that this offset corresponds to `qa_qd_num` within a `qadata` structure, which I can confirm from my understanding of the relevant data structures.
This means I am dereferencing `ip->i_qadata`, where `ip->i_qadata` *should* contain the address of the `qadata` structure, but currently, it's holding a NULL pointer. That's the root cause: accessing memory through a null pointer.
Now, the crucial question is *why* is `ip->i_qadata` null? Where is this pointer being set? The call stack indicates that the issue originates within `gfs2_quota_lock_check`, which, in turn, is calling `gfs2_quota_lock`. I'll need to dig deeper into the code around those two functions to track down why the `i_qadata` pointer isn't being initialized or is being prematurely set to NULL in this particular scenario. Understanding the logic within those two functions is the key to resolving this bug and preventing future issues of this type.
|
| 13/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/quota.c Name:gfs2_quota_lock_check]
Results: map[SourceCode: 40: static inline int gfs2_quota_lock_check(struct gfs2_inode *ip,
41: struct gfs2_alloc_parms *ap)
42: {
43: struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
44: int ret;
45:
46: ap->allowed = UINT_MAX; /* Assume we are permitted a whole lot */
47: if (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF ||
48: capable(CAP_SYS_RESOURCE))
49: return 0;
50: ret = gfs2_quota_lock(ip, NO_UID_QUOTA_CHANGE, NO_GID_QUOTA_CHANGE);
51: if (ret)
52: return ret;
53: if (sdp->sd_args.ar_quota == GFS2_QUOTA_ACCOUNT)
54: return 0;
55: ret = gfs2_quota_check(ip, ip->i_inode.i_uid, ip->i_inode.i_gid, ap);
56: if (ret)
57: gfs2_quota_unlock(ip);
58: return ret;
59: }
SourceFile:fs/gfs2/quota.h]
|
| 14/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 21240
output: 37
thoughts: 37
Thoughts: Here's my analysis of the provided information, as if these are my immediate thoughts:
**Analyzing `gfs2_quota_lock_check`'s Call Site**
Okay, so I'm digging into GFS2's quota management, and I see that the `gfs2_quota_lock_check` function is called from `do_grow`. Specifically, it looks like it's happening right there in `fs/gfs2/bmap.c` at line 2117. This `do_grow` function is interesting because it deals with expanding a file or a filesystem, usually. That means this quota check is most likely associated with block allocation. Hmm, this is probably to ensure a user or group doesn't exceed their quota limits when they're trying to add more blocks to their files, as I would expect. The fact that it is a block map operation makes this even more interesting. I bet this check happens at some key point during the expansion process. It would be valuable to understand the exact context, and determine when in the `do_grow` process this lock check is performed. I will need to look into this more, but my current understanding is that it is a check that would prevent a quota violation from occurring.
|
| 15/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/bmap.c Name:do_grow]
Results: map[SourceCode:2106: static int do_grow(struct inode *inode, u64 size)
2107: {
2108: struct gfs2_glock *gl = gfs2_inode_glock(inode);
2109: struct gfs2_inode *ip = GFS2_I(inode);
2110: struct gfs2_sbd *sdp = GFS2_SB(inode);
2111: struct gfs2_alloc_parms ap = { .target = 1, };
2112: struct buffer_head *dibh;
2113: int error;
2114: int unstuff = 0;
2115:
2116: if (gfs2_is_stuffed(ip) && size > gfs2_max_stuffed_size(ip)) {
2117: error = gfs2_quota_lock_check(ip, &ap);
2118: if (error)
2119: return error;
2120:
2121: error = gfs2_inplace_reserve(ip, &ap);
2122: if (error)
2123: goto do_grow_qunlock;
2124: unstuff = 1;
2125: }
2126:
2127: if (!unstuff) {
2128: error = gfs2_clear_beyond_eof(inode, size);
2129: if (error)
2130: goto do_grow_qunlock;
2131: }
2132:
2133: error = gfs2_trans_begin(sdp, RES_DINODE + RES_STATFS + RES_RG_BIT +
2134: (unstuff &&
2135: gfs2_is_jdata(ip) ? RES_JDATA : 0) +
2136: (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF ?
2137: 0 : RES_QUOTA), 0);
2138: if (error)
2139: goto do_grow_release;
2140:
2141: if (unstuff) {
2142: error = gfs2_unstuff_dinode(ip);
2143: if (error)
2144: goto do_end_trans;
2145: }
2146:
2147: error = gfs2_meta_inode_buffer(ip, &dibh);
2148: if (error)
2149: goto do_end_trans;
2150:
2151: truncate_setsize(inode, size);
2152: inode_set_mtime_to_ts(&ip->i_inode, inode_set_ctime_current(&ip->i_inode));
2153: gfs2_trans_add_meta(gl, dibh);
2154: gfs2_dinode_out(ip, dibh->b_data);
2155: brelse(dibh);
2156:
2157: do_end_trans:
2158: gfs2_trans_end(sdp);
2159: do_grow_release:
2160: if (unstuff) {
2161: gfs2_inplace_release(ip);
2162: do_grow_qunlock:
2163: gfs2_quota_unlock(ip);
2164: }
2165: return error;
2166: }
SourceFile:fs/gfs2/bmap.c]
|
| 16/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 22578
output: 42
thoughts: 18
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:
**My Immediate Thought Process on `do_grow` and `gfs2_setattr_size`**
Right, so `do_grow` is being called from within `gfs2_setattr_size`. That immediately flags some important implications. The `gfs2_setattr_size` function, being a part of the GFS2 filesystem, is directly responsible for modifying the size attribute of a file or inode. Therefore, it's pretty clear that `do_grow` is involved in expanding the *logical* size of a file, not just the metadata updates.
My initial assumption would be that `do_grow` handles the actual allocation of new blocks on disk, or perhaps the extension of existing extents, based on the requested size change within `gfs2_setattr_size`. I need to look closer at their interactions to identify if I should expect to see any interaction with the resource group. Are we allocating new extents or is this file growing using existing space? How does it handle potential fragmentation? I'd want to understand the locking involved here – `gfs2_setattr_size` likely needs to serialize size changes, but `do_grow` might introduce additional locking requirements, particularly if it's interacting with the block allocator or dealing with extent maps.
I also wonder if `do_grow` is responsible for handling any file system quotas or space limits enforcement. Or perhaps that is handled higher up in the call stack. Understanding where that validation occurs is critical for ensuring data integrity and preventing potential denial-of-service scenarios. I need to keep in mind, too, the specifics of how GFS2 manages its allocation maps, and how `do_grow` integrates with those mechanisms.
Finally, I'll be looking for how it deals with the transaction system. Is this a single, atomic operation or is it part of a larger, potentially multi-step transaction? Understanding the journaling and recovery mechanisms is critical to this, especially for large file growths where failures could lead to significant data loss or corruption if not handled correctly. I also want to verify if this is an atomic transaction or if it requires journaling.
|
| 17/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/bmap.c Name:gfs2_setattr_size]
Results: map[SourceCode:2180: int gfs2_setattr_size(struct inode *inode, u64 newsize)
2181: {
2182: struct gfs2_inode *ip = GFS2_I(inode);
2183: int ret;
2184:
2185: BUG_ON(!S_ISREG(inode->i_mode));
2186:
2187: ret = inode_newsize_ok(inode, newsize);
2188: if (ret)
2189: return ret;
2190:
2191: inode_dio_wait(inode);
2192:
2193: ret = gfs2_qa_get(ip);
2194: if (ret)
2195: goto out;
2196:
2197: if (newsize >= inode->i_size) {
2198: ret = do_grow(inode, newsize);
2199: goto out;
2200: }
2201:
2202: ret = do_shrink(inode, newsize);
2203: out:
2204: gfs2_rs_delete(ip);
2205: gfs2_qa_put(ip);
2206: return ret;
2207: }
SourceFile:fs/gfs2/bmap.c]
|
| 18/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 23510
output: 41
thoughts: 42
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm well-versed in GFS2 internals:
**Diving into `gfs2_qa_get`**
Right, so `gfs2_setattr_size` calls `gfs2_qa_get(ip)`. That immediately makes me curious. I know `setattr_size` is all about file size manipulation, which often involves handling quotas. So, `gfs2_qa_get` *must* be connected to quota accounting, probably fetching quota-related information for the inode `ip`. Time to see exactly what `gfs2_qa_get` is up to in the code. I'll need to understand how it interacts with the quota subsystem within GFS2, and how it's integrated with file size changes. I'm guessing it likely checks or updates quota usage before allowing the size change. Let's see if my hunch is right.
|
| 19/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/bmap.c Name:gfs2_qa_get]
Results: map[SourceCode: 553: /**
554: * gfs2_qa_get - make sure we have a quota allocations data structure,
555: * if necessary
556: * @ip: the inode for this reservation
557: */
558: int gfs2_qa_get(struct gfs2_inode *ip)
559: {
560: struct gfs2_sbd *sdp = GFS2_SB(&ip->i_inode);
561: struct inode *inode = &ip->i_inode;
562:
563: if (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF)
564: return 0;
565: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
566: return 0;
567:
568: spin_lock(&inode->i_lock);
569: if (ip->i_qadata == NULL) {
570: struct gfs2_qadata *tmp;
571:
572: spin_unlock(&inode->i_lock);
573: tmp = kmem_cache_zalloc(gfs2_qadata_cachep, GFP_NOFS);
574: if (!tmp)
575: return -ENOMEM;
576:
577: spin_lock(&inode->i_lock);
578: if (ip->i_qadata == NULL)
579: ip->i_qadata = tmp;
580: else
581: kmem_cache_free(gfs2_qadata_cachep, tmp);
582: }
583: ip->i_qadata->qa_ref++;
584: spin_unlock(&inode->i_lock);
585: return 0;
586: }
SourceFile:fs/gfs2/quota.c]
|
| 20/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 24357
output: 59
thoughts: 1272
Thoughts: **Analyzing a Potential NULL Pointer Dereference in GFS2 Quota Handling**
Okay, let's break this down. It seems we have a possible NULL pointer dereference situation in the GFS2 quota code, specifically related to how system files and quota checks interact. The core issue is this: `gfs2_quota_lock` might be called when `ip->i_qadata` is NULL, which could lead to a crash.
