[moderation] [exfat?] KCSAN: data-race in fat32_ent_put / fat_mirror_bhs (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c9cfc122f037 Merge tag 'for-6.18-rc4-tag' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16a51bcd980000
kernel config: https://syzkaller.appspot.com/x/.config?x=d2e0bc0ae94545f7
dashboard link: https://syzkaller.appspot.com/bug?extid=1e151cbdd1bc1701c8c0
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
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/5f366eb5a67c/disk-c9cfc122.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/34bb36eef674/vmlinux-c9cfc122.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cc542f2a2978/bzImage-c9cfc122.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1e151c...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in fat32_ent_put / fat_mirror_bhs

read-write to 0xffff88813590d130 of 4 bytes by task 12571 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:2145
block_write_begin fs/buffer.c:2256 [inline]
cont_write_begin+0x5fc/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:229
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:229
generic_perform_write+0x184/0x490 mm/filemap.c:4242
__generic_file_write_iter+0x9e/0x120 mm/filemap.c:4359
generic_file_write_iter+0x8d/0x2f0 mm/filemap.c:4385
__kernel_write_iter+0x2d6/0x540 fs/read_write.c:619
dump_emit_page fs/coredump.c:1298 [inline]
dump_user_range+0x61e/0x8f0 fs/coredump.c:1372
elf_core_dump+0x1de7/0x1f80 fs/binfmt_elf.c:2111
coredump_write+0xb12/0xe30 fs/coredump.c:1049
vfs_coredump+0x143a/0x20d0 fs/coredump.c:1170
get_signal+0xd84/0xf70 kernel/signal.c:3019
arch_do_signal_or_restart+0x96/0x440 arch/x86/kernel/signal.c:337
exit_to_user_mode_loop kernel/entry/common.c:40 [inline]
exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
irqentry_exit_to_user_mode+0x5b/0xa0 kernel/entry/common.c:73
irqentry_exit+0x12/0x50 kernel/entry/common.c:191
asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618

read to 0xffff88813590d000 of 512 bytes by task 12575 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+0x15d/0x440 fs/fat/misc.c:136
fat_add_cluster fs/fat/inode.c:112 [inline]
__fat_get_block fs/fat/inode.c:154 [inline]
fat_get_block+0x46c/0x5e0 fs/fat/inode.c:189
__block_write_begin_int+0x400/0xf90 fs/buffer.c:2145
block_write_begin fs/buffer.c:2256 [inline]
cont_write_begin+0x5fc/0x970 fs/buffer.c:2594
fat_write_begin+0x4f/0xe0 fs/fat/inode.c:229
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:229
generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2485
fat_cont_expand+0x3e/0x170 fs/fat/file.c:223
fat_fallocate+0x177/0x1c0 fs/fat/file.c:301
vfs_fallocate+0x3b6/0x400 fs/open.c:342
ksys_fallocate fs/open.c:366 [inline]
__do_sys_fallocate fs/open.c:371 [inline]
__se_sys_fallocate fs/open.c:369 [inline]
__x64_sys_fallocate+0x7a/0xd0 fs/open.c:369
x64_sys_call+0x2518/0x3000 arch/x86/include/generated/asm/syscalls_64.h:286
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: 12575 Comm: syz.4.2335 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
==================================================================


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