| 0/0 |
2026/06/27 00:50 |
flow |
assessment-security |
2m
Results: map[DenialOfService:false Explanation:**Analysis of the Bug Report**
**1. Bug Mechanism and Impact:**
The report from the Kernel Concurrency Sanitizer (KCSAN) highlights a data race
on the `i_mtime_nsec` field of an `inode` structure.
- **Writer:** One thread is creating a symlink in a shared memory filesystem
(tmpfs/shmem) via `shmem_symlink`. As part of this operation, it updates the
modification time of the parent directory by calling `inode_set_mtime_to_ts()`,
which writes to `inode->i_mtime_nsec`.
- **Reader:** Another thread is concurrently querying the attributes of the same
directory via a `stat` family system call (`newfstatat`). This calls
`generic_fillattr()`, which reads the modification time via
`inode_get_mtime_nsec()`.
Because these accesses are not synchronized (e.g., via locks or
`READ_ONCE`/`WRITE_ONCE` macros), KCSAN flags this as a data race.
**2. Exploitability:**
This is a **benign data race**. The only consequence of this race is that the
reader thread might observe a slightly outdated, newer, or partially torn value
for the directory's modification timestamp (nanoseconds part).
- It does not cause memory corruption (like out-of-bounds accesses or
use-after-frees).
- It does not leak sensitive kernel pointers or memory contents.
- It does not allow control-flow hijacking or privilege escalation.
Therefore, this bug is **not exploitable**.
**3. Denial of Service (DoS):**
In a production kernel (where KCSAN is disabled), this data race will occur
silently without triggering any warnings, panics, or hangs. It does not corrupt
critical kernel data structures or leak resources. Thus, it cannot be used to
cause a Denial of Service.
**4. Accessibility:**
The bug can be easily triggered by any **unprivileged user process**. All it
requires is the ability to create a symlink in a tmpfs directory (like `/tmp`)
and concurrently call `stat` on that directory. No special capabilities (like
`CAP_SYS_ADMIN`) or user namespaces are required.
**Conclusion:**
This is a harmless concurrency artifact on a timestamp field. It is accessible
to unprivileged users but poses no security or stability risks to the system. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/06/27 00:50 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/06/27 00:50 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/a0b4f042315857ea9a83ecf601aa5c8580cb5d42]
|
| 3/1 |
2026/06/27 00:50 |
action |
kernel-builder |
0m
Results: map[KernelObj:/app/workdir/cache/build/914521359e38fcf508cdbc7fb48d1e6b923660f3]
|
| 4/1 |
2026/06/27 00:50 |
action |
codesearch-prepare |
0m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/06/27 00:51 |
agent |
expert |
2m
Model:
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true 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:
==================================================================
BUG: KCSAN: data-race in generic_fillattr / shmem_symlink
write to 0xffff8881047393fc of 4 bytes by task 3288 on cpu 0:
inode_set_mtime_to_ts include/linux/fs.h:1655 [inline]
shmem_symlink+0x369/0x3a0 mm/shmem.c:4066
vfs_symlink+0xfd/0x220 fs/namei.c:5656
filename_symlinkat+0xe8/0x2b0 fs/namei.c:5681
__do_sys_symlink fs/namei.c:5708 [inline]
__se_sys_symlink+0x42/0x1b0 fs/namei.c:5704
__x64_sys_symlink+0x31/0x40 fs/namei.c:5704
x64_sys_call+0x2465/0x3020 arch/x86/include/generated/asm/syscalls_64.h:89
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
read to 0xffff8881047393fc of 4 bytes by task 3280 on cpu 1:
inode_get_mtime_nsec include/linux/fs.h:1641 [inline]
inode_get_mtime include/linux/fs.h:1647 [inline]
generic_fillattr+0x214/0x340 fs/stat.c:102
shmem_getattr+0x181/0x200 mm/shmem.c:1302
vfs_getattr_nosec+0x146/0x1e0 fs/stat.c:213
vfs_getattr+0x48/0x60 fs/stat.c:262
vfs_statx_path+0x28/0x140 fs/stat.c:299
vfs_statx+0xc4/0x170 fs/stat.c:356
vfs_fstatat+0x118/0x170 fs/stat.c:373
__do_sys_newfstatat fs/stat.c:538 [inline]
__se_sys_newfstatat+0x55/0x3e0 fs/stat.c:532
__x64_sys_newfstatat+0x55/0x70 fs/stat.c:532
x64_sys_call+0x2c64/0x3020 arch/x86/include/generated/asm/syscalls_64.h:263
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
value changed: 0x0d7320d7 -> 0x0d7e64da
Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3280 Comm: udevd Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================
Reply: **Analysis of the Bug Report**
**1. Bug Mechanism and Impact:**
The report from the Kernel Concurrency Sanitizer (KCSAN) highlights a data race on the `i_mtime_nsec` field of an `inode` structure.