I see that `gfs2_qa_get` allocates `ip->i_qadata` only if quotas are enabled ( `sdp->sd_args.ar_quota != GFS2_QUOTA_OFF`). If quotas are disabled, or if the file is a system file (`ip->i_diskflags & GFS2_DIF_SYSTEM`), it returns 0 without allocating the `i_qadata`.
Now, the `gfs2_quota_lock_check` function has a crucial check. If quotas are off OR the process has `CAP_SYS_RESOURCE`, it returns early, effectively bypassing the `gfs2_quota_lock`. Conversely, if the system *doesn't* have `CAP_SYS_RESOURCE`, the code proceeds to `gfs2_quota_lock`.
In `gfs2_quota_lock`, it *only* checks if quotas are off; it *doesn't* check for `GFS2_DIF_SYSTEM`. This is the crux. So, here's the scenario:
1. We have quotas *enabled* (`sdp->sd_args.ar_quota != GFS2_QUOTA_OFF`).
2. The file is a system file (`ip->i_diskflags & GFS2_DIF_SYSTEM`).
3. The process *lacks* `CAP_SYS_RESOURCE`.
In this specific situation, `gfs2_qa_get` will *not* allocate `ip->i_qadata` because the file is a system file. Consequently, `gfs2_quota_lock` is then called and tries to use the uninitialized (NULL) `ip->i_qadata`, leading to a NULL pointer dereference.
The reproducer confirms this. It mounts a GFS2 image with `quota=account` (meaning quotas *are* enabled) and then drops `CAP_SYS_RESOURCE`. Then it attempts to truncate a file (`./file0`) that appears to be a system file. The `truncate` operation calls `gfs2_setattr_size`, which in turn leads to the `gfs2_quota_lock` call via the `do_grow` function and ultimately crashes.
I need to confirm if a user can even operate on `GFS2_DIF_SYSTEM` files, however, my prior experience suggests that these files are hidden or inaccessible to standard users. The crafted image likely has normal files marked with `GFS2_DIF_SYSTEM`, which is the root cause of the bug. It seems we need to verify the ability of a user to modify files that have this flag and then, fix the `gfs2_quota_lock` code.
|
| 21/2 |
2026/09/02 02:06 |
tool |
codesearch-find-references |
0m
Error:
Args: map[ContextFile:fs/gfs2/bmap.c IncludeSnippetLines:0 Name:GFS2_DIF_SYSTEM SourceTreePrefix:fs/gfs2]
Results: map[References:<nil> TruncatedOutput:false]
|
| 22/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 26383
output: 32
thoughts: 15
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in my code and this is a typical debugging scenario:
**Identifying and Locating a Macro**
Right, a macro. That's a good starting point. I can instantly see that this section of code revolves around a macro. Now, I need to understand its behavior and usage. The most straightforward approach is to find all instances of that macro within the codebase. Therefore, the next step is definitely a `grep` – that's the fastest way to get a quick overview of where this macro is defined and, more importantly, *used* throughout the project. This will reveal the context. Using grep will allow me to instantly understand how the macro is expanded, what arguments it takes, and where the errors are located.
|
| 23/2 |
2026/09/02 02:06 |
tool |
grepper |
0m
Args: map[Expression:GFS2_DIF_SYSTEM PathPrefix:fs/gfs2]
Results: map[Output:fs/gfs2/inode.c=260=struct inode *gfs2_lookup_by_inum(struct gfs2_sbd *sdp, u64 no_addr,
--
fs/gfs2/inode.c-273- error = -EIO;
fs/gfs2/inode.c:274: if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c-275- goto fail_iput;
--
fs/gfs2/inode.c=704=static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
--
fs/gfs2/inode.c-821- /* Force SYSTEM flag on all files and subdirs of a SYSTEM directory */
fs/gfs2/inode.c:822: if (dip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c:823: ip->i_diskflags |= GFS2_DIF_SYSTEM;
fs/gfs2/inode.c-824-
--
fs/gfs2/quota.c=558=int gfs2_qa_get(struct gfs2_inode *ip)
--
fs/gfs2/quota.c-564- return 0;
fs/gfs2/quota.c:565: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-566- return 0;
--
fs/gfs2/quota.c=1301=void gfs2_quota_change(struct gfs2_inode *ip, s64 change,
--
fs/gfs2/quota.c-1310- return;
fs/gfs2/quota.c:1311: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-1312- return;
]
|
| 24/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 27119
output: 42
thoughts: 31
Thoughts: Here's my take on those thoughts:
**Okay, Let's Pinpoint This Definition**
Alright, let me just double-check that... right, *of course*. I knew it was somewhere fundamental. So, `GFS2_DIF_SYSTEM`... that's declared in `include/uapi/linux/gfs2_ondisk.h`. Good, good. That's precisely where I'd expect it to be, in the user-space API definitions for the on-disk structures of GFS2. Now I can move on to the next step.
|
| 25/2 |
2026/09/02 02:06 |
tool |
grepper |
0m
Args: map[Expression:GFS2_DIF_SYSTEM PathPrefix:include/uapi/linux/gfs2_ondisk.h]
Results: map[Output:include/uapi/linux/gfs2_ondisk.h=232=enum {
--
include/uapi/linux/gfs2_ondisk.h-258-#define GFS2_DIF_SYNC 0x00000100
include/uapi/linux/gfs2_ondisk.h:259:#define GFS2_DIF_SYSTEM 0x00000200 /* New in gfs2 */
include/uapi/linux/gfs2_ondisk.h-260-#define GFS2_DIF_TOPDIR 0x00000400 /* New in gfs2 */
]
|
| 26/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 27492
output: 36
thoughts: 142
Thoughts: **My Analysis of the GFS2_DIF_SYSTEM Flag**
Alright, let's break this down. So, `GFS2_DIF_SYSTEM` is clearly an on-disk flag, a bit set directly within the file's metadata. My initial thought is the immediate vulnerability; a carefully crafted image could easily manipulate this flag on what should be a regular file. This is a classic attack vector, attempting to masquerade a user file as system file.
The next question, the crucial one, is how the system reacts when a file with this set flag is *opened*. Does it actually prevent the open operation outright? The code snippet from `gfs2_lookup_by_inum` is illuminating. Ah, the check! We see the logic:
```c
error = -EIO;
if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
goto fail_iput;
```
It looks like `gfs2_lookup_by_inum` *does* prevent looking up a "system" file given its inode number, typically done through a file handle. It's effectively blocking that method of access via an inode number lookup. That's a good first line of defense, but what about the broader question: what happens with regular path-based lookups? This `gfs2_lookup_by_inum` only addresses inode number-based lookups. The crucial question that remains is the path lookup. Is there a similar check when resolving a path? If not, the system remains vulnerable. This is where I'll need to drill deeper into the rest of the file system code, specifically how path lookups are handled, to confirm whether the check is bypassed in those cases. That could be the real vulnerability, or conversely a place for a better defense.
|
| 27/2 |
2026/09/02 02:06 |
tool |
grepper |
0m
Args: map[Expression:GFS2_DIF_SYSTEM PathPrefix:fs/gfs2/inode.c]
Results: map[Output:fs/gfs2/inode.c=260=struct inode *gfs2_lookup_by_inum(struct gfs2_sbd *sdp, u64 no_addr,
--
fs/gfs2/inode.c-273- error = -EIO;
fs/gfs2/inode.c:274: if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c-275- goto fail_iput;
--
fs/gfs2/inode.c=704=static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
--
fs/gfs2/inode.c-821- /* Force SYSTEM flag on all files and subdirs of a SYSTEM directory */
fs/gfs2/inode.c:822: if (dip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c:823: ip->i_diskflags |= GFS2_DIF_SYSTEM;
fs/gfs2/inode.c-824-
]
|
| 28/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 28356
output: 39
thoughts: 22
Thoughts: Here's my understanding of the situation, from a technical perspective:
**My Perspective on the GFS2 Lookup Process**
Okay, so I'm digging into how GFS2 handles file lookups. It looks like the standard path lookup sequence, the one we'd expect, funnels through `gfs2_lookup`. That's the main entry point for the path resolution. But the key piece here is that `gfs2_lookup` *then* calls `gfs2_lookupi`. So, `gfs2_lookupi` is the core workhorse function doing the heavy lifting, the actual meat of the lookup operation, while `gfs2_lookup` acts as the initial wrapper and entry point for this lookup routine. It's a pretty standard design pattern, isolating the internal workings from the initial public entry point. I'll need to look at `gfs2_lookupi` in detail to understand the real guts of how the inode is resolved from its path component.
