KASAN: slab-out-of-bounds Read in write_tag_3

Based on kernel commit 05441d697f7f:

Root Cause Analysis

Vulnerability Overview

The bug is a slab out-of-bounds read (and potential out-of-bounds write) in the eCryptfs subsystem caused by missing payload size validation when retrieving an authentication token (struct ecryptfs_auth_tok) from a user or encrypted key.

Involved Components

Source Files:
Functions:
- ecryptfs_get_key_payload_data() (fs/ecryptfs/ecryptfs_kernel.h:113)
- ecryptfs_verify_auth_tok_from_key() (fs/ecryptfs/keystore.c:453)
- write_tag_3_packet() (fs/ecryptfs/keystore.c:2161)
- ecryptfs_generate_key_packet_set() (fs/ecryptfs/keystore.c:2386)
Structures:
- struct ecryptfs_auth_tok (sizeof = 740 bytes)
- struct user_key_payload (header size 24 bytes + data[])
- struct ecryptfs_password (part of union in struct ecryptfs_auth_tok)

Technical Mechanism & Trigger Flow

Key Creation with Short Payload: A user-defined key is added via add_key("user", "syz", payload, 0xc9, ...) where datalen = 201 (0xc9). In user_preparse() (security/keys/user_defined.c), upayload is allocated via kmalloc_flex() for sizeof(struct user_key_payload) + datalen = $24 + 201 = 225$ bytes, which lands in the kmalloc-256 slab cache.
Mount and Key Lookup: An eCryptfs filesystem is mounted with ecryptfs_sig=syz. During mount (ecryptfs_init_global_auth_toks()) and subsequent file creation (ecryptfs_create() $\to$ ecryptfs_initialize_file() $\to$ ecryptfs_write_metadata() $\to$ ecryptfs_generate_key_packet_set() $\to$ ecryptfs_find_global_auth_tok_for_sig()), eCryptfs verifies the key using ecryptfs_verify_auth_tok_from_key().
Missing Payload Length Validation: In ecryptfs_verify_auth_tok_from_key():
```
(*auth_tok) = ecryptfs_get_key_payload_data(auth_tok_key);
```
ecryptfs_get_key_payload_data() fetches ukp = user_key_payload_locked(key) and immediately returns:
```
return (struct ecryptfs_auth_tok *)ukp->data;
```
Neither ecryptfs_get_key_payload_data() nor ecryptfs_verify_auth_tok_from_key() checks whether ukp->datalen >= sizeof(struct ecryptfs_auth_tok) (or the corresponding payload->decrypted_datalen for encrypted keys). ecryptfs_verify_auth_tok_from_key() only checks that (*auth_tok)->version matches ECRYPTFS_VERSION_MAJOR/MINOR (at offset 0) and (*auth_tok)->token_type is valid (at offset 2).
Out-of-Bounds Memory Access: sizeof(struct ecryptfs_auth_tok) is 740 bytes. Inside struct ecryptfs_auth_tok:
- session_key is at offset 8 bytes (size 588 bytes).
- reserved is at offset 596 bytes (size 32 bytes).
- token (union containing struct ecryptfs_password) starts at offset 628 bytes.
- Within struct ecryptfs_password, salt[ECRYPTFS_SALT_SIZE] is at offset 101 bytes.
- Thus, auth_tok->token.password.salt resides at offset $628 + 101 = 729$ bytes into ukp->data.
Adding the 24-byte user_key_payload header yields an offset of $24 + 729 = 753$ bytes from the start of the allocated user_key_payload object in kmalloc-256. Since $753 = 2 \times 256 + 241$, this pointer lands 2 slab slots past the user_key_payload object, specifically at offset 241 (0xf1) within a third, unrelated 248-byte slab object in kmalloc-256 (in the crash report, an allocated fib_info object [ffff888043e1bc00, ffff888043e1bcf8)).
Crash Manifestation: In write_tag_3_packet() (fs/ecryptfs/keystore.c:2353):
```
memcpy(&dest[(*packet_size)], auth_tok->token.password.salt, ECRYPTFS_SALT_SIZE);
```
An 8-byte memcpy starting at offset 241 attempts to read bytes 241 through 248 ($241 + 8 = 249 > 248$), crossing 1 byte past the end of the 248-byte allocation into the KASAN redzone at ffff888043e1bcf8, triggering the reported KASAN: slab-out-of-bounds read.