- **Writer:** One thread is creating a symlink in a shared memory filesystem (tmpfs/shmem) via `shmem_symlink`. As part of this operation, it updates the modification time of the parent directory by calling `inode_set_mtime_to_ts()`, which writes to `inode->i_mtime_nsec`.
- **Reader:** Another thread is concurrently querying the attributes of the same directory via a `stat` family system call (`newfstatat`). This calls `generic_fillattr()`, which reads the modification time via `inode_get_mtime_nsec()`.
Because these accesses are not synchronized (e.g., via locks or `READ_ONCE`/`WRITE_ONCE` macros), KCSAN flags this as a data race.
**2. Exploitability:**
This is a **benign data race**. The only consequence of this race is that the reader thread might observe a slightly outdated, newer, or partially torn value for the directory's modification timestamp (nanoseconds part).
- It does not cause memory corruption (like out-of-bounds accesses or use-after-frees).
- It does not leak sensitive kernel pointers or memory contents.
- It does not allow control-flow hijacking or privilege escalation.
Therefore, this bug is **not exploitable**.
**3. Denial of Service (DoS):**
In a production kernel (where KCSAN is disabled), this data race will occur silently without triggering any warnings, panics, or hangs. It does not corrupt critical kernel data structures or leak resources. Thus, it cannot be used to cause a Denial of Service.
**4. Accessibility:**
The bug can be easily triggered by any **unprivileged user process**. All it requires is the ability to create a symlink in a tmpfs directory (like `/tmp`) and concurrently call `stat` on that directory. No special capabilities (like `CAP_SYS_ADMIN`) or user namespaces are required.
**Conclusion:**
This is a harmless concurrency artifact on a timestamp field. It is accessible to unprivileged users but poses no security or stability risks to the system.
|
| 6/2 |
2026/06/27 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 6702
output: 41
thoughts: 146
Thoughts: **Analyzing a KCSAN Data-Race Report**
Okay, so I've got a KCSAN data-race report here. Looks like the core issue is a race condition between `generic_fillattr` and `shmem_symlink`. That's an interesting combination, let's see... the write is happening inside `inode_set_mtime_to_ts`, which makes sense given that it's called from `shmem_symlink`, which likely means we're modifying the inode's mtime somehow when creating the symbolic link. The read, on the other hand, is happening through `inode_get_mtime_nsec`, which is part of the `generic_fillattr` function. So, `generic_fillattr` is reading the modification time (mtime) of the inode, and `shmem_symlink` is, at least sometimes, writing to it concurrently.
The report also specifies that the size of the value changed is 4 bytes. That strongly suggests this involves the nanosecond component, `tv_nsec`, of the `mtime` field within the `inode` structure. Knowing the exact field is important, of course.
My next step is definitely to dive straight into the source code for both `inode_set_mtime_to_ts` and `inode_get_mtime_nsec`. I need to understand precisely how they're accessing and manipulating the `mtime` field. Are there any existing locking mechanisms around `inode->i_mtime`? Or maybe it's only protected under certain scenarios? I need to understand the memory ordering as well. Given my experience, I'm expecting some sort of potential weak ordering issue; there's always a possibility that an improper memory barrier is causing the problem. I'll need to confirm that.