|
| 29/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/inode.c Name:gfs2_lookupi]
Results: map[SourceCode: 324: struct inode *gfs2_lookupi(struct inode *dir, const struct qstr *name,
325: int is_root)
326: {
327: struct gfs2_glock *gl = gfs2_inode_glock(dir);
328: struct super_block *sb = dir->i_sb;
329: struct gfs2_holder d_gh;
330: int error = 0;
331: struct inode *inode = NULL;
332:
333: gfs2_holder_mark_uninitialized(&d_gh);
334: if (!name->len || name->len > GFS2_FNAMESIZE)
335: return ERR_PTR(-ENAMETOOLONG);
336:
337: if ((name->len == 1 && memcmp(name->name, ".", 1) == 0) ||
338: (name->len == 2 && memcmp(name->name, "..", 2) == 0 &&
339: dir == d_inode(sb->s_root))) {
340: igrab(dir);
341: return dir;
342: }
343:
344: if (gfs2_glock_is_locked_by_me(gl) == NULL) {
345: error = gfs2_glock_nq_init(gl, LM_ST_SHARED, 0, &d_gh);
346: if (error)
347: return ERR_PTR(error);
348: }
349:
350: if (!is_root) {
351: error = gfs2_permission(&nop_mnt_idmap, dir, MAY_EXEC);
352: if (error)
353: goto out;
354: }
355:
356: inode = gfs2_dir_search(dir, name, false);
357: if (IS_ERR(inode))
358: error = PTR_ERR(inode);
359: out:
360: if (gfs2_holder_initialized(&d_gh))
361: gfs2_glock_dq_uninit(&d_gh);
362: if (error == -ENOENT)
363: return NULL;
364: return inode ? inode : ERR_PTR(error);
365: }
SourceFile:fs/gfs2/inode.c]
|
| 30/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 29345
output: 41
thoughts: 461
Thoughts: **My Analysis of the GFS2 Null Pointer Dereference**
Okay, so `gfs2_dir_search` is returning an inode without bothering to check if the `GFS2_DIF_SYSTEM` flag is set. That's a potential vulnerability right there. It seems a regular user could potentially open a file marked as a system file if it's placed in a regular directory. That's a bad start.
But wait, if `GFS2_DIF_SYSTEM` *is* set, then `gfs2_qa_get` smartly returns 0, and that means it doesn't allocate `ip->i_qadata`. And that's where things get interesting. Then, `gfs2_setattr_size` calls `do_grow`, which in turn calls `gfs2_quota_lock_check`. So far, so good, but now `gfs2_quota_lock_check` does a `capable(CAP_SYS_RESOURCE)` check. If the user *doesn't* have that capability, it calls `gfs2_quota_lock`, and this is where it's all going to go wrong because `gfs2_quota_lock` doesn't check the `GFS2_DIF_SYSTEM` flag. Instead, it checks only if `sdp->sd_args.ar_quota` is `GFS2_QUOTA_OFF`.
Since quota is on, it *then* proceeds to dereference `ip->i_qadata`, which, as we established earlier, is NULL! Eureka! It's a classic NULL pointer dereference.
The critical line is reading `ip->i_qadata->qa_qd_num`. I see it clearly; `ip->i_qadata` is NULL, which means we are dereferencing a NULL pointer at offset 0x100. It reads from address 0x100, which is very, very low.
In modern Linux, due to address space layout randomization and protections like `mmap_min_addr` (often 65536 or higher), mapping address 0 is not allowed. Which is why reading from 0x100 is always going to cause a page fault, leading to a kernel oops or panic. It's a straightforward Denial of Service.
The upshot: this isn't an exploitable security vulnerability for privilege escalation or code execution, because we can't control what's at address 0x100 and there is no way for the attacker to map memory there to control the read value. We can crash the kernel, but that's about it. Still, a DoS is a DoS, and we need to report this.
Now, a crucial question, can a normal user even set `GFS2_DIF_SYSTEM` on a file in the first place, or do we need a malicious filesystem to trigger this? I need to investigate that part further.
|
| 31/2 |
2026/09/02 02:06 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/file.c Name:gfs2_fileattr_set]
Results: map[SourceCode: 280: int gfs2_fileattr_set(struct mnt_idmap *idmap,
281: struct dentry *dentry, struct file_kattr *fa)
282: {
283: struct inode *inode = d_inode(dentry);
284: u32 fsflags = fa->flags, gfsflags = 0;
285: u32 mask;
286: int i;
287:
288: if (d_is_special(dentry))
289: return -ENOTTY;
290:
291: if (fileattr_has_fsx(fa))
292: return -EOPNOTSUPP;
293:
294: for (i = 0; i < ARRAY_SIZE(fsflag_gfs2flag); i++) {
295: if (fsflags & fsflag_gfs2flag[i].fsflag) {
296: fsflags &= ~fsflag_gfs2flag[i].fsflag;
297: gfsflags |= fsflag_gfs2flag[i].gfsflag;
298: }
299: }
300: if (fsflags || gfsflags & ~GFS2_FLAGS_USER_SET)
301: return -EINVAL;
302:
303: mask = GFS2_FLAGS_USER_SET;
304: if (S_ISDIR(inode->i_mode)) {
305: mask &= ~GFS2_DIF_JDATA;
306: } else {
307: /* The GFS2_DIF_TOPDIR flag is only valid for directories. */
308: if (gfsflags & GFS2_DIF_TOPDIR)
309: return -EINVAL;
310: mask &= ~(GFS2_DIF_TOPDIR | GFS2_DIF_INHERIT_JDATA);
311: }
312:
313: return do_gfs2_set_flags(inode, gfsflags, mask);
314: }
SourceFile:fs/gfs2/file.c]
|
| 32/2 |
2026/09/02 02:06 |
llm |
expert |
0m
Model:
Tokens:
input: 31101
output: 33
thoughts: 14
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Examining GFS2_FLAGS_USER_SET**
Okay, let's dive in. I need to take a look at `GFS2_FLAGS_USER_SET`. I'm going to quickly check the definition and context of this flag. It's likely related to user-settable attributes within the GFS2 filesystem. I'm immediately thinking about potential implications for security, consistency, and how this flag influences metadata updates. I need to understand whether this flag affects what users can directly manipulate, what its intended use cases are, and if any restrictions are in place. Does this flag have any interactions with other flags or access control mechanisms within the kernel? I am interested in how the flag's behavior is influenced by the underlying storage structure, and how it translates to changes in disk layout or journaling. I'll need to examine the relevant code paths that read, write, and interpret this flag to ensure there are no surprises or potential vulnerabilities in our implementation. It's time to dig in and understand this flag's role completely.
|
| 33/2 |
2026/09/02 02:07 |
tool |
grepper |
0m
Args: map[Expression:GFS2_FLAGS_USER_SET PathPrefix:fs/gfs2]
Results: map[Output:fs/gfs2/file.c=185=void gfs2_set_inode_flags(struct inode *inode)
--
fs/gfs2/file.c-204-/* Flags that can be set by user space */
fs/gfs2/file.c:205:#define GFS2_FLAGS_USER_SET (GFS2_DIF_JDATA| \
fs/gfs2/file.c-206- GFS2_DIF_IMMUTABLE| \
--
fs/gfs2/file.c=280=int gfs2_fileattr_set(struct mnt_idmap *idmap,
--
fs/gfs2/file.c-299- }
fs/gfs2/file.c:300: if (fsflags || gfsflags & ~GFS2_FLAGS_USER_SET)
fs/gfs2/file.c-301- return -EINVAL;
fs/gfs2/file.c-302-
fs/gfs2/file.c:303: mask = GFS2_FLAGS_USER_SET;
fs/gfs2/file.c-304- if (S_ISDIR(inode->i_mode)) {
]
|
| 34/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 31705
output: 41
thoughts: 18
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into GFS2_FLAGS_USER_SET**
Okay, so I'm looking at `GFS2_FLAGS_USER_SET`. It's that flag again, and I need to solidify my understanding of it. I should probably start by refreshing my memory on the *exact* definition and how it is implemented. I'm going to pull up the definitive documentation or code reference. I need the canonical source, not just some random blog post. I want to see the complete specification. I can't rely on assumptions here; I need the authoritative statement on what this flag *actually* does, what it influences, and how it interacts with other GFS2 components. That definition needs to be absolutely clear in my mind before I start any real analysis or look at how it's used elsewhere in the kernel. Let's get that definition.
|
| 35/2 |
2026/09/02 02:07 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:fs/gfs2/file.c Name:GFS2_FLAGS_USER_SET]
Results: map[SourceCode: SourceFile:]
|
| 36/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 31984
output: 37
thoughts: 31
Thoughts: Okay, here's my interpretation of that thought process, written as a first-person summary, keeping in mind I'm an expert in this field:
**Decoding the GFS2 File Macro**
Alright, this is pointing me directly at a macro. A quick glance tells me we're likely dealing with something kernel-level, given the file path in question. Immediately, my mind jumps to `fs/gfs2/file.c` – a crucial spot for understanding how GFS2, the Global File System version 2, interacts with files. I'm already anticipating something related to file I/O or metadata handling, most likely.
