[moderation] [exfat?] KCSAN: data-race in fat32_ent_put / fat_mirror_bhs (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e98f34af6116 Merge tag 'i2c-for-6.19-final' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1442aa52580000
kernel config: https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
dashboard link: https://syzkaller.appspot.com/bug?extid=e4ddb4d04ca84e14a5b5
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
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/49a5e6aae7b9/disk-e98f34af.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/605a872be1a9/vmlinux-e98f34af.xz
kernel image: https://storage.googleapis.com/syzbot-assets/dfd740d44724/bzImage-e98f34af.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e4ddb4...@syzkaller.appspotmail.com

loop6: detected capacity change from 0 to 128
==================================================================
BUG: KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

read-write to 0xffff88811c64213c of 4 bytes by task 8412 on cpu 0:
fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:191
fat_alloc_clusters+0x57b/0xc50 fs/fat/fatent.c:503
fat_add_cluster fs/fat/inode.c:108 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x258/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
block_write_begin fs/buffer.c:2256 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
cont_expand_zero fs/buffer.c:2522 [inline]
cont_write_begin+0x1b0/0x970 fs/buffer.c:2584
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
generic_perform_write+0x183/0x490 mm/filemap.c:4314
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4431
generic_file_write_iter+0x8d/0x310 mm/filemap.c:4457
__kernel_write_iter+0x319/0x590 fs/read_write.c:619
dump_emit_page fs/coredump.c:1298 [inline]
dump_user_range+0xa7d/0xdb0 fs/coredump.c:1372
elf_core_dump+0x21a2/0x2330 fs/binfmt_elf.c:2111
coredump_write+0xacf/0xdf0 fs/coredump.c:1049
do_coredump fs/coredump.c:1126 [inline]
vfs_coredump+0x26bc/0x3120 fs/coredump.c:1200
get_signal+0xd7b/0xf60 kernel/signal.c:3019
arch_do_signal_or_restart+0x96/0x450 arch/x86/kernel/signal.c:337
__exit_to_user_mode_loop kernel/entry/common.c:41 [inline]
exit_to_user_mode_loop+0x6a/0x6f0 kernel/entry/common.c:75
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode_work include/linux/entry-common.h:159 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:194 [inline]
do_syscall_64+0x1d3/0x2a0 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811c642000 of 512 bytes by task 8404 on cpu 1:
fat_mirror_bhs+0x1df/0x320 fs/fat/fatent.c:395
fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:423
fat_chain_add+0x16c/0x490 fs/fat/misc.c:136
fat_add_cluster fs/fat/inode.c:113 [inline]
__fat_get_block fs/fat/inode.c:155 [inline]
fat_get_block+0x46c/0x5e0 fs/fat/inode.c:190
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
block_write_begin fs/buffer.c:2256 [inline]
cont_write_begin+0x5fe/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
cont_expand_zero fs/buffer.c:2522 [inline]
cont_write_begin+0x1b0/0x970 fs/buffer.c:2584
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:230
generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2485
fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
fat_setattr+0x2a4/0x8a0 fs/fat/file.c:513
notify_change+0x872/0x8f0 fs/attr.c:546
do_truncate fs/open.c:68 [inline]
do_ftruncate+0x349/0x3d0 fs/open.c:195
do_sys_ftruncate fs/open.c:207 [inline]
__do_sys_ftruncate fs/open.c:212 [inline]
__se_sys_ftruncate fs/open.c:210 [inline]
__x64_sys_ftruncate+0x68/0xc0 fs/open.c:210
x64_sys_call+0x2e35/0x3000 arch/x86/include/generated/asm/syscalls_64.h:78
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xc0/0x2a0 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: 8404 Comm: syz.6.1731 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
==================================================================


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