|
| 7/2 |
2026/06/27 00:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/fs.h Name:inode_set_mtime_to_ts]
Results: map[SourceCode:1651: static inline struct timespec64 inode_set_mtime_to_ts(struct inode *inode,
1652: struct timespec64 ts)
1653: {
1654: inode->i_mtime_sec = ts.tv_sec;
1655: inode->i_mtime_nsec = ts.tv_nsec;
1656: return ts;
1657: }
SourceFile:include/linux/fs.h]
|
| 8/2 |
2026/06/27 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 7272
output: 39
thoughts: 74
Thoughts: Okay, here's my take on that KCSAN data-race report, from a high-level perspective, as I'd approach it:
**Deconstructing a KCSAN Race: Inode MTIME Madness**
Right, so KCSAN's flagged a data race. It's between `inode_set_mtime_to_ts` and `inode_get_mtime_nsec`. That's interesting, and it immediately sets off some alarms in my head. We're talking about modifications and reads of the inode's modification time. This is a common area for subtle concurrency bugs, and the fact that KCSAN found something means we should pay close attention.
The report pinpoints the write as originating from `shmem_symlink`, which makes sense. We're creating a symbolic link, so there's a good chance we're updating modification times. The read is in `generic_fillattr`, which suggests we're getting attributes – maybe `stat` or similar. Okay, so one path is creating, and the other is reporting.
The next thing I'd do is immediately dive into `inode_get_mtime_nsec` to understand the read side of the race. This is where the vulnerability is. I'll need to know: what lock it's holding, what assumptions it's making about the data's consistency, and whether it's even aware of any concurrent writers. I'll be looking for potential atomicity issues or missing memory barriers. From there, I'll go to the write side and study the code flow.
|
| 9/2 |
2026/06/27 00:51 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/fs.h Name:inode_get_mtime_nsec]
Results: map[SourceCode:1639: static inline long inode_get_mtime_nsec(const struct inode *inode)
1640: {
1641: return inode->i_mtime_nsec;
1642: }
SourceFile:include/linux/fs.h]
|
| 10/2 |
2026/06/27 00:51 |
llm |
expert |
0m
Model:
Tokens:
input: 7737
output: 35
thoughts: 325
Thoughts: **Analyzing the Data Race Bug**
Okay, so I'm looking at this bug report, and it seems like we have a data race condition here, specifically between `inode_set_mtime_to_ts` and `inode_get_mtime_nsec`. The write is occurring in `shmem_symlink`, specifically when it calls `inode_set_mtime_to_ts`, where `inode->i_mtime_nsec` is being modified. The read, on the other hand, comes from `generic_fillattr` which internally calls `inode_get_mtime_nsec`.
This is a pretty standard inode timestamp data race, nothing novel. `shmem_symlink` is, in effect, updating the modification time of either the directory itself or the symlink being created. Meanwhile, `generic_fillattr` is reading that very same timestamp for a `stat` call.
Since we're dealing with `i_mtime_nsec`, which is, at worst, a 64-bit value, though the report states it's only 4 bytes. This means it's a benign data race. The most likely outcome is `stat` might report a "torn" read – a partially updated nanosecond value – or perhaps an old or a newer timestamp value, but it's not going to crash the system.
Given that, this isn't exploitable. It's not going to lead to a denial-of-service condition. It can be triggered by unprivileged processes; creating symlinks and then statting them is all that's required. It can also be accessed from user namespaces, which is not really a concern.
Importantly, it's not a VM guest or host trigger, not network-related, not triggered by peripherals, and not a malicious filesystem thing. This is purely in `tmpfs/shmem` land. The real question is: Is `shmem_symlink` the only function that's updating `i_mtime_nsec`, or does this race exist in other parts of the kernel as well? I'll probably need to take a closer look at `shmem_symlink` to be certain.
