[syzbot] [mm?] [fs?] KCSAN: data-race in __filemap_add_folio / invalidate_bdev (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 80e54e84911a Linux 6.14-rc6
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1664a7a8580000
kernel config: https://syzkaller.appspot.com/x/.config?x=958433697845b9a6
dashboard link: https://syzkaller.appspot.com/bug?extid=f2aaf773187f5cae54f3
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e94728c052e3/disk-80e54e84.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/742f05e27746/vmlinux-80e54e84.xz
kernel image: https://storage.googleapis.com/syzbot-assets/90d418e775f7/bzImage-80e54e84.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f2aaf7...@syzkaller.appspotmail.com

EXT4-fs (loop0): unmounting filesystem 00000000-0000-0000-0000-000000000000.
==================================================================
BUG: KCSAN: data-race in __filemap_add_folio / invalidate_bdev

read-write to 0xffff888100630570 of 8 bytes by task 3291 on cpu 0:
__filemap_add_folio+0x430/0x6f0 mm/filemap.c:929
filemap_add_folio+0x9c/0x1b0 mm/filemap.c:981
page_cache_ra_unbounded+0x1c1/0x350 mm/readahead.c:276
do_page_cache_ra mm/readahead.c:328 [inline]
force_page_cache_ra mm/readahead.c:357 [inline]
page_cache_sync_ra+0x252/0x680 mm/readahead.c:585
filemap_get_pages+0x2ca/0x11a0 mm/filemap.c:2580
filemap_read+0x230/0x8c0 mm/filemap.c:2691
blkdev_read_iter+0x228/0x2d0 block/fops.c:796
new_sync_read fs/read_write.c:484 [inline]
vfs_read+0x5cc/0x6f0 fs/read_write.c:565
ksys_read+0xe8/0x1b0 fs/read_write.c:708
__do_sys_read fs/read_write.c:717 [inline]
__se_sys_read fs/read_write.c:715 [inline]
__x64_sys_read+0x42/0x50 fs/read_write.c:715
x64_sys_call+0x2874/0x2dc0 arch/x86/include/generated/asm/syscalls_64.h:1
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff888100630570 of 8 bytes by task 3306 on cpu 1:
invalidate_bdev+0x25/0x70 block/bdev.c:99
ext4_put_super+0x571/0x810 fs/ext4/super.c:1356
generic_shutdown_super+0xe5/0x220 fs/super.c:642
kill_block_super+0x2a/0x70 fs/super.c:1710
ext4_kill_sb+0x44/0x80 fs/ext4/super.c:7368
deactivate_locked_super+0x7d/0x1c0 fs/super.c:473
deactivate_super+0x9f/0xb0 fs/super.c:506
cleanup_mnt+0x268/0x2e0 fs/namespace.c:1413
__cleanup_mnt+0x19/0x20 fs/namespace.c:1420
task_work_run+0x13a/0x1a0 kernel/task_work.c:227
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
exit_to_user_mode_prepare include/linux/entry-common.h:329 [inline]
__syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
syscall_exit_to_user_mode+0xa8/0x120 kernel/entry/common.c:218
do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x000000000000000d -> 0x000000000000000e

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 3306 Comm: syz-executor Not tainted 6.14.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================


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[Matthew Wilcox]

On Mon, Mar 10, 2025 at 02:40:26AM -0700, syzbot wrote:
> Reported-by: syzbot+f2aaf7...@syzkaller.appspotmail.com
>
> EXT4-fs (loop0): unmounting filesystem 00000000-0000-0000-0000-000000000000.
> ==================================================================
> BUG: KCSAN: data-race in __filemap_add_folio / invalidate_bdev
>
> read-write to 0xffff888100630570 of 8 bytes by task 3291 on cpu 0:
> __filemap_add_folio+0x430/0x6f0 mm/filemap.c:929

This is a write to mapping->nrpages with the i_pages lock held, as it
should be.

> filemap_add_folio+0x9c/0x1b0 mm/filemap.c:981
> page_cache_ra_unbounded+0x1c1/0x350 mm/readahead.c:276
> do_page_cache_ra mm/readahead.c:328 [inline]
> force_page_cache_ra mm/readahead.c:357 [inline]
> page_cache_sync_ra+0x252/0x680 mm/readahead.c:585
> filemap_get_pages+0x2ca/0x11a0 mm/filemap.c:2580
> filemap_read+0x230/0x8c0 mm/filemap.c:2691
> blkdev_read_iter+0x228/0x2d0 block/fops.c:796
> new_sync_read fs/read_write.c:484 [inline]
> vfs_read+0x5cc/0x6f0 fs/read_write.c:565
> ksys_read+0xe8/0x1b0 fs/read_write.c:708
>

> read to 0xffff888100630570 of 8 bytes by task 3306 on cpu 1:
> invalidate_bdev+0x25/0x70 block/bdev.c:99

This is a read of mapping->nrpages with no lock held. So we could
silence this warning by making this a READ_ONCE or data_race().

The problem is that I'm not sure this is the right answer. Obviously
here we only care about zero-vs-non-zero, but what if we race with
0 being incremented to 1? Should there be some locking higher up
that prevents this? Or is this "yes, root can do this and screw
themselves"?

[Matthew Wilcox]

On Mon, Mar 10, 2025 at 09:54:00AM -0700, Yuan Tan wrote:
> Syzbot reported a data-race in __filemap_add_folio / invalidate_bdev[1]
> due to concurrent access to mapping->nrpages.
> Adds a lock around the access to nrpages.

Did you even read my analysis? This is a grossly inappropriate fix.
NAK.

[syzbot]

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