[moderation] [exfat?] KCSAN: data-race in copy_folio_from_iter_atomic / fat16_ent_put (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9aa622dbaab8 Merge tag 'for-linus' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=171a5a9c580000
kernel config: https://syzkaller.appspot.com/x/.config?x=425d0ddf0525df30
dashboard link: https://syzkaller.appspot.com/bug?extid=65c35fdc0b13ed286a96
compiler: Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
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/87039a551b46/disk-9aa622db.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e4d1036b4cb1/vmlinux-9aa622db.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1f867c50f079/bzImage-9aa622db.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+65c35f...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in copy_folio_from_iter_atomic / fat16_ent_put

write to 0xffff88811c9f2362 of 2 bytes by task 4241 on cpu 1:
fat16_ent_put+0x28/0x60 fs/fat/fatent.c:185
fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
fat_chain_add+0x170/0x4c0 fs/fat/misc.c:138
fat_add_cluster fs/fat/inode.c:113 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x468/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2030
block_write_begin fs/buffer.c:2144 [inline]
cont_write_begin+0x5bf/0x920 fs/buffer.c:2506
fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
generic_perform_write+0x186/0x490 mm/filemap.c:4374
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4491
generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4517
__kernel_write_iter+0x30f/0x590 fs/read_write.c:621
dump_emit_page fs/coredump.c:1293 [inline]
dump_user_range+0xa73/0xd00 fs/coredump.c:1367
elf_core_dump+0x21c0/0x2340 fs/binfmt_elf.c:2106
coredump_write+0xafb/0xdd0 fs/coredump.c:1047
do_coredump fs/coredump.c:1126 [inline]
vfs_coredump+0x2762/0x3270 fs/coredump.c:1195
get_signal+0xd33/0xf10 kernel/signal.c:3097
arch_do_signal_or_restart+0x96/0x480 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:66 [inline]
exit_to_user_mode_loop kernel/entry/common.c:101 [inline]
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
irqentry_exit_to_user_mode_prepare include/linux/irq-entry-common.h:244 [inline]
irqentry_exit_to_user_mode include/linux/irq-entry-common.h:315 [inline]
irqentry_exit+0x14d/0x610 kernel/entry/common.c:165
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:595

read to 0xffff88811c9f2200 of 512 bytes by task 411 on cpu 0:
memcpy_from_iter lib/iov_iter.c:85 [inline]
iterate_bvec include/linux/iov_iter.h:123 [inline]
iterate_and_advance2 include/linux/iov_iter.h:306 [inline]
iterate_and_advance include/linux/iov_iter.h:330 [inline]
__copy_from_iter lib/iov_iter.c:261 [inline]
copy_folio_from_iter_atomic+0x728/0x10a0 lib/iov_iter.c:491
generic_perform_write+0x2c4/0x490 mm/filemap.c:4392
shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3443
lo_rw_aio+0x6ab/0x760 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1949 [inline]
loop_process_work+0x569/0xad0 drivers/block/loop.c:1984
loop_workfn+0x31/0x40 drivers/block/loop.c:2008
process_one_work kernel/workqueue.c:3396 [inline]
process_scheduled_works+0x56a/0xa50 kernel/workqueue.c:3479
worker_thread+0x569/0x750 kernel/workqueue.c:3560
kthread+0x223/0x270 kernel/kthread.c:436
ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 411 Comm: kworker/u8:8 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
Workqueue: loop4 loop_workfn
==================================================================


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