|
| 11/2 |
2026/06/27 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/shmem.c Name:shmem_symlink]
Results: map[SourceCode:4016: static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
4017: struct dentry *dentry, const char *symname)
4018: {
4019: int error;
4020: int len;
4021: struct inode *inode;
4022: struct folio *folio;
4023: char *link;
4024:
4025: len = strlen(symname) + 1;
4026: if (len > PAGE_SIZE)
4027: return -ENAMETOOLONG;
4028:
4029: inode = shmem_get_inode(idmap, dir->i_sb, dir, S_IFLNK | 0777, 0,
4030: mk_vma_flags(VMA_NORESERVE_BIT));
4031: if (IS_ERR(inode))
4032: return PTR_ERR(inode);
4033:
4034: error = security_inode_init_security(inode, dir, &dentry->d_name,
4035: shmem_initxattrs, NULL);
4036: if (error && error != -EOPNOTSUPP)
4037: goto out_iput;
4038:
4039: error = simple_offset_add(shmem_get_offset_ctx(dir), dentry);
4040: if (error)
4041: goto out_iput;
4042:
4043: inode->i_size = len-1;
4044: if (len <= SHORT_SYMLINK_LEN) {
4045: link = kmemdup(symname, len, GFP_KERNEL);
4046: if (!link) {
4047: error = -ENOMEM;
4048: goto out_remove_offset;
4049: }
4050: inode->i_op = &shmem_short_symlink_operations;
4051: inode_set_cached_link(inode, link, len - 1);
4052: } else {
4053: inode_nohighmem(inode);
4054: inode->i_mapping->a_ops = &shmem_aops;
4055: error = shmem_get_folio(inode, 0, 0, &folio, SGP_WRITE);
4056: if (error)
4057: goto out_remove_offset;
4058: inode->i_op = &shmem_symlink_inode_operations;
4059: memcpy(folio_address(folio), symname, len);
4060: folio_mark_uptodate(folio);
4061: folio_mark_dirty(folio);
4062: folio_unlock(folio);
4063: folio_put(folio);
4064: }
4065: dir->i_size += BOGO_DIRENT_SIZE;
4066: inode_set_mtime_to_ts(dir, inode_set_ctime_current(dir));
4067: inode_inc_iversion(dir);
4068: d_make_persistent(dentry, inode);
4069: return 0;
4070:
4071: out_remove_offset:
4072: simple_offset_remove(shmem_get_offset_ctx(dir), dentry);
4073: out_iput:
4074: iput(inode);
4075: return error;
4076: }
SourceFile:mm/shmem.c]
|
| 12/2 |
2026/06/27 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 9243
output: 33
thoughts: 0
|
| 13/2 |
2026/06/27 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/stat.c Name:generic_fillattr]
Results: map[SourceCode: 65: /**
66: * generic_fillattr - Fill in the basic attributes from the inode struct
67: * @idmap: idmap of the mount the inode was found from
68: * @request_mask: statx request_mask
69: * @inode: Inode to use as the source
70: * @stat: Where to fill in the attributes
71: *
72: * Fill in the basic attributes in the kstat structure from data that's to be
73: * found on the VFS inode structure. This is the default if no getattr inode
74: * operation is supplied.
75: *
76: * If the inode has been found through an idmapped mount the idmap of
77: * the vfsmount must be passed through @idmap. This function will then
78: * take care to map the inode according to @idmap before filling in the
79: * uid and gid filds. On non-idmapped mounts or if permission checking is to be
80: * performed on the raw inode simply pass @nop_mnt_idmap.
81: */
82: void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
83: struct inode *inode, struct kstat *stat)
84: {
85: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
86: vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
87:
88: stat->dev = inode->i_sb->s_dev;
89: stat->ino = inode->i_ino;
90: stat->mode = inode->i_mode;
91: stat->nlink = inode->i_nlink;
92: stat->uid = vfsuid_into_kuid(vfsuid);
93: stat->gid = vfsgid_into_kgid(vfsgid);
94: stat->rdev = inode->i_rdev;
95: stat->size = i_size_read(inode);
96: stat->atime = inode_get_atime(inode);
97:
98: if (is_mgtime(inode)) {
99: fill_mg_cmtime(stat, request_mask, inode);
100: } else {
101: stat->ctime = inode_get_ctime(inode);
102: stat->mtime = inode_get_mtime(inode);
103: }
104:
105: stat->blksize = i_blocksize(inode);
106: stat->blocks = inode->i_blocks;
107:
108: if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
109: stat->result_mask |= STATX_CHANGE_COOKIE;
110: stat->change_cookie = inode_query_iversion(inode);
111: }
112:
113: }
SourceFile:fs/stat.c]
|
| 14/2 |
2026/06/27 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 10140
output: 19
thoughts: 336
Thoughts: **Analyzing a Benign Data Race in the Filesystem**
Okay, so I'm looking at this bug report and it seems like we have a classic data race, specifically in `inode_set_mtime_to_ts` (called by `shmem_symlink`) and `inode_get_mtime_nsec` (called by `generic_fillattr`). Essentially, one thread is creating a symlink within a tmpfs/shmem directory, which naturally updates the directory's modification time (mtime). Another thread, potentially concurrently, is calling `stat` on that same directory, thus reading the mtime. The race is directly on `inode->i_mtime_nsec`.