My next move is simple: I'll jump straight to the relevant lines, 204 through 210. That should give me the immediate context I need. This specific range is likely where this macro is defined or, at the very least, where its initial usage reveals its function. No need to waste time scanning the entire file – precision is key, and a focused read will be more efficient. I'm expecting a snippet of code that will provide clues about what this macro is doing in the context of GFS2's file system operations. Let's see what we're looking at.
|
| 37/2 |
2026/09/02 02:07 |
tool |
read-file |
0m
Args: map[File:fs/gfs2/file.c FirstLine:204 LineCount:10]
Results: map[Contents: 204: /* Flags that can be set by user space */
205: #define GFS2_FLAGS_USER_SET (GFS2_DIF_JDATA| \
206: GFS2_DIF_IMMUTABLE| \
207: GFS2_DIF_APPENDONLY| \
208: GFS2_DIF_NOATIME| \
209: GFS2_DIF_SYNC| \
210: GFS2_DIF_TOPDIR| \
211: GFS2_DIF_INHERIT_JDATA)
212:
213: /**
]
|
| 38/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 32508
output: 32
thoughts: 374
Thoughts: **My Analysis of GFS2 and the GFS2_DIF_SYSTEM Flag**
Okay, so here's what I'm thinking about this `GFS2_DIF_SYSTEM` flag. It's not part of the `GFS2_FLAGS_USER_SET`, which means your average user shouldn't be able to mess with it directly using `ioctl` or `fileattr_set`. That makes sense; it's probably intended for internal filesystem housekeeping stuff like quotas, journals, and the like. This also means, if it *is* set, then it's either the filesystem itself doing it, or someone's got a maliciously crafted filesystem image.
Wait a second... if it's for those internal files, can a regular user even *access* them, let alone `truncate` them? Usually, these internal files are hidden, right? They're not going to be sitting around in the normal directory tree. Okay, let's see... looking at `gfs2_lookup_by_inum`, it explicitly blocks lookups of these system files. Bingo! It checks the `GFS2_DIF_SYSTEM` flag and bails with an error:
```c
error = -EIO;
if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
goto fail_iput;
```
So, you can't use something like `open_by_handle_at` to get at them. They aren't going to be linked in the root directory or any regular directory.
So, the only way to hit this bug is by mounting a specifically malicious GFS2 image. This image would have to have a directory entry that's pointing to an inode which *either* has the `GFS2_DIF_SYSTEM` flag set, or even more sneaky, the inode itself could be a normal file but has that flag set.
Now, mounting a filesystem usually requires `CAP_SYS_ADMIN` in the initial user namespace, or in a user namespace if the filesystem supports `FS_USERNS_MOUNT`. The big question now is, is GFS2 one of those that supports `FS_USERNS_MOUNT`? That's the next thing to find out.
|
| 39/2 |
2026/09/02 02:07 |
tool |
grepper |
0m
Error:
Args: map[Expression:FS_USERNS_MOUNT PathPrefix:fs/gfs2]
Results:
|
| 40/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 33415
output: 60
thoughts: 174
Thoughts: **Analyzing the GFS2 Mount Vulnerability**
Okay, so GFS2 definitely doesn't support `FS_USERNS_MOUNT`, meaning if you want to mount it, you need `CAP_SYS_ADMIN` in the initial user namespace. That limits the attack vectors quite a bit, but it's still a serious issue. Basically, we're talking about two scenarios:
1. A root user, who already has the privilege, does something malicious, which isn't ideal but not unheard of.
2. A system automatically mounts a malicious USB drive. Desktop environments are often configured this way, making this a much bigger concern. This is a supply chain vulnerability, basically.
Wait a second, are those the *only* possibilities? I need to think this through. Is there any other way a user could interact with GFS2 and possibly trigger this without requiring that root privilege? Could a system file somehow be exposed to userspace in a way that allows a crafted interaction? I know there's `sysfs` and `debugfs` for GFS2, but those aren't regular files that could be truncated. They don't expose system files as user-writable resources. So, is there anything else? Ah, what about that `gfs2_quota_lock_check` function? Where else is it called from? Maybe that's another vector for attack? Gotta check the call sites for that one.
|
| 41/2 |
2026/09/02 02:07 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:fs/gfs2/quota.h IncludeSnippetLines:0 Name:gfs2_quota_lock_check SourceTreePrefix:fs/gfs2]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:__gfs2_fallocate SourceFile:fs/gfs2/file.c SourceLine:1324] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:alloc_dinode SourceFile:fs/gfs2/inode.c SourceLine:426] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:do_grow SourceFile:fs/gfs2/bmap.c SourceLine:2117] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:ea_alloc_skeleton SourceFile:fs/gfs2/xattr.c SourceLine:757] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_iomap_begin_write SourceFile:fs/gfs2/bmap.c SourceLine:1041] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_link SourceFile:fs/gfs2/inode.c SourceLine:1114] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_page_mkwrite SourceFile:fs/gfs2/file.c SourceLine:488] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_rename SourceFile:fs/gfs2/inode.c SourceLine:1674] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:link_dinode SourceFile:fs/gfs2/inode.c SourceLine:641]] TruncatedOutput:false]
|
| 42/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 34447
output: 32
thoughts: 532
Thoughts: **My Analysis of the GFS2 Quota Lock Check Vulnerability**
Okay, so I've been tracing the calls to `gfs2_quota_lock_check`, and it seems like all the paths that call it also expect `ip->i_qadata` to be allocated *if* quota is enabled. That makes sense, but I need to be sure that `ip->i_qadata` *is* allocated before any of these calls. `gfs2_qa_get` is the function responsible for that allocation, and I see it called earlier, for instance, in `gfs2_setattr_size`.
Aha, here's a wrinkle. If the `ip->i_diskflags & GFS2_DIF_SYSTEM` bit is set, `gfs2_qa_get` returns immediately without allocating `ip->i_qadata`. Now, if `gfs2_quota_lock_check` is subsequently called, it calls `gfs2_quota_lock`, and *that* dereferences `ip->i_qadata`. Bingo! This is where the NULL pointer dereference happens.
So, any operation that modifies a file (fallocate, write, link, rename, you name it) on a file with the `GFS2_DIF_SYSTEM` flag set will trigger this, *provided* quota is enabled. Ah, but, wait... there's a crucial condition: the user must *not* have `CAP_SYS_RESOURCE`.
```c
if (sdp->sd_args.ar_quota == GFS2_QUOTA_OFF ||
capable(CAP_SYS_RESOURCE))
return 0;
```
This is significant. If the user *does* have `CAP_SYS_RESOURCE` (e.g., root), the check short-circuits, and we *avoid* the crash. So, this is a vulnerability specifically triggered by an unprivileged user operating on a system file.
But, how can an unprivileged user even *access* a system file? I have to consider the attack vector here. System files, by design, are not normally accessible. The only scenario that comes to mind is the mounting of a malicious filesystem image, perhaps through USB auto-mount or something similar. If such an image is mounted, the files *within* it could be owned by the unprivileged user, and the unprivileged user could then open them and call truncate or write functions on them. That's the likely exploit path. If the `GFS2_DIF_SYSTEM` flag is set on the file within the malicious image, the kernel will crash.
Now I need to eliminate other possible attack vectors. Are there other functions like `gfs2_set_flags` that allow a normal user to set `GFS2_DIF_SYSTEM`? I have to rule this out by checking them.
|
| 43/2 |
2026/09/02 02:07 |
tool |
grepper |
0m
Args: map[Expression:GFS2_DIF_SYSTEM PathPrefix:fs/gfs2]
Results: map[Output:fs/gfs2/inode.c=260=struct inode *gfs2_lookup_by_inum(struct gfs2_sbd *sdp, u64 no_addr,
--
fs/gfs2/inode.c-273- error = -EIO;
fs/gfs2/inode.c:274: if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c-275- goto fail_iput;
--
fs/gfs2/inode.c=704=static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
--
fs/gfs2/inode.c-821- /* Force SYSTEM flag on all files and subdirs of a SYSTEM directory */
fs/gfs2/inode.c:822: if (dip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c:823: ip->i_diskflags |= GFS2_DIF_SYSTEM;
fs/gfs2/inode.c-824-
--
fs/gfs2/quota.c=558=int gfs2_qa_get(struct gfs2_inode *ip)
--
fs/gfs2/quota.c-564- return 0;
fs/gfs2/quota.c:565: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-566- return 0;
--
fs/gfs2/quota.c=1301=void gfs2_quota_change(struct gfs2_inode *ip, s64 change,
--
fs/gfs2/quota.c-1310- return;
fs/gfs2/quota.c:1311: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-1312- return;
]
|
| 44/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 36152
output: 32
thoughts: 635
Thoughts: **My Assessment of the GFS2 `GFS2_DIF_SYSTEM` Bug**
Okay, here's the breakdown of what I'm seeing. This is in the `gfs2_create_inode` function, specifically related to the `GFS2_DIF_SYSTEM` flag. The code ensures that if a directory is marked as `GFS2_DIF_SYSTEM`, any files or subdirectories created within it also inherit that flag. The immediate question is, how can a regular user even *get* a file into this situation? SYSTEM directories are internal, used by GFS2 itself for things like quota or journaling, and therefore shouldn't be accessible to normal users.
So, the crux of this revolves around a malicious filesystem image. This bug is triggered when an unprivileged user, without `CAP_SYS_RESOURCE`, modifies a file with the `GFS2_DIF_SYSTEM` flag. Truncating a file is the stated trigger. That sounds reasonable.
Let's address the key properties.
1. **Malicious Filesystem Trigger:** Yes, absolutely. A specially crafted GFS2 image is required. The image must contain a file (or directory) with the `GFS2_DIF_SYSTEM` flag set, or a normal directory with this flag, but accessible to a standard user.
2. **Denial of Service (DoS):** Yes, this is a definite DoS. It's a NULL pointer dereference, which will cause a kernel oops or panic.
3. **Exploitable:** No, and this is important. It's a *pure* NULL pointer dereference. The attacker cannot control the value being read at the NULL address. This means privilege escalation or code execution is not possible, it's just a crash.
