KCSAN: data-race in _copy_to_iter / fat12_ent_put (8)

Status: moderation: reported on 2026/04/16 19:03
Subsystems: exfat
Labels: prio:low
[Documentation on labels]
Reported-by: syzbot+816dcf406a59578d6d84@syzkaller.appspotmail.com
First crash: 145d, last: 15d
▶ ▼ ✨ AI Jobs (2)
ID Workflow Result Correct Bug Created Started Finished Revision
2f9c2b3e assessment-security DenialOfService: ❌ Exploitable: ❌ FilesystemTrigger: ✅ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ✅ UserNamespace: ✅ VMGuestTrigger: ✅ VMHostTrigger: ❌ ❓ KCSAN: data-race in _copy_to_iter / fat12_ent_put (8) 2026/05/26 05:58 2026/05/26 05:58 2026/05/26 06:32 c69befb3
e2e60dae assessment-kcsan Benign: ✅ Confident: ✅ ❓ KCSAN: data-race in _copy_to_iter / fat12_ent_put (8) 2026/04/16 20:59 2026/04/16 20:59 2026/04/16 21:01 6285b005
▶ ▼ Similar bugs (7)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put exfat 6 6 1460d 1491d 0/29 auto-obsoleted due to no activity on 2022/10/14 22:33
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (7) exfat 6 5 237d 372d 0/29 auto-obsoleted due to no activity on 2026/03/12 02:36
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (4) exfat 6 5 1200d 1230d 0/29 auto-obsoleted due to no activity on 2023/07/02 02:14
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (5) exfat 6 21 1020d 1005d 0/29 auto-obsoleted due to no activity on 2023/12/28 22:56
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (6) exfat 6 1 923d 923d 0/29 auto-obsoleted due to no activity on 2024/04/04 01:15
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (3) exfat 6 3 1274d 1290d 0/29 auto-obsoleted due to no activity on 2023/04/24 07:39
upstream KCSAN: data-race in _copy_to_iter / fat12_ent_put (2) exfat 6 3 1390d 1424d 0/29 auto-obsoleted due to no activity on 2022/12/31 00:12

Sample crash report:
==================================================================
BUG: KCSAN: data-race in _copy_to_iter / fat12_ent_put

read-write to 0xffff88810813123a of 1 bytes by task 6033 on cpu 0:
 fat12_ent_put+0xc4/0x180 fs/fat/fatent.c:165
 fat_alloc_clusters+0x55e/0xc40 fs/fat/fatent.c:508
 fat_add_cluster+0x46/0xe0 fs/fat/inode.c:108
 fat_fallocate+0x11e/0x1c0 fs/fat/file.c:295
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ioctl_preallocate fs/ioctl.c:289 [inline]
 file_ioctl+0x4e1/0x5b0 fs/ioctl.c:-1
 do_vfs_ioctl+0x7c4/0xe50 fs/ioctl.c:576
 __do_sys_ioctl fs/ioctl.c:595 [inline]
 __se_sys_ioctl+0x82/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x239d/0x2550 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888108131200 of 512 bytes by task 3286 on cpu 1:
 instrument_copy_to_user include/linux/instrumented.h:130 [inline]
 copy_to_user_iter lib/iov_iter.c:24 [inline]
 iterate_ubuf include/linux/iov_iter.h:30 [inline]
 iterate_and_advance2 include/linux/iov_iter.h:302 [inline]
 iterate_and_advance include/linux/iov_iter.h:330 [inline]
 _copy_to_iter+0x130/0xea0 lib/iov_iter.c:197
 copy_page_to_iter+0x174/0x210 lib/iov_iter.c:374
 copy_folio_to_iter include/linux/uio.h:204 [inline]
 filemap_read+0x3ed/0xa20 mm/filemap.c:2875
 blkdev_read_iter+0x21e/0x2d0 block/fops.c:832
 new_sync_read fs/read_write.c:493 [inline]
 vfs_read+0x699/0x7c0 fs/read_write.c:574
 ksys_read+0xdc/0x1a0 fs/read_write.c:716
 __do_sys_read fs/read_write.c:725 [inline]
 __se_sys_read fs/read_write.c:723 [inline]
 __x64_sys_read+0x40/0x50 fs/read_write.c:723
 x64_sys_call+0x2135/0x2550 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3286 Comm: udevd Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
==================================================================

Crashes (4):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/25 13:17 upstream 66498c75b4f8 a9a1fc32 .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in _copy_to_iter / fat12_ent_put
2026/07/29 21:20 upstream fc02acf6ac0c 6ef5c1e2 .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in _copy_to_iter / fat12_ent_put
2026/06/05 14:48 upstream ddd664bbff63 48b6c3fa .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in _copy_to_iter / fat12_ent_put
2026/04/16 19:02 upstream 1d51b370a0f8 4743f87d .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in _copy_to_iter / fat12_ent_put
* Struck through repros no longer work on HEAD.