The report indicates a 4-byte write and a 4-byte read, which isn't entirely surprising. The values changed, as expected, but this kind of thing isn't usually exploitable.
My initial thought is that this is a benign race, a timestamp being read while it's being written. The worst case scenario is `stat` returning a slightly inconsistent timestamp, maybe old seconds with new nanoseconds, or vice-versa. And although a torn read could happen on a 32-bit architecture, here the system is 64-bit and we have a 4-byte read and write, so it's likely we're seeing either the old or the new value of `i_mtime_nsec` being read.
I need to confirm if `i_mtime_nsec` is a `long` or a `u32`. This could impact the likelihood of any observable issues. Time to check the `struct inode` definition...
|
| 15/2 |
2026/06/27 00:52 |
tool |
codesearch-struct-layout |
0m
Args:
Results: map[Fields:[map[Name:i_mode OffsetBits:0 SizeBits:16] map[Name:i_opflags OffsetBits:16 SizeBits:16] map[Name:i_flags OffsetBits:32 SizeBits:32] map[Name:i_acl OffsetBits:64 SizeBits:64] map[Name:i_default_acl OffsetBits:128 SizeBits:64] map[Name:i_uid OffsetBits:192 SizeBits:32] map[Name:i_gid OffsetBits:224 SizeBits:32] map[Name:i_op OffsetBits:256 SizeBits:64] map[Name:i_sb OffsetBits:320 SizeBits:64] map[Name:i_mapping OffsetBits:384 SizeBits:64] map[Name:i_security OffsetBits:448 SizeBits:64] map[Name:i_ino OffsetBits:512 SizeBits:64] map[Name: OffsetBits:576 SizeBits:32] map[Name:i_rdev OffsetBits:608 SizeBits:32] map[Name:i_size OffsetBits:640 SizeBits:64] map[Name:i_atime_sec OffsetBits:704 SizeBits:64] map[Name:i_mtime_sec OffsetBits:768 SizeBits:64] map[Name:i_ctime_sec OffsetBits:832 SizeBits:64] map[Name:i_atime_nsec OffsetBits:896 SizeBits:32] map[Name:i_mtime_nsec OffsetBits:928 SizeBits:32] map[Name:i_ctime_nsec OffsetBits:960 SizeBits:32] map[Name:i_generation OffsetBits:992 SizeBits:32] map[Name:i_lock OffsetBits:1024 SizeBits:32] map[Name:i_bytes OffsetBits:1056 SizeBits:16] map[Name:i_blkbits OffsetBits:1072 SizeBits:8] map[Name:i_write_hint OffsetBits:1080 SizeBits:8] map[Name:i_blocks OffsetBits:1088 SizeBits:64] map[Name:i_state OffsetBits:1152 SizeBits:32] map[Name:i_rwsem OffsetBits:1216 SizeBits:256] map[Name:dirtied_when OffsetBits:1472 SizeBits:64] map[Name:dirtied_time_when OffsetBits:1536 SizeBits:64] map[Name:i_hash OffsetBits:1600 SizeBits:128] map[Name:i_io_list OffsetBits:1728 SizeBits:128] map[Name:i_wb OffsetBits:1856 SizeBits:64] map[Name:i_wb_frn_winner OffsetBits:1920 SizeBits:32] map[Name:i_wb_frn_avg_time OffsetBits:1952 SizeBits:16] map[Name:i_wb_frn_history OffsetBits:1968 SizeBits:16] map[Name:i_lru OffsetBits:1984 SizeBits:128] map[Name:i_sb_list OffsetBits:2112 SizeBits:128] map[Name:i_wb_list OffsetBits:2240 SizeBits:128] map[Name: OffsetBits:2368 SizeBits:128] map[Name:i_version OffsetBits:2496 SizeBits:64] map[Name:i_sequence OffsetBits:2560 SizeBits:64] map[Name:i_count OffsetBits:2624 SizeBits:32] map[Name:i_dio_count OffsetBits:2656 SizeBits:32] map[Name:i_writecount OffsetBits:2688 SizeBits:32] map[Name:i_readcount OffsetBits:2720 SizeBits:32] map[Name: OffsetBits:2752 SizeBits:64] map[Name:i_flctx OffsetBits:2816 SizeBits:64] map[Name:i_data OffsetBits:2880 SizeBits:1216] map[Name: OffsetBits:4096 SizeBits:128] map[Name: OffsetBits:4224 SizeBits:64] map[Name:i_fsnotify_mask OffsetBits:4288 SizeBits:32] map[Name:i_fsnotify_marks OffsetBits:4352 SizeBits:64] map[Name:i_private OffsetBits:4416 SizeBits:64]]]