4. **Unprivileged:** This is the interesting point. Mounting a GFS2 image itself requires `CAP_SYS_ADMIN` in the initial namespace, because GFS2 is not a `FS_USERNS_MOUNT` filesystem. So, a normal user can't just mount a GFS2 image. However, if there's an auto-mount mechanism (like with USB drives) on the system, an unprivileged user with physical access could plug in a drive and potentially trigger the bug *after* the auto-mount completes. This falls under both the "Malicious Filesystem" and the "Peripheral Trigger" categories. Since mounting requires root, but triggering requires unprivileged user, the bug is accessible *after* an administrator has set up the malicious GFS2 filesystem.
The syzkaller reproducer makes this even clearer: it uses `syz_mount_image`, which, in that environment, runs as root. However, the critical vulnerability is accessible by an unprivileged user after the malicious filesystem is mounted.
Therefore, since the vulnerability is accessible *after* the initial privileged mount is complete, my answer to the question "Can the bug be reached from a typical (non-root) user process that does NOT have any special capabilities..." is a *conditional yes*. If the GFS2 image is mounted by an administrator (or through auto-mount), then the vulnerability is accessible to an unprivileged user without any capabilities.
|
| 45/2 |
2026/09/02 02:07 |
tool |
grepper |
0m
Error:
Args: map[Expression:FS_USERNS_MOUNT PathPrefix:fs/gfs2]
Results:
|
| 46/2 |
2026/09/02 02:07 |
llm |
expert |
0m
Model:
Tokens:
input: 37514
output: 58
thoughts: 578
Thoughts: **My Assessment of the GFS2 Quota Lock Vulnerability**
Okay, let's break this down systematically. The initial assessment is straightforward: "Accessible From Unprivileged Processes" is NO, because an unprivileged user can't mount GFS2. Similarly, "Accessible From User Namespaces" is NO, since GFS2 doesn't play nice with user namespaces. "Malicious Filesystem Trigger" is definitely YES. And "Peripheral Trigger" is YES, USB auto-mounts are a prime target for these kinds of attacks. "Exploitable" is NO, and I assess "Denial Of Service" to be YES.
Now, the important part: I need to determine if we can reach this bug *without* the obvious malicious filesystem vector. Let's see... the core issue revolves around `ip->i_qadata` being NULL when `gfs2_quota_lock` gets called. From the provided C code, `ip->i_qadata` *should* be initialized through `gfs2_qa_get`. Let's check `gfs2_qa_get` specifically...
Alright, if `sdp->sd_args.ar_quota == GFS2_QUOTA_OFF`, the function simply returns, which is fine. Similarly, if `ip->i_diskflags & GFS2_DIF_SYSTEM` is set, it also returns. Otherwise, it attempts to allocate `gfs2_qadata` using `kmem_cache_zalloc`. If that allocation fails, it returns `-ENOMEM`. The important observation is that if `kmem_cache_zalloc` fails, `gfs2_setattr_size` will return `-ENOMEM` which prevents `do_grow` from ever being called.
Therefore, `ip->i_qadata` can *only* be NULL if either quotas are explicitly off (`sdp->sd_args.ar_quota == GFS2_QUOTA_OFF`) or if the `GFS2_DIF_SYSTEM` flag is set. The quota check only occurs if `sdp->sd_args.ar_quota != GFS2_QUOTA_OFF`. Therefore, the only way to reach `gfs2_quota_lock` with `ip->i_qadata == NULL` is if `ip->i_diskflags & GFS2_DIF_SYSTEM` is true, AND quotas are on (`sdp->sd_args.ar_quota != GFS2_QUOTA_OFF`), AND the user lacks `CAP_SYS_RESOURCE`.
Now, the final point: Is there *any* other code path that might call `gfs2_quota_lock` without going through `gfs2_qa_get` first? This is the crucial question to determine the full scope of the vulnerability.
|
| 47/2 |
2026/09/02 02:08 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:fs/gfs2/quota.c IncludeSnippetLines:0 Name:gfs2_quota_lock SourceTreePrefix:fs/gfs2]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_quota_lock_check SourceFile:fs/gfs2/quota.h SourceLine:50] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:setattr_chown SourceFile:fs/gfs2/inode.c SourceLine:2064]] TruncatedOutput:false]
|
| 48/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 38884
output: 38
thoughts: 50
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience:
**Diving into the Quota Lock in GFS2**
Alright, my internal radar just pinged – I noticed something interesting in the GFS2 code: `setattr_chown` at `fs/gfs2/inode.c:2064` makes a direct call to `gfs2_quota_lock`. Now *that's* something I need to investigate further. It's not necessarily unusual, but it does flag a potential area to explore in terms of locking strategies and potential contention, especially around user and group ID changes. I need to refresh my memory on the intricacies of `setattr_chown` within the GFS2 context, specifically how it interacts with the quota subsystem. My immediate thoughts go to the performance implications of acquiring the quota lock here – are we talking about a short-duration lock, or is there a chance of blocking? I'll need to look closely at the surrounding code and the intended behavior.
|
| 49/2 |
2026/09/02 02:08 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/inode.c Name:setattr_chown]
Results: map[SourceCode:2038: static int setattr_chown(struct inode *inode, struct iattr *attr)
2039: {
2040: struct gfs2_inode *ip = GFS2_I(inode);
2041: struct gfs2_sbd *sdp = GFS2_SB(inode);
2042: kuid_t ouid, nuid;
2043: kgid_t ogid, ngid;
2044: int error;
2045: struct gfs2_alloc_parms ap = {};
2046:
2047: ouid = inode->i_uid;
2048: ogid = inode->i_gid;
2049: nuid = attr->ia_uid;
2050: ngid = attr->ia_gid;
2051:
2052: if (!(attr->ia_valid & ATTR_UID) || uid_eq(ouid, nuid))
2053: ouid = nuid = NO_UID_QUOTA_CHANGE;
2054: if (!(attr->ia_valid & ATTR_GID) || gid_eq(ogid, ngid))
2055: ogid = ngid = NO_GID_QUOTA_CHANGE;
2056: error = gfs2_qa_get(ip);
2057: if (error)
2058: return error;
2059:
2060: error = gfs2_rindex_update(sdp);
2061: if (error)
2062: goto out;
2063:
2064: error = gfs2_quota_lock(ip, nuid, ngid);
2065: if (error)
2066: goto out;
2067:
2068: ap.target = gfs2_get_inode_blocks(&ip->i_inode);
2069:
2070: if (!uid_eq(ouid, NO_UID_QUOTA_CHANGE) ||
2071: !gid_eq(ogid, NO_GID_QUOTA_CHANGE)) {
2072: error = gfs2_quota_check(ip, nuid, ngid, &ap);
2073: if (error)
2074: goto out_gunlock_q;
2075: }
2076:
2077: error = gfs2_trans_begin(sdp, RES_DINODE + 2 * RES_QUOTA, 0);
2078: if (error)
2079: goto out_gunlock_q;
2080:
2081: error = gfs2_setattr_simple(inode, attr);
2082: if (error)
2083: goto out_end_trans;
2084:
2085: if (!uid_eq(ouid, NO_UID_QUOTA_CHANGE) ||
2086: !gid_eq(ogid, NO_GID_QUOTA_CHANGE)) {
2087: gfs2_quota_change(ip, -(s64)ap.target, ouid, ogid);
2088: gfs2_quota_change(ip, ap.target, nuid, ngid);
2089: }
2090:
2091: out_end_trans:
2092: gfs2_trans_end(sdp);
2093: out_gunlock_q:
2094: gfs2_quota_unlock(ip);
2095: out:
2096: gfs2_qa_put(ip);
2097: return error;
2098: }
SourceFile:fs/gfs2/inode.c]
|
| 50/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 40212
output: 58
thoughts: 289
Thoughts: **Analyzing a Potential Vulnerability in GFS2 Filesystem Chown Implementation**
Okay, so I'm looking at `setattr_chown` and immediately see it calls `gfs2_qa_get(ip)`. Interesting. If `ip->i_diskflags & GFS2_DIF_SYSTEM` is set, `gfs2_qa_get` simply returns 0. No quota data is allocated in this case. Then, crucially, `gfs2_quota_lock(ip, nuid, ngid)` gets called.
Wait a second... `gfs2_quota_lock` *doesn't* seem to check for `capable(CAP_SYS_RESOURCE)`. That's a huge oversight! So, even if I, as root, call `chown` on a file flagged with `GFS2_DIF_SYSTEM`, it's going to trigger this code path without any privilege checks. This sounds like a potential crash scenario. The lack of a `CAP_SYS_RESOURCE` check in `gfs2_quota_lock` is definitely a concern. Root calling chown on a "system" file will lead to a crash!
But hold on, this does require a file with the `GFS2_DIF_SYSTEM` flag, right? And that's usually only set during filesystem image creation, correct? So, it should be hard to exploit.
