syzbot


KCSAN: data-race in _copy_to_iter / vsnprintf

Status: moderation: reported on 2026/03/20 12:25
Subsystems: ext4
[Documentation on labels]
Reported-by: syzbot+be7bd9b03a9be40d9ac5@syzkaller.appspotmail.com
First crash: 2d10h, last: 2d10h
✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision Error
c02b0531-eb98-4e6a-806c-1b42d2f53db7 assessment-kcsan Benign: ✅  Confident: ✅  KCSAN: data-race in _copy_to_iter / vsnprintf 2026/03/20 12:24 2026/03/20 12:24 2026/03/20 12:26 f84caa2f84dea47b51a188e8d6267f295e89bbcb

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

write to 0xffff888105f0b055 of 1 bytes by task 6874 on cpu 0:
 vsnprintf+0x7ee/0x860 lib/vsprintf.c:2980
 snprintf+0x86/0xb0 lib/vsprintf.c:3042
 ext4_multi_mount_protect+0x4e4/0x600 fs/ext4/mmp.c:382
 __ext4_fill_super fs/ext4/super.c:5464 [inline]
 ext4_fill_super+0x22e6/0x37c0 fs/ext4/super.c:5791
 get_tree_bdev_flags+0x291/0x300 fs/super.c:1694
 get_tree_bdev+0x1f/0x30 fs/super.c:1717
 ext4_get_tree+0x1c/0x30 fs/ext4/super.c:5823
 vfs_get_tree+0x57/0x1d0 fs/super.c:1754
 fc_mount fs/namespace.c:1193 [inline]
 do_new_mount_fc fs/namespace.c:3763 [inline]
 do_new_mount+0x288/0x8d0 fs/namespace.c:3839
 path_mount+0x4d0/0xbc0 fs/namespace.c:4159
 do_mount fs/namespace.c:4172 [inline]
 __do_sys_mount fs/namespace.c:4361 [inline]
 __se_sys_mount+0x28c/0x2e0 fs/namespace.c:4338
 __x64_sys_mount+0x67/0x80 fs/namespace.c:4338
 x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888105f0b000 of 512 bytes by task 6698 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+0x18f/0x2d0 lib/iov_iter.c:374
 copy_folio_to_iter include/linux/uio.h:204 [inline]
 filemap_read+0x407/0xa10 mm/filemap.c:2856
 blkdev_read_iter+0x22d/0x2e0 block/fops.c:855
 new_sync_read fs/read_write.c:493 [inline]
 vfs_read+0x6c0/0x7f0 fs/read_write.c:574
 ksys_read+0xdc/0x1a0 fs/read_write.c:717
 __do_sys_read fs/read_write.c:726 [inline]
 __se_sys_read fs/read_write.c:724 [inline]
 __x64_sys_read+0x40/0x50 fs/read_write.c:724
 x64_sys_call+0x2886/0x3020 arch/x86/include/generated/asm/syscalls_64.h:1
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 6698 Comm: udevd Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/03/20 12:24 upstream 8a30aeb0d1b4 2f245add .config console log report info [disk image] [vmlinux] [kernel image] ci2-upstream-kcsan-gce KCSAN: data-race in _copy_to_iter / vsnprintf
* Struck through repros no longer work on HEAD.