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| 16/2 |
2026/06/27 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 11765
output: 37
thoughts: 0
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| 17/2 |
2026/06/27 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:include/linux/fs.h Name:inode_get_mtime]
Results: map[SourceCode:1644: static inline struct timespec64 inode_get_mtime(const struct inode *inode)
1645: {
1646: struct timespec64 ts = { .tv_sec = inode_get_mtime_sec(inode),
1647: .tv_nsec = inode_get_mtime_nsec(inode) };
1648: return ts;
1649: }
SourceFile:include/linux/fs.h]
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| 18/2 |
2026/06/27 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 11941
output: 66
thoughts: 966
Thoughts: **Analyzing a Benign Data Race in the Filesystem**
Okay, so I've got a data race on my hands, specifically in `generic_fillattr` and `shmem_symlink`, dealing with the `i_mtime_nsec` field of an inode. It seems `shmem_symlink` is writing to it via `inode_set_mtime_to_ts`, and `generic_fillattr` is reading it using `inode_get_mtime_nsec`. Essentially, `shmem_symlink` updates the directory's modification time when creating a symlink, while `generic_fillattr` reads that same modification time when `stat` is called.
This looks like a benign data race, and my initial assessment is that it's not exploitable. It's a timestamp, a 32-bit integer, being read and written, and the concern would be garbage data. I'm not seeing any pointers or any path to memory corruption or denial of service; at worst, `stat` might report a slightly inaccurate timestamp.
Alright, let's confirm this by thinking it through. Unprivileged users can create symlinks in tmpfs/shmem and `stat` files, so it's definitely accessible. It's just a 32-bit integer, and the reader is just returning it via `stat` so there's no way to overwrite a pointer or anything exploitable in that respect. It won't cause denial of service; it's a KCSAN warning at most, but it will be a normal memory access in a production build without KCSAN.
It's accessible from unprivileged processes and user namespaces; creating a symlink in `/tmp` (which is often tmpfs) and stat-ing it concurrently should be sufficient. Also, it's not a VM guest, VM host, network, remote, peripheral or malicious filesystem trigger, this is in an in-memory filesystem.
So, let's finalize the assessment: `Exploitable: False`, `DenialOfService: False`, `Unprivileged: True`, `UserNamespace: True`, and all the other triggers: `False`.
Wait a second, let me think a bit more. This is a 32-bit field. On a 32-bit architecture, a 64-bit read/write might be torn, but that isn't the case here. Even in a torn read, it's just a garbage timestamp, being read by `generic_fillattr`, passed into `struct kstat`, then into `vfs_getattr` and `vfs_statx` and finally copied to userspace via `__do_sys_newfstatat`. So it's still just a bad timestamp, not a security issue, and it is a 32-bit field so the read and writes are atomic.
Okay, let's double check the KCSAN report. The write and read are both 4 bytes, which is good. The timestamp values being changed are nanoseconds. I'm confident in my analysis. I will set the results.