I need to confirm if there are other ways to set `GFS2_DIF_SYSTEM`. I've looked at the obvious places. I checked `gfs2_fileattr_set` and it doesn't allow setting the `GFS2_DIF_SYSTEM`. What about `gfs2_set_flags`? Is there a possibility there? I need to check it and make sure. I'll need to keep digging to find a way to escalate privilege in any real scenario.
|
| 51/2 |
2026/09/02 02:08 |
tool |
codesearch-find-references |
0m
Error:
Args: map[ContextFile:fs/gfs2/file.c IncludeSnippetLines:0 Name:GFS2_DIF_SYSTEM SourceTreePrefix:fs/gfs2]
Results: map[References:<nil> TruncatedOutput:false]
|
| 52/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 40986
output: 32
thoughts: 16
Thoughts: Here's my summary, as though I'm thinking these thoughts:
**Time for Another Search**
Okay, time to circle back and use `grep` again. I need to make sure I've got a comprehensive picture of where `GFS2_DIF_SYSTEM` is defined, used, or even just mentioned. I've been digging through the codebase, but a fresh `grep` is always a good sanity check. Gotta be thorough. Let's see what else might be lurking out there that I missed the first time around...
|
| 53/2 |
2026/09/02 02:08 |
tool |
grepper |
0m
Args: map[Expression:GFS2_DIF_SYSTEM PathPrefix:fs/gfs2]
Results: map[Output:fs/gfs2/inode.c=260=struct inode *gfs2_lookup_by_inum(struct gfs2_sbd *sdp, u64 no_addr,
--
fs/gfs2/inode.c-273- error = -EIO;
fs/gfs2/inode.c:274: if (GFS2_I(inode)->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c-275- goto fail_iput;
--
fs/gfs2/inode.c=704=static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
--
fs/gfs2/inode.c-821- /* Force SYSTEM flag on all files and subdirs of a SYSTEM directory */
fs/gfs2/inode.c:822: if (dip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/inode.c:823: ip->i_diskflags |= GFS2_DIF_SYSTEM;
fs/gfs2/inode.c-824-
--
fs/gfs2/quota.c=558=int gfs2_qa_get(struct gfs2_inode *ip)
--
fs/gfs2/quota.c-564- return 0;
fs/gfs2/quota.c:565: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-566- return 0;
--
fs/gfs2/quota.c=1301=void gfs2_quota_change(struct gfs2_inode *ip, s64 change,
--
fs/gfs2/quota.c-1310- return;
fs/gfs2/quota.c:1311: if (ip->i_diskflags & GFS2_DIF_SYSTEM)
fs/gfs2/quota.c-1312- return;
]
|
| 54/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 41672
output: 58
thoughts: 1737
Thoughts: **Analysis of the GFS2 Bug**
Okay, let's break this down. It seems we've got a kernel bug within the GFS2 file system. The `GFS2_DIF_SYSTEM` flag on a normal file is only set during filesystem image creation, meaning it's not something a user can normally flip. This strongly suggests a malicious filesystem image is the root cause.
The vulnerability manifests as a NULL pointer dereference, triggered by code expecting `ip->i_qadata` to be valid when it isn't. The analysis points to the `truncate` syscall as the trigger, and specifically, functions like `gfs2_setattr_size`, `do_grow`, and `gfs2_quota_lock_check`, eventually leading to `gfs2_quota_lock`. It's crucial we see how it works in practice.
Here's the interesting part: root calling `truncate` *doesn't* crash the kernel because of the `capable(CAP_SYS_RESOURCE)` check, effectively short-circuiting the path to the problematic code. However, an unprivileged user *can* trigger the crash. This is a denial-of-service vulnerability due to the unprivileged process triggering the kernel panic.
Now, about accessibility. The bug itself is in a filesystem function, so it clearly involves filesystem interactions. The prompt is asking if this can be reached by a *typical* user who doesn't have any elevated privileges. The key here is that mounting the malicious filesystem is necessary to trigger the bug. Even though the final trigger (e.g. `truncate`) comes from an unprivileged process, the setup (mounting) requires root privileges. It's a bit ambiguous, because if the filesystem is *already* mounted, like on a USB drive with auto-mount, then an unprivileged user might be able to trigger it. So, while an unprivileged user *could* trigger the bug if the filesystem is mounted, a typical user can't mount a filesystem themselves. I'm going to categorize it as "NO" - *unless* we assume a pre-mounted malicious filesystem. Syzkaller had to mount the filesystem as root initially.
Let's quickly verify no other way can cause this. I'm focusing on whether `ip->i_qadata` could be NULL in other scenarios. The code in `gfs2_qa_get` seems pretty solid – it allocates `ip->i_qadata` when needed, or returns an error. It looks like this can only happen if the malicious file image does not call `gfs2_qa_get` before triggering the bug, or fails to get quota information. Looking at the code confirms this.
Therefore, the full reproducer does indeed require `CAP_SYS_ADMIN` to mount the malicious filesystem, and is "NO".
|
| 55/2 |
2026/09/02 02:08 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:fs/gfs2/quota.c IncludeSnippetLines:0 Name:gfs2_quota_lock SourceTreePrefix:fs/gfs2]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_quota_lock_check SourceFile:fs/gfs2/quota.h SourceLine:50] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:setattr_chown SourceFile:fs/gfs2/inode.c SourceLine:2064]] TruncatedOutput:false]
|
| 56/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 44197
output: 60
thoughts: 48
Thoughts: Okay, here's my interpretation of those thoughts, presented as my own mental process, given my expertise:
**Quota Locking Call Flow Analysis**
Right, so we've already thoroughly examined `setattr_chown`, and we confirmed that it calls `gfs2_qa_get` *before* it tries to grab the `gfs2_quota_lock`. That's good; it confirms that specific order which is vital.
But that's just one function. I need to widen the scope of this investigation. The crucial next step is to analyze the other locations, the *callers* of `gfs2_quota_lock_check`. I need to understand if the same safeguarding behavior is present, or if there's any other code path that might call `gfs2_quota_lock_check` in a way that *doesn't* first ensure that `gfs2_qa_get` is called. Those other call paths are my key concern now. Any alternate call paths might bypass the current expected behavior.
|
| 57/2 |
2026/09/02 02:08 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:fs/gfs2/quota.h IncludeSnippetLines:0 Name:gfs2_quota_lock_check SourceTreePrefix:fs/gfs2]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:__gfs2_fallocate SourceFile:fs/gfs2/file.c SourceLine:1324] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:alloc_dinode SourceFile:fs/gfs2/inode.c SourceLine:426] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:do_grow SourceFile:fs/gfs2/bmap.c SourceLine:2117] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:ea_alloc_skeleton SourceFile:fs/gfs2/xattr.c SourceLine:757] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_iomap_begin_write SourceFile:fs/gfs2/bmap.c SourceLine:1041] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_link SourceFile:fs/gfs2/inode.c SourceLine:1114] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_page_mkwrite SourceFile:fs/gfs2/file.c SourceLine:488] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_rename SourceFile:fs/gfs2/inode.c SourceLine:1674] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:link_dinode SourceFile:fs/gfs2/inode.c SourceLine:641]] TruncatedOutput:false]
|
| 58/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 45020
output: 40
thoughts: 45
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, and written as if they were my own:
**Digging into gfs2_qa_get and Quota Lock Order**
Alright, I'm diving into the code and my gut feeling is that we need to confirm something critical. Specifically, I need to verify that `gfs2_qa_get` consistently precedes `gfs2_quota_lock_check` in its call sequence. This is a common pattern for resource management, so I want to make sure we haven't inadvertently broken anything or introduced a subtle race condition. My primary concern revolves around quota accounting and lock ordering in critical operations. So, let's start with `__gfs2_fallocate`.
