syzbot


KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

Status: moderation: reported on 2026/05/10 02:31
Subsystems: exfat
[Documentation on labels]
Reported-by: syzbot+6244b6da154efe0aedb6@syzkaller.appspotmail.com
First crash: 2d07h, last: 2d07h
✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision Error
ea86efda-5eb8-485a-a901-887adaa67f9d assessment-kcsan Benign: ✅ KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync 2026/05/10 03:58 2026/05/10 03:59 2026/05/10 04:02 29233ece713919081e9069c2a18be92526041f39

Sample crash report:
==================================================================
BUG: KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync

write to 0xffff88811b0b8a80 of 8 bytes by task 5174 on cpu 0:
 __remove_assoc_queue fs/buffer.c:495 [inline]
 mmb_sync+0x14d/0x600 fs/buffer.c:566
 fat_file_fsync+0xba/0x110 fs/fat/file.c:197
 vfs_fsync_range+0xc5/0xe0 fs/sync.c:186
 generic_write_sync include/linux/fs.h:2654 [inline]
 generic_file_write_iter+0x1ba/0x310 mm/filemap.c:4472
 iter_file_splice_write+0x6c4/0xa80 fs/splice.c:736
 do_splice_from fs/splice.c:936 [inline]
 direct_splice_actor+0x156/0x2a0 fs/splice.c:1159
 splice_direct_to_actor+0x311/0x670 fs/splice.c:1103
 do_splice_direct_actor fs/splice.c:1202 [inline]
 do_splice_direct+0x119/0x1a0 fs/splice.c:1228
 do_sendfile+0x382/0x650 fs/read_write.c:1372
 __do_sys_sendfile64 fs/read_write.c:1433 [inline]
 __se_sys_sendfile64 fs/read_write.c:1419 [inline]
 __x64_sys_sendfile64+0x105/0x150 fs/read_write.c:1419
 x64_sys_call+0x2dc4/0x3020 arch/x86/include/generated/asm/syscalls_64.h:41
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b0b8a80 of 8 bytes by task 5175 on cpu 1:
 mmb_mark_buffer_dirty+0x2e/0x140 fs/buffer.c:720
 fat16_ent_put+0x52/0x60 fs/fat/fatent.c:186
 fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:508
 fat_add_cluster fs/fat/inode.c:108 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x25e/0x5f0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2042
 block_write_begin fs/buffer.c:2153 [inline]
 cont_write_begin+0x5fe/0x970 fs/buffer.c:2491
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2419 [inline]
 cont_write_begin+0x1b0/0x970 fs/buffer.c:2481
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2382
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:226
 fat_setattr+0x2a4/0x870 fs/fat/file.c:516
 notify_change+0x872/0x8f0 fs/attr.c:556
 do_truncate+0x14f/0x190 fs/open.c:68
 vfs_truncate+0x1e8/0x220 fs/open.c:118
 ksys_truncate+0xa2/0x140 fs/open.c:142
 __do_sys_truncate fs/open.c:154 [inline]
 __se_sys_truncate fs/open.c:152 [inline]
 __x64_sys_truncate+0x31/0x40 fs/open.c:152
 x64_sys_call+0x19fb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:77
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff88811b0e31f8 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 5175 Comm: syz.4.473 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/05/10 02:31 upstream e92b2872d0b1 29233ece .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in mmb_mark_buffer_dirty / mmb_sync
* Struck through repros no longer work on HEAD.