ID	Workflow	Result	Correct	Bug	Created	Started	Finished	Revision
fd872f7f	assessment-security	DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ FuzzingArtifact: ❌ MemoryCorruption: ✅ NetworkNeedsSetup: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌	❓	KASAN: slab-out-of-bounds Read in write_tag_3_packet	2026/11/04 04:25	2026/11/04 04:26	2026/11/04 05:00	d02c3949

assessment-security

DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ FuzzingArtifact: ❌ MemoryCorruption: ✅ NetworkNeedsSetup: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌

❓

KASAN: slab-out-of-bounds Read in write_tag_3_packet

2026/11/04 04:25

2026/11/04 04:26

2026/11/04 05:00

d02c3949

Title	Replies (including bot)	Last reply
[syzbot] [ecryptfs?] KASAN: slab-out-of-bounds Read in write_tag_3_packet	0 (1)	2026/11/05 11:22

Title

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[syzbot] [ecryptfs?] KASAN: slab-out-of-bounds Read in write_tag_3_packet

0 (1)

2026/11/05 11:22

================================================================== BUG: KASAN: slab-out-of-bounds in write_tag_3_packet+0xe4f/0x1430 fs/ecryptfs/keystore.c:2353 Read of size 8 at addr ffff888043e1bcf1 by task syz.0.17/6007 CPU: 3 UID: 0 PID: 6007 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 Call Trace: <TASK> __dump_stack lib/dump_stack.c:94 [inline] dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120 print_address_description mm/kasan/report.c:378 [inline] print_report+0x13d/0x4b0 mm/kasan/report.c:482 kasan_report+0xdf/0x1c0 mm/kasan/report.c:595 check_region_inline mm/kasan/generic.c:186 [inline] kasan_check_range+0x10f/0x1e0 mm/kasan/generic.c:200 __asan_memcpy+0x23/0x60 mm/kasan/shadow.c:105 write_tag_3_packet+0xe4f/0x1430 fs/ecryptfs/keystore.c:2353 ecryptfs_generate_key_packet_set+0x7ae/0x1880 fs/ecryptfs/keystore.c:2422 ecryptfs_write_headers_virt fs/ecryptfs/crypto.c:955 [inline] ecryptfs_write_metadata+0x590/0x930 fs/ecryptfs/crypto.c:1065 ecryptfs_initialize_file+0xb6/0x180 fs/ecryptfs/inode.c:252 ecryptfs_create+0x393/0x550 fs/ecryptfs/inode.c:286 lookup_open+0xd8a/0x19e0 fs/namei.c:4575 open_last_lookups fs/namei.c:4774 [inline] path_openat+0xe75/0x23f0 fs/namei.c:5013 do_file_open+0x20e/0x430 fs/namei.c:5045 do_sys_openat2+0x163/0x1d0 fs/open.c:1442 do_sys_open fs/open.c:1448 [inline] __do_compat_sys_openat fs/open.c:1510 [inline] __se_compat_sys_openat fs/open.c:1508 [inline] __ia32_compat_sys_openat+0x12d/0x210 fs/open.c:1508 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:79 [inline] __do_fast_syscall_32+0x14b/0x8c0 arch/x86/entry/syscall_32.c:282 do_fast_syscall_32+0x32/0x70 arch/x86/entry/syscall_32.c:307 entry_SYSENTER_compat_after_hwframe+0x84/0x8e RIP: 0023:0xf73eefbc Code: d2 74 05 c1 e8 0c 89 02 8b 5d fc 31 c0 c9 c3 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 2e 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00 58 b8 RSP: 002b:00000000ffad3b4c EFLAGS: 00000292 ORIG_RAX: 0000000000000127 RAX: ffffffffffffffda RBX: 00000000ffffff9c RCX: 0000000080000580 RDX: 0000000000101042 RSI: 000000000000009d RDI: 0000000000000000 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 </TASK> Allocated by task 5949: kasan_save_stack+0x30/0x50 mm/kasan/common.c:57 kasan_save_track+0x14/0x30 mm/kasan/common.c:78 poison_kmalloc_redzone mm/kasan/common.c:409 [inline] __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:426 kasan_kmalloc include/linux/kasan.h:263 [inline] __do_kmalloc_node mm/slub.c:5531 [inline] __kmalloc_noprof+0x32f/0x840 mm/slub.c:5556 _kmalloc_noprof include/linux/slab.h:995 [inline] _kzalloc_noprof include/linux/slab.h:1312 [inline] fib_create_info+0x5bf/0x4640 net/ipv4/fib_semantics.c:1432 fib_table_insert+0x169/0x1c70 net/ipv4/fib_trie.c:1210 fib_magic+0x4d4/0x5c0 net/ipv4/fib_frontend.c:1147 fib_add_ifaddr+0x3a1/0x560 net/ipv4/fib_frontend.c:1184 fib_netdev_event+0x3d6/0x710 net/ipv4/fib_frontend.c:1529 notifier_call_chain+0x99/0x3f0 kernel/notifier.c:86 call_netdevice_notifiers_info+0xbe/0x110 net/core/dev.c:2313 call_netdevice_notifiers_extack net/core/dev.c:2351 [inline] call_netdevice_notifiers net/core/dev.c:2365 [inline] __dev_notify_flags+0x22c/0x4b0 net/core/dev.c:10032 netif_change_flags+0x108/0x160 net/core/dev.c:10061 do_setlink.isra.0+0x1b9f/0x3f50 net/core/rtnetlink.c:3294 rtnl_changelink net/core/rtnetlink.c:3942 [inline] __rtnl_newlink net/core/rtnetlink.c:4113 [inline] rtnl_newlink+0x13c1/0x2620 net/core/rtnetlink.c:4252 rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:7169 netlink_rcv_skb+0x159/0x420 net/netlink/af_netlink.c:2584 netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline] netlink_unicast+0x668/0x950 net/netlink/af_netlink.c:1361 netlink_sendmsg+0x9ac/0x10c0 net/netlink/af_netlink.c:1920 sock_sendmsg_nosec net/socket.c:800 [inline] __sock_sendmsg net/socket.c:818 [inline] __sys_sendto+0x492/0x4e0 net/socket.c:2283 __do_compat_sys_socketcall net/compat.c:474 [inline] __se_compat_sys_socketcall net/compat.c:422 [inline] __ia32_compat_sys_socketcall+0x59a/0x770 net/compat.c:422 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:79 [inline] do_int80_emulation+0x166/0x650 arch/x86/entry/syscall_32.c:166 asm_int80_emulation+0x1a/0x20 arch/x86/include/asm/idtentry.h:598 The buggy address belongs to the object at ffff888043e1bc00 which belongs to the cache kmalloc-256 of size 256 The buggy address is located 241 bytes inside of allocated 248-byte region [ffff888043e1bc00, ffff888043e1bcf8) The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x43e1a head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0 flags: 0x4fff00000000040(head|node=1|zone=1|lastcpupid=0x7ff) page_type: f5(slab) raw: 04fff00000000040 ffff88801d442b40 dead000000000100 dead000000000122 raw: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000 head: 04fff00000000040 ffff88801d442b40 dead000000000100 dead000000000122 head: 0000000000000000 0000000000100010 00000000f5000000 0000000000000000 head: 04fff00000000001 ffffffffffffff81 00000000ffffffff 00000000ffffffff head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002 page dumped because: kasan: bad access detected page_owner tracks the page as allocated page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd2820(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5864, tgid 5864 (syz-executor), ts 41561072516 set_page_owner include/linux/page_owner.h:33 [inline] post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1856 prep_new_page mm/page_alloc.c:1864 [inline] get_page_from_freelist+0x1113/0x35e0 mm/page_alloc.c:3935 __alloc_frozen_pages_noprof+0x2bd/0x3280 mm/page_alloc.c:5444 alloc_slab_page mm/slub.c:3384 [inline] allocate_slab mm/slub.c:3499 [inline] new_slab+0xb1/0x6d0 mm/slub.c:3550 refill_objects+0xe3/0x410 mm/slub.c:7539 refill_sheaf mm/slub.c:2917 [inline] __pcs_replace_empty_main+0x35e/0x650 mm/slub.c:4811 alloc_from_pcs mm/slub.c:4887 [inline] slab_alloc_node mm/slub.c:5021 [inline] __kmalloc_cache_noprof+0x4ef/0x6d0 mm/slub.c:5676 _kmalloc_noprof include/linux/slab.h:991 [inline] _kzalloc_noprof include/linux/slab.h:1312 [inline] ____ip_mc_inc_group+0x40b/0x10d0 net/ipv4/igmp.c:1601 br_ip4_multicast_join_snoopers.isra.0+0x152/0x400 net/bridge/br_multicast.c:4263 br_multicast_join_snoopers+0x8b/0x120 