|
| 59/2 |
2026/09/02 02:08 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/file.c Name:__gfs2_fallocate]
Results: map[SourceCode:1264: static long __gfs2_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
1265: {
1266: struct inode *inode = file_inode(file);
1267: struct gfs2_sbd *sdp = GFS2_SB(inode);
1268: struct gfs2_inode *ip = GFS2_I(inode);
1269: struct gfs2_alloc_parms ap = {};
1270: unsigned int data_blocks = 0, ind_blocks = 0, rblocks;
1271: loff_t bytes, max_bytes, max_blks;
1272: int error;
1273: const loff_t pos = offset;
1274: const loff_t count = len;
1275: loff_t bsize_mask = ~((loff_t)sdp->sd_sb.sb_bsize - 1);
1276: loff_t next = (offset + len - 1) >> sdp->sd_sb.sb_bsize_shift;
1277: loff_t max_chunk_size = UINT_MAX & bsize_mask;
1278:
1279: next = (next + 1) << sdp->sd_sb.sb_bsize_shift;
1280:
1281: if (!(mode & FALLOC_FL_KEEP_SIZE)) {
1282: error = gfs2_clear_beyond_eof(inode, offset + len);
1283: if (error)
1284: return error;
1285: }
1286:
1287: offset &= bsize_mask;
1288:
1289: len = next - offset;
1290: bytes = sdp->sd_max_rg_data * sdp->sd_sb.sb_bsize / 2;
1291: if (!bytes)
1292: bytes = UINT_MAX;
1293: bytes &= bsize_mask;
1294: if (bytes == 0)
1295: bytes = sdp->sd_sb.sb_bsize;
1296:
1297: gfs2_size_hint(file, offset, len);
1298:
1299: gfs2_write_calc_reserv(ip, PAGE_SIZE, &data_blocks, &ind_blocks);
1300: ap.min_target = data_blocks + ind_blocks;
1301:
1302: while (len > 0) {
1303: if (len < bytes)
1304: bytes = len;
1305: if (!gfs2_write_alloc_required(ip, offset, bytes)) {
1306: len -= bytes;
1307: offset += bytes;
1308: continue;
1309: }
1310:
1311: /* We need to determine how many bytes we can actually
1312: * fallocate without exceeding quota or going over the
1313: * end of the fs. We start off optimistically by assuming
1314: * we can write max_bytes */
1315: max_bytes = (len > max_chunk_size) ? max_chunk_size : len;
1316:
1317: /* Since max_bytes is most likely a theoretical max, we
1318: * calculate a more realistic 'bytes' to serve as a good
1319: * starting point for the number of bytes we may be able
1320: * to write */
1321: gfs2_write_calc_reserv(ip, bytes, &data_blocks, &ind_blocks);
1322: ap.target = data_blocks + ind_blocks;
1323:
1324: error = gfs2_quota_lock_check(ip, &ap);
1325: if (error)
1326: return error;
1327: /* ap.allowed tells us how many blocks quota will allow
1328: * us to write. Check if this reduces max_blks */
1329: max_blks = UINT_MAX;
1330: if (ap.allowed)
1331: max_blks = ap.allowed;
1332:
1333: error = gfs2_inplace_reserve(ip, &ap);
1334: if (error)
1335: goto out_qunlock;
1336:
1337: /* check if the selected rgrp limits our max_blks further */
1338: if (ip->i_res.rs_reserved < max_blks)
1339: max_blks = ip->i_res.rs_reserved;
1340:
1341: /* Almost done. Calculate bytes that can be written using
1342: * max_blks. We also recompute max_bytes, data_blocks and
1343: * ind_blocks */
1344: calc_max_reserv(ip, &max_bytes, &data_blocks,
1345: &ind_blocks, max_blks);
1346:
1347: rblocks = RES_DINODE + ind_blocks + RES_STATFS + RES_QUOTA +
1348: RES_RG_HDR + gfs2_rg_blocks(ip, data_blocks + ind_blocks);
1349: if (gfs2_is_jdata(ip))
1350: rblocks += data_blocks ? data_blocks : 1;
1351:
1352: error = gfs2_trans_begin(sdp, rblocks,
1353: PAGE_SIZE >> inode->i_blkbits);
1354: if (error)
1355: goto out_trans_fail;
1356:
1357: error = fallocate_chunk(inode, offset, max_bytes);
1358: gfs2_trans_end(sdp);
1359:
1360: if (error)
1361: goto out_trans_fail;
1362:
1363: len -= max_bytes;
1364: offset += max_bytes;
1365: gfs2_inplace_release(ip);
1366: gfs2_quota_unlock(ip);
1367: }
1368:
1369: if (!(mode & FALLOC_FL_KEEP_SIZE) && (pos + count) > inode->i_size)
1370: i_size_write(inode, pos + count);
1371: file_update_time(file);
1372: mark_inode_dirty(inode);
1373:
1374: if ((file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host))
1375: return vfs_fsync_range(file, pos, pos + count - 1,
1376: (file->f_flags & __O_SYNC) ? 0 : 1);
1377: return 0;
1378:
1379: out_trans_fail:
1380: gfs2_inplace_release(ip);
1381: out_qunlock:
1382: gfs2_quota_unlock(ip);
1383: return error;
1384: }
SourceFile:fs/gfs2/file.c]
|
| 60/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 47302
output: 39
thoughts: 20
Thoughts: Here's my understanding of the situation, framed as my own thought process, aimed at an expert audience like yourself:
**Understanding the Allocation Chain in GFS2**
Alright, so the `__gfs2_fallocate` function is the direct recipient of a call from `gfs2_fallocate`. This is crucial because it immediately tells me something about the architectural layering here. `gfs2_fallocate` likely acts as a higher-level interface, perhaps handling things like VFS integration, permission checks, error handling, and argument validation. It presents a more user-facing API. Then, under the hood, it delegates the actual, low-level allocation work to `__gfs2_fallocate`. I'd expect `__gfs2_fallocate` to be the core workhorse responsible for the nuts and bolts of allocating blocks within the GFS2 file system structure, perhaps interacting directly with the on-disk metadata or block allocation structures. My immediate questions would revolve around the specifics of this delegation – what's being passed down as context, are there any assumptions that `__gfs2_fallocate` makes about the state of the system or the arguments passed to it? The implications of this split need to be analyzed to understand the benefits and potential trade-offs of this approach. It looks like the system design is very intentional here.
|
| 61/2 |
2026/09/02 02:08 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/file.c Name:gfs2_fallocate]
Results: map[SourceCode:1386: static long gfs2_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
1387: {
1388: struct inode *inode = file_inode(file);
1389: struct gfs2_sbd *sdp = GFS2_SB(inode);
1390: struct gfs2_glock *gl = gfs2_inode_glock(inode);
1391: struct gfs2_inode *ip = GFS2_I(inode);
1392: struct gfs2_holder gh;
1393: int ret;
1394:
1395: if (mode & ~(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE))
1396: return -EOPNOTSUPP;
1397: /* fallocate is needed by gfs2_grow to reserve space in the rindex */
1398: if (gfs2_is_jdata(ip) && inode != sdp->sd_rindex)
1399: return -EOPNOTSUPP;
1400:
1401: inode_lock(inode);
1402:
1403: gfs2_holder_init(gl, LM_ST_EXCLUSIVE, 0, &gh);
1404: ret = gfs2_glock_nq(&gh);
1405: if (ret)
1406: goto out_uninit;
1407:
1408: if (!(mode & FALLOC_FL_KEEP_SIZE) &&
1409: (offset + len) > inode->i_size) {
1410: ret = inode_newsize_ok(inode, offset + len);
1411: if (ret)
1412: goto out_unlock;
1413: }
1414:
1415: ret = get_write_access(inode);
1416: if (ret)
1417: goto out_unlock;
1418:
1419: if (mode & FALLOC_FL_PUNCH_HOLE) {
1420: ret = __gfs2_punch_hole(file, offset, len);
1421: } else {
1422: ret = __gfs2_fallocate(file, mode, offset, len);
1423: if (ret)
1424: gfs2_rs_deltree(&ip->i_res);
1425: }
1426:
1427: put_write_access(inode);
1428: out_unlock:
1429: gfs2_glock_dq(&gh);
1430: out_uninit:
1431: gfs2_holder_uninit(&gh);
1432: inode_unlock(inode);
1433: return ret;
1434: }
SourceFile:fs/gfs2/file.c]
|
| 62/2 |
2026/09/02 02:08 |
llm |
expert |
0m
Model:
Tokens:
input: 48460
output: 58
thoughts: 128
Thoughts: **My Immediate Concerns Regarding `gfs2_fallocate` and Quota Accounting**
Okay, hold on a second. It just hit me: `gfs2_fallocate` *doesn't* actually call `gfs2_qa_get`. That's a potential problem, a subtle but significant oversight. I need to verify this immediately. Let's dig into the code, specifically within both `gfs2_fallocate` and `__gfs2_fallocate`, and double-check; are we actually calling `gfs2_qa_get` anywhere in there?
Nope, nothing. I see nothing. This is bad, this is *very* bad. If `gfs2_qa_get` isn't being invoked, then we have a gaping hole in our quota accounting. My mind's racing... This means that `ip->i_qadata`, the quota data associated with an inode, *could* remain NULL even for perfectly normal, everyday files! That's a major vulnerability. We could be completely missing quota information during allocation.
I need to confirm *when* `ip->i_qadata` is initialized. Is it allocated when the inode is created or when it is read from disk? I need to understand the lifecycle of this data structure to grasp the implications of this oversight fully. We must figure out where the quota accounting is going wrong.
|
| 63/2 |
2026/09/02 02:09 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:fs/gfs2/quota.c IncludeSnippetLines:0 Name:gfs2_qa_get SourceTreePrefix:fs/gfs2]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_create_inode SourceFile:fs/gfs2/inode.c SourceLine:725] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_create_inode SourceFile:fs/gfs2/inode.c SourceLine:788] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_link SourceFile:fs/gfs2/inode.c SourceLine:1061] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_open_common SourceFile:fs/gfs2/file.c SourceLine:653] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_quota_hold SourceFile:fs/gfs2/quota.c SourceLine:609] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_rename SourceFile:fs/gfs2/inode.c SourceLine:1523] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_set_acl SourceFile:fs/gfs2/acl.c SourceLine:119] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_set_dqblk SourceFile:fs/gfs2/quota.c SourceLine:1746] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_setattr SourceFile:fs/gfs2/inode.c SourceLine:2121] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_setattr_size SourceFile:fs/gfs2/bmap.c SourceLine:2193] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:gfs2_xattr_set SourceFile:fs/gfs2/xattr.c SourceLine:1252] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:setattr_chown SourceFile:fs/gfs2/inode.c SourceLine:2056]] TruncatedOutput:false]
|
| 64/2 |
2026/09/02 02:09 |
llm |
expert |
0m
Model:
Tokens:
input: 49602
output: 40
thoughts: 24
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience and written in the first person:
**Initial Observation: gfs2_qa_get and gfs2_open_common**
Okay, so I'm digging into this GFS2 code, and it's looking like `gfs2_qa_get` gets called within `gfs2_open_common`. This immediately triggers a mental check. Given the naming conventions, `gfs2_qa_get` likely deals with some kind of quota or allocation information, and `gfs2_open_common` is the core routine for, well, opening a file. This is interesting. The fact that the quota retrieval is performed during a file open suggests that quota enforcement or reservation could be implemented at the open time. I'll need to look at what `gfs2_qa_get` is actually retrieving and how it's being used in `gfs2_open_common`. Is it just checking if we _can_ open the file given quota constraints? Or is it doing more? Is it pre-reserving resources? Or maybe it is setting up the quotas on an opened file, and checking, or enforcing during close, or even during write. I'm going to have to analyze the code flow from there. My gut says this is an important point in the access control mechanism.
|
| 65/2 |
2026/09/02 02:09 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/gfs2/file.c Name:gfs2_open_common]
Results: map[SourceCode: 630: int gfs2_open_common(struct inode *inode, struct file *file)
631: {
632: struct gfs2_file *fp;
633: int ret;
634:
635: if (S_ISREG(inode->i_mode)) {
636: ret = generic_file_open(inode, file);
637: if (ret)
638: return ret;
639:
640: if (!gfs2_is_jdata(GFS2_I(inode)))
641: file->f_mode |= FMODE_CAN_ODIRECT;
642: }
643:
644: fp = kzalloc_obj(struct gfs2_file, GFP_NOFS);
645: if (!fp)
646: return -ENOMEM;
647:
648: mutex_init(&fp->f_fl_mutex);
649:
650: gfs2_assert_warn(GFS2_SB(inode), !file->private_data);
651: file->private_data = fp;
652: if (file->f_mode & FMODE_WRITE) {
653: ret = gfs2_qa_get(GFS2_I(inode));
654: if (ret)
655: goto fail;
656: }
657: return 0;
658:
659: fail:
660: kfree(file->private_data);
661: file->private_data = NULL;
662: return ret;
663: }
SourceFile:fs/gfs2/file.c]
|
| 66/2 |
2026/09/02 02:09 |
llm |
expert |
0m
Model:
Tokens:
input: 50463
output: 66
thoughts: 2336
Thoughts: **Analysis of the GFS2 Quota Bug**
Okay, so here's what I've figured out. A vulnerability exists in the GFS2 file system related to quota handling and the `GFS2_DIF_SYSTEM` flag. Let me break it down.
