[moderation] [bpf?] KCSAN: data-race in alloc_htab_elem / bcmp

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 8f0b4cce4481 Linux 6.19-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1644c11a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=568abfa605e4f389ea53
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [and...@kernel.org a...@kernel.org b...@vger.kernel.org dan...@iogearbox.net edd...@gmail.com hao...@google.com john.fa...@gmail.com jo...@kernel.org kps...@kernel.org linux-...@vger.kernel.org marti...@linux.dev s...@fomichev.me so...@kernel.org yongho...@linux.dev]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/0397a0339bfd/disk-8f0b4cce.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/1c266f6e8d6b/vmlinux-8f0b4cce.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a748ab0188b2/bzImage-8f0b4cce.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+568abf...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in alloc_htab_elem / bcmp

write to 0xffff88811ae3c270 of 4 bytes by task 8146 on cpu 0:
alloc_htab_elem+0x226/0x420 kernel/bpf/hashtab.c:1029
htab_map_update_elem+0x312/0x980 kernel/bpf/hashtab.c:1149
bpf_map_update_value+0x4f4/0x570 kernel/bpf/syscall.c:294
generic_map_update_batch+0x3eb/0x540 kernel/bpf/syscall.c:2038
bpf_map_do_batch+0x25c/0x380 kernel/bpf/syscall.c:5647
__sys_bpf+0x5f8/0x7c0 kernel/bpf/syscall.c:-1
__do_sys_bpf kernel/bpf/syscall.c:6274 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6272 [inline]
__x64_sys_bpf+0x41/0x50 kernel/bpf/syscall.c:6272
x64_sys_call+0x28e1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811ae3c271 of 1 bytes by task 8149 on cpu 1:
memcmp lib/string.c:694 [inline]
bcmp+0x52/0x90 lib/string.c:715
memcmp include/linux/fortify-string.h:727 [inline]
lookup_nulls_elem_raw kernel/bpf/hashtab.c:639 [inline]
__htab_map_lookup_elem+0xd0/0x150 kernel/bpf/hashtab.c:668
bpf_prog_f32ce9a4f916f6a1+0x48/0x59
bpf_dispatcher_nop_func include/linux/bpf.h:1376 [inline]
__bpf_prog_run include/linux/filter.h:723 [inline]
bpf_prog_run include/linux/filter.h:730 [inline]
__bpf_trace_run kernel/trace/bpf_trace.c:2075 [inline]
bpf_trace_run3+0x10f/0x1e0 kernel/trace/bpf_trace.c:2117
__traceiter_kmem_cache_free+0x38/0x60 include/trace/events/kmem.h:117
__do_trace_kmem_cache_free include/trace/events/kmem.h:117 [inline]
trace_kmem_cache_free include/trace/events/kmem.h:117 [inline]
kmem_cache_free+0x327/0x3a0 mm/slub.c:6778
putname+0x90/0x100 fs/namei.c:297
__free_putname include/linux/fs.h:2512 [inline]
do_sys_openat2+0xd7/0x150 fs/open.c:1431
do_sys_open fs/open.c:1436 [inline]
__do_sys_openat fs/open.c:1452 [inline]
__se_sys_openat fs/open.c:1447 [inline]
__x64_sys_openat+0xf2/0x120 fs/open.c:1447
x64_sys_call+0x2b07/0x3000 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00 -> 0x2f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 8149 Comm: syz.1.1240 Tainted: G W syzkaller #0 PREEMPT(voluntary)
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


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