[syzbot] [exfat?] KCSAN: data-race in fat32_ent_put / fat_mirror_bhs (4)

Hello,

syzbot found the following issue on:

HEAD commit:    3d6d817622b0 Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15263225580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=33c5ac947f6be69e
dashboard link: https://syzkaller.appspot.com/bug?extid=9435f33de7962a2cb62d
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/8a0f9e563bc1/disk-3d6d8176.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a3d8fcb60747/vmlinux-3d6d8176.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f5e1d8d94487/bzImage-3d6d8176.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

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

read-write to 0xffff88811db1234c of 4 bytes by task 4861 on cpu 1:
 fat32_ent_put+0x4e/0x90 fs/fat/fatent.c:194
 fat_ent_write+0x6c/0xe0 fs/fat/fatent.c:422
 fat_chain_add+0x174/0x4a0 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+0x468/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
 block_write_begin fs/buffer.c:2234 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 cont_expand_zero fs/buffer.c:2524 [inline]
 cont_write_begin+0x18d/0x920 fs/buffer.c:2586
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_cont_expand_simple+0xb0/0x150 fs/buffer.c:2487
 fat_cont_expand+0x3e/0x170 fs/fat/file.c:227
 fat_fallocate+0x177/0x1c0 fs/fat/file.c:305
 vfs_fallocate+0x3ac/0x400 fs/open.c:338
 ksys_fallocate fs/open.c:362 [inline]
 __do_sys_fallocate fs/open.c:367 [inline]
 __se_sys_fallocate fs/open.c:365 [inline]
 __x64_sys_fallocate+0x7a/0xd0 fs/open.c:365
 x64_sys_call+0x298e/0x3020 arch/x86/include/generated/asm/syscalls_64.h:286
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811db12000 of 2048 bytes by task 4866 on cpu 0:
 fat_mirror_bhs+0x1df/0x330 fs/fat/fatent.c:399
 fat_ent_write+0xd0/0xe0 fs/fat/fatent.c:428
 fat_chain_add+0x174/0x4a0 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+0x468/0x5e0 fs/fat/inode.c:190
 __block_write_begin_int+0x400/0xf90 fs/buffer.c:2123
 block_write_begin fs/buffer.c:2234 [inline]
 cont_write_begin+0x5bf/0x920 fs/buffer.c:2596
 fat_write_begin+0x52/0xe0 fs/fat/inode.c:230
 generic_perform_write+0x186/0x490 mm/filemap.c:4364
 __generic_file_write_iter+0x9e/0x120 mm/filemap.c:4481
 generic_file_write_iter+0x77/0x1f0 mm/filemap.c:4507
 __kernel_write_iter+0x30f/0x590 fs/read_write.c:621
 dump_emit_page fs/coredump.c:1298 [inline]
 dump_user_range+0xa73/0xd00 fs/coredump.c:1372
 elf_core_dump+0x21c0/0x2340 fs/binfmt_elf.c:2109
 coredump_write+0xaf9/0xdd0 fs/coredump.c:1052
 do_coredump fs/coredump.c:1131 [inline]
 vfs_coredump+0x2770/0x3290 fs/coredump.c:1200
 get_signal+0xd33/0xf10 kernel/signal.c:3031
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 4866 Comm: syz.3.376 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
==================================================================
syz.3.376 (4866) used greatest stack depth: 9320 bytes left


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