When a file is opened for writing, `gfs2_qa_get` is called. If the file has `GFS2_DIF_SYSTEM` set, this function returns 0 without allocating any quota data (`ip->i_qadata`). Now, the problem arises with the `truncate` system call. `gfs2_setattr_size`, called by `truncate`, also calls `gfs2_qa_get`, which returns 0 in this scenario, *still* without allocating `ip->i_qadata`. Later, `do_grow` calls `gfs2_quota_lock_check`, which then calls `gfs2_quota_lock`, and *this* is where the dereference of `ip->i_qadata` happens. Since `ip->i_qadata` is null, it crashes.
If it's a *normal* file (no `GFS2_DIF_SYSTEM`), the process works as expected. `gfs2_qa_get` allocates `ip->i_qadata`. Subsequent calls to `truncate` and the quota lock mechanism use the valid data. This bug *only* happens when `GFS2_DIF_SYSTEM` is true. This flag is only settable by the filesystem itself (on internal files) or a malicious image.
So, the conclusion is clear: this bug requires a file with the `GFS2_DIF_SYSTEM` flag. It seems I can trigger this by using the `truncate` syscall on the malicious image if I have write access to it.
I've determined that the bug is:
* **Exploitable: NO**
* **Denial of Service: YES**
* **Accessible From Unprivileged Processes: NO** (mounting requires root)
* **Accessible From User Namespaces: NO** (GFS2 cannot be mounted in userns)
* **Malicious Filesystem Trigger: YES**
* **Peripheral Trigger: YES** (USB auto-mount)
* **Network Trigger: NO**
* **Remote Trigger: NO**
* **VM Guest Trigger: NO**
* **VM Host Trigger: NO**
I also verified that any error in the `gfs2_qa_get` due to memory allocation failure won't trigger the bug. It fails at `open` or `truncate`, but does not make it to the problematic code. Also, quota being turned on/off dynamically is not an option.
Now, I've got a good handle on how the code crashes. It's the dereference of the NULL `ip->i_qadata` pointer within the `gfs2_quota_lock` function. The crash occurs at `movzbl (%rcx,%rbx,1),%ecx`, which is a KASAN shadow memory check, and this confirms it is not an exploitable memory corruption, because it attempts to read address 0x100 which has not been mapped and will cause an immediate page fault.
I'm confident that this is a Malicious Filesystem Trigger vulnerability. The bug is caused by a crafted filesystem image with the `GFS2_DIF_SYSTEM` flag set on a file, in conjunction with the `truncate` system call. The user must be able to write to the file.
|
| 67/2 |
2026/09/02 02:09 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:true NetworkTrigger:false PeripheralTrigger:true RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 68/2 |
2026/09/02 02:09 |
llm |
expert |
0m
Model:
Tokens:
input: 53696
output: 809
thoughts: 514
Thoughts: **My Analysis of the GFS2 Quota Lock Bug**
Okay, the analysis is complete, time to synthesize this into a final response. Here's what I've got:
This is a pretty straightforward NULL pointer dereference, a classic. The root cause lies within the `gfs2_quota_lock` function. Specifically, it's attempting to access `ip->i_qadata->qa_qd_num` without adequate null pointer protection. The core vulnerability is that `ip->i_qadata` *can* be NULL under specific conditions.
Normally, the `gfs2_qa_get` function is responsible for allocating `ip->i_qadata` prior to any operations that would need quota locks, like `truncate`, `fallocate`, or `chown`. However, the code explicitly skips this allocation if the inode in question has the `GFS2_DIF_SYSTEM` flag set. That's the crux of the issue.
The scenario is this: An unprivileged user, without `CAP_SYS_RESOURCE`, triggers an operation, let's say a `truncate`, on a file that – for some reason – has the `GFS2_DIF_SYSTEM` flag set. The operation will call `gfs2_setattr_size`, which in turn, calls `gfs2_qa_get`. Because of the `GFS2_DIF_SYSTEM` flag, `gfs2_qa_get` does nothing and returns 0. The execution continues down the call stack to `do_grow` -> `gfs2_quota_lock_check` -> `gfs2_quota_lock`. Here, `gfs2_quota_lock` *naively* assumes `ip->i_qadata` is valid and dereferences it, leading to the NULL pointer dereference and a kernel oops.
The interesting aspect is the `GFS2_DIF_SYSTEM` flag itself. It's not normally settable by userspace on regular files; the kernel filters it out in `gfs2_fileattr_set`. That means this is going to require a specifically crafted GFS2 filesystem image. To exploit this, you need to mount an image where a *normal* (accessible) file has the `GFS2_DIF_SYSTEM` flag set.
The good news is that the dereference is a read from offset 0x100 (`qa_qd_num`), which sits right in the zero page. It will reliably cause a Denial of Service via a kernel panic, but, importantly, it's not directly exploitable for privilege escalation or code execution. The attacker just gets a crash.
The other good news, from a security perspective, is that GFS2 can't be mounted by unprivileged users or within user namespaces. So, the attack requires either root privileges to mount the malicious image or physical access to trigger an auto-mount (perhaps via a USB drive). Once the image is mounted, an unprivileged user can trigger the crash simply by running `truncate` on the specifically crafted file.
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Results: map[Explanation:**Bug Analysis:**
The bug is a NULL pointer dereference in `gfs2_quota_lock()` when trying to
access `ip->i_qadata->qa_qd_num`.
In the GFS2 filesystem, the `ip->i_qadata` structure is allocated on-demand by
calling `gfs2_qa_get()` before performing operations that might require quota
locks (such as `truncate`, `fallocate`, `chown`, etc.). However, `gfs2_qa_get()`
has an explicit check to skip allocation and return success (`0`) if the inode
has the `GFS2_DIF_SYSTEM` flag set:
```c
if (ip->i_diskflags & GFS2_DIF_SYSTEM)
return 0;
```
If an operation like `truncate` is performed on a file with the
`GFS2_DIF_SYSTEM` flag, `gfs2_setattr_size()` will call `gfs2_qa_get()` (which
does nothing and returns 0), and then proceed to call `do_grow()` ->
`gfs2_quota_lock_check()`.
If the user does not have the `CAP_SYS_RESOURCE` capability,
`gfs2_quota_lock_check()` will call `gfs2_quota_lock()`. Unlike `gfs2_qa_get()`,
`gfs2_quota_lock()` does not check for the `GFS2_DIF_SYSTEM` flag and
unconditionally dereferences `ip->i_qadata`, which is `NULL`, leading to the
crash.
**Trigger and Exploitability:**
The `GFS2_DIF_SYSTEM` flag is an internal filesystem flag. It cannot be set by
userspace on normal files (it is explicitly filtered out in
`gfs2_fileattr_set()`). Therefore, this bug can only be triggered by mounting a
maliciously crafted GFS2 filesystem image where a normal, accessible file has
the `GFS2_DIF_SYSTEM` flag set.
The crash occurs when reading from offset `0x100` (the offset of `qa_qd_num` in
`struct gfs2_qadata`). Because this address falls within the unmapped zero page
(protected by `mmap_min_addr`), it will reliably cause a page fault and a kernel
oops/panic (Denial of Service). It is not exploitable for privilege escalation
or arbitrary code execution.
Because GFS2 does not support unprivileged user namespace mounts
(`FS_USERNS_MOUNT` is not set), an unprivileged user cannot mount the malicious
image themselves. The attack requires either root privileges to mount the image,
or physical access to a system that auto-mounts external media (e.g., a USB
drive). Once the image is mounted, an unprivileged user can trigger the crash by
simply calling `truncate` on the crafted file.
**Conclusions:**
* **Exploitable:** No. It is a pure NULL pointer dereference (read from `0x100`)
which cannot be mapped by an attacker.
* **Denial of Service:** Yes. It reliably crashes the kernel.
* **Accessible From Unprivileged Processes:** No. While the final trigger
(`truncate`) is done by an unprivileged user, the required setup (mounting the
GFS2 image) requires `CAP_SYS_ADMIN` in the initial namespace.
* **Accessible From User Namespaces:** No. GFS2 cannot be mounted inside a user
namespace.
* **Malicious Filesystem Trigger:** Yes. The bug relies on a crafted inode with
the `GFS2_DIF_SYSTEM` flag.
* **Peripheral Trigger:** Yes. Can be triggered by plugging in a malicious USB
drive on systems with auto-mount enabled.]
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