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| 19/2 |
2026/06/27 00:53 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:true UserNamespace:true VMGuestTrigger:false VMHostTrigger:false]
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| 20/2 |
2026/06/27 00:53 |
llm |
expert |
0m
Model:
Tokens:
input: 12733
output: 509
thoughts: 487
Thoughts: **Benign Data Race in Timestamp Handling**
Okay, so I've got this KCSAN report flagged, and it's pointing to a data race. My initial assessment is that it's a pretty classic one: a race condition on the modification timestamp (`i_mtime_nsec`) within an `inode` structure. Task 1, specifically `shmem_symlink`, is writing to that timestamp, using `inode_set_mtime_to_ts`. Meanwhile, Task 2, `generic_fillattr`, is concurrently reading the same timestamp, using `inode_get_mtime_nsec`.
The value change observed (from `0x0d7320d7` to `0x0d7e64da`) confirms the race – it's definitely being modified while read operations are also present. However, and this is important, it seems to be *benign*. It looks like a torn read scenario is the worst-case, or just an out-of-date timestamp.
This means it won't allow memory corruption, escalation of privilege, or leak any data other than a possibly slightly incorrect timestamp, if that can even be counted. No exploits here, and I don't see any denial-of-service potential either. It's just a KCSAN report, which is a debug tool anyway, and in production, the kernel will just silently read the potentially inconsistent value.
What I've got here is unprivileged access to trigger the condition; creating a symlink and calling `stat` simultaneously should do it. No special capabilities are needed.
Let's break it down to be sure:
* The data race is on the `i_mtime_nsec` field of the `inode`.
* A write is occurring in a symlink creation (`shmem_symlink`). This is updating the directory modification time.
* Concurrently, a read happens in the `stat` system call (`generic_fillattr`), which needs that directory modification time for its function.
* Since the read and write are unsynchronized, KCSAN picks it up as a data race.
* The impact is *minimal*: the timestamp in the `stat` call might be off slightly.
* There's no memory corruption, no control-flow issues, and absolutely no exploitable conditions.
* This also means this shouldn't cause any problems in production, just an inaccurate timestamp.
* I can create a symlink, and run stat on it to hit this issue, and the KCSAN report confirms.
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| 21/1 |
2026/06/27 00:53 |
action |
format-explanation |
0m
Results: map[Explanation:**Analysis of the Bug Report**
**1. Bug Mechanism and Impact:**
The report from the Kernel Concurrency Sanitizer (KCSAN) highlights a data race
on the `i_mtime_nsec` field of an `inode` structure.
- **Writer:** One thread is creating a symlink in a shared memory filesystem
(tmpfs/shmem) via `shmem_symlink`. As part of this operation, it updates the
modification time of the parent directory by calling `inode_set_mtime_to_ts()`,
which writes to `inode->i_mtime_nsec`.
- **Reader:** Another thread is concurrently querying the attributes of the same
directory via a `stat` family system call (`newfstatat`). This calls
`generic_fillattr()`, which reads the modification time via
`inode_get_mtime_nsec()`.
Because these accesses are not synchronized (e.g., via locks or
`READ_ONCE`/`WRITE_ONCE` macros), KCSAN flags this as a data race.
**2. Exploitability:**
This is a **benign data race**. The only consequence of this race is that the
reader thread might observe a slightly outdated, newer, or partially torn value
for the directory's modification timestamp (nanoseconds part).
- It does not cause memory corruption (like out-of-bounds accesses or
use-after-frees).
- It does not leak sensitive kernel pointers or memory contents.
- It does not allow control-flow hijacking or privilege escalation.
Therefore, this bug is **not exploitable**.
**3. Denial of Service (DoS):**
In a production kernel (where KCSAN is disabled), this data race will occur
silently without triggering any warnings, panics, or hangs. It does not corrupt
critical kernel data structures or leak resources. Thus, it cannot be used to
cause a Denial of Service.
**4. Accessibility:**
The bug can be easily triggered by any **unprivileged user process**. All it
requires is the ability to create a symlink in a tmpfs directory (like `/tmp`)
and concurrently call `stat` on that directory. No special capabilities (like
`CAP_SYS_ADMIN`) or user namespaces are required.
**Conclusion:**
This is a harmless concurrency artifact on a timestamp field. It is accessible
to unprivileged users but poses no security or stability risks to the system.]
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