net/bridge/br_multicast.c:4283 br_dev_open+0x112/0x150 net/bridge/br_device.c:175 __dev_open+0x3ad/0x8f0 net/core/dev.c:1762 __dev_change_flags+0x596/0x850 net/core/dev.c:9991 netif_change_flags+0x8d/0x160 net/core/dev.c:10056 do_setlink.isra.0+0x1b9f/0x3f50 net/core/rtnetlink.c:3294 rtnl_changelink net/core/rtnetlink.c:3942 [inline] __rtnl_newlink net/core/rtnetlink.c:4113 [inline] rtnl_newlink+0x13c1/0x2620 net/core/rtnetlink.c:4252 page last free pid 5856 tgid 5856 ts 40658746840 stack trace: reset_page_owner include/linux/page_owner.h:26 [inline] __free_pages_prepare mm/page_alloc.c:1403 [inline] __free_frozen_pages+0x7b4/0x1110 mm/page_alloc.c:2954 qlink_free mm/kasan/quarantine.c:163 [inline] qlist_free_all+0x47/0xf0 mm/kasan/quarantine.c:179 kasan_quarantine_reduce+0x1a0/0x1f0 mm/kasan/quarantine.c:286 __kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:361 kasan_slab_alloc include/linux/kasan.h:253 [inline] slab_post_alloc_hook mm/slub.c:4720 [inline] slab_alloc_node mm/slub.c:5033 [inline] kmem_cache_alloc_node_noprof+0x282/0x6e0 mm/slub.c:5105 __alloc_skb+0x140/0x710 net/core/skbuff.c:705 alloc_skb include/linux/skbuff.h:1390 [inline] nlmsg_new include/net/netlink.h:1055 [inline] rtmsg_ifinfo_build_skb+0x81/0x260 net/core/rtnetlink.c:4617 rtmsg_ifinfo_event net/core/rtnetlink.c:4659 [inline] rtmsg_ifinfo_event net/core/rtnetlink.c:4649 [inline] rtmsg_ifinfo+0xa4/0x1b0 net/core/rtnetlink.c:4668 netif_close_many+0x3ca/0x810 net/core/dev.c:1866 unregister_netdevice_many_notify+0x563/0x1e00 net/core/dev.c:12694 unregister_netdevice_many net/core/dev.c:12796 [inline] unregister_netdevice_queue net/core/dev.c:12592 [inline] unregister_netdevice_queue+0x30b/0x3c0 net/core/dev.c:12578 unregister_netdevice include/linux/netdevice.h:3509 [inline] __tun_detach+0x1386/0x1850 drivers/net/tun.c:641 tun_detach drivers/net/tun.c:657 [inline] tun_chr_close+0xc2/0x220 drivers/net/tun.c:3608 __fput+0x3fb/0xb50 fs/file_table.c:512 fput_close_sync+0x118/0x250 fs/file_table.c:618 filp_close_sync+0x75/0xa0 fs/open.c:1573 Memory state around the buggy address: ffff888043e1bb80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff888043e1bc00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >ffff888043e1bc80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fc ^ ffff888043e1bd00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff888043e1bd80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ================================================================== ---------------- Code disassembly (best guess): 0: d2 74 05 c1 shlb %cl,-0x3f(%rbp,%rax,1) 4: e8 0c 89 02 8b call 0x8b028915 9: 5d pop %rbp a: fc cld b: 31 c0 xor %eax,%eax d: c9 leave e: c3 ret f: 90 nop 10: 90 nop 11: 90 nop 12: 90 nop 13: 90 nop 14: 90 nop 15: 90 nop 16: 90 nop 17: 90 nop 18: 90 nop 19: 90 nop 1a: 90 nop 1b: 90 nop 1c: 90 nop 1d: 90 nop 1e: 0f 1f 00 nopl (%rax) 21: 51 push %rcx 22: 52 push %rdx 23: 55 push %rbp 24: 89 e5 mov %esp,%ebp 26: 0f 34 sysenter 28: cd 80 int $0x80 * 2a: 5d pop %rbp <-- trapping instruction 2b: 5a pop %rdx 2c: 59 pop %rcx 2d: c3 ret 2e: 90 nop 2f: 2e 8d b4 26 00 00 00 cs lea 0x0(%rsi,%riz,1),%esi 36: 00 37: 8d b4 26 00 00 00 00 lea 0x0(%rsi,%riz,1),%esi 3e: 58 pop %rax 3f: b8 .byte 0xb8

Crashes (1):
Time	Kernel	Commit	Syzkaller	Config	Log	Report	Syz repro	C repro	VM info	Assets (help?)	Manager	Title
2026/11/04 04:25	upstream	05441d697f7f	5af5b6c6	.config	console log	report	syz / log	C		[disk image (non-bootable)] [vmlinux] [kernel image]	ci-qemu-upstream-386	KASAN: slab-out-of-bounds Read in write_tag_3_packet

Crashes (1):

Assets (help?)