[moderation] [exfat?] KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 7595b66ae9de Merge tag 'selinux-pr-20250624' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=105ab70c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=39acdbb3c95d37e5
dashboard link: https://syzkaller.appspot.com/bug?extid=643ffc17fded52bc7c07
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
CC: [hiro...@mail.parknet.co.jp linki...@kernel.org linux-...@vger.kernel.org linux-...@vger.kernel.org sj155...@samsung.com]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/95c3cc8c3471/disk-7595b66a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/40d1c5a0506e/vmlinux-7595b66a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/636329b317bc/bzImage-7595b66a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+643ffc...@syzkaller.appspotmail.com

loop3: detected capacity change from 0 to 128
==================================================================
BUG: KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

read-write to 0xffff88810ad1a134 of 4 bytes by task 8303 on cpu 0:
fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:191
fat_alloc_clusters+0x4d1/0xa80 fs/fat/fatent.c:503
fat_add_cluster fs/fat/inode.c:107 [inline]
__fat_get_block fs/fat/inode.c:154 [inline]
fat_get_block+0x258/0x5e0 fs/fat/inode.c:189
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2151
block_write_begin fs/buffer.c:2262 [inline]
cont_write_begin+0x5fc/0x970 fs/buffer.c:2601
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:228
generic_perform_write+0x181/0x490 mm/filemap.c:4112
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4229
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4255
__kernel_write_iter+0x253/0x4c0 fs/read_write.c:619
dump_emit_page fs/coredump.c:1112 [inline]
dump_user_range+0x407/0x8c0 fs/coredump.c:1186
elf_core_dump+0x1dc2/0x1f80 fs/binfmt_elf.c:2075
do_coredump+0x1dfd/0x27b0 fs/coredump.c:950
get_signal+0xd85/0xf70 kernel/signal.c:3019
arch_do_signal_or_restart+0x96/0x480 arch/x86/kernel/signal.c:337
exit_to_user_mode_loop kernel/entry/common.c:111 [inline]
exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline]
irqentry_exit_to_user_mode+0x5e/0xa0 kernel/entry/common.c:184
irqentry_exit+0x12/0x50 kernel/entry/common.c:287
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623

read to 0xffff88810ad1a000 of 512 bytes by task 8301 on cpu 1:
fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
fat_alloc_clusters+0x98b/0xa80 fs/fat/fatent.c:543
fat_add_new_entries fs/fat/dir.c:1230 [inline]
fat_add_entries+0x97b/0xfc0 fs/fat/dir.c:1388
vfat_add_entry+0x19a2/0x1b90 fs/fat/namei_vfat.c:674
vfat_create+0xa8/0x190 fs/fat/namei_vfat.c:768
lookup_open fs/namei.c:3717 [inline]
open_last_lookups fs/namei.c:3816 [inline]
path_openat+0x1102/0x2170 fs/namei.c:4052
do_filp_open+0x109/0x230 fs/namei.c:4082
do_sys_openat2+0xa6/0x110 fs/open.c:1437
do_sys_open fs/open.c:1452 [inline]
__do_sys_openat fs/open.c:1468 [inline]
__se_sys_openat fs/open.c:1463 [inline]
__x64_sys_openat+0xf2/0x120 fs/open.c:1463
x64_sys_call+0x1af/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:258
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd2/0x200 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: 8301 Comm: syz.3.1491 Not tainted 6.16.0-rc3-syzkaller-00044-g7595b66ae9de #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.