[PATCH V2] inotify: Fix possible deadlock in fsnotify_destroy_mark

Lizhi Xu
2024-09-27T13:38:48Z

[Syzbot reported]
WARNING: possible circular locking dependency detected
6.11.0-rc4-syzkaller-00019-gb311c1b497e5 #0 Not tainted
------------------------------------------------------
kswapd0/78 is trying to acquire lock:
ffff88801b8d8930 (&group->mark_mutex){+.+.}-{3:3}, at: fsnotify_group_lock include/linux/fsnotify_backend.h:270 [inline]
ffff88801b8d8930 (&group->mark_mutex){+.+.}-{3:3}, at: fsnotify_destroy_mark+0x38/0x3c0 fs/notify/mark.c:578

but task is already holding lock:
ffffffff8ea2fd60 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat mm/vmscan.c:6841 [inline]
ffffffff8ea2fd60 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0xbb4/0x35a0 mm/vmscan.c:7223

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (fs_reclaim){+.+.}-{0:0}:
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5759
__fs_reclaim_acquire mm/page_alloc.c:3818 [inline]
fs_reclaim_acquire+0x88/0x140 mm/page_alloc.c:3832
might_alloc include/linux/sched/mm.h:334 [inline]
slab_pre_alloc_hook mm/slub.c:3939 [inline]
slab_alloc_node mm/slub.c:4017 [inline]
kmem_cache_alloc_noprof+0x3d/0x2a0 mm/slub.c:4044
inotify_new_watch fs/notify/inotify/inotify_user.c:599 [inline]
inotify_update_watch fs/notify/inotify/inotify_user.c:647 [inline]
__do_sys_inotify_add_watch fs/notify/inotify/inotify_user.c:786 [inline]
__se_sys_inotify_add_watch+0x72e/0x1070 fs/notify/inotify/inotify_user.c:729
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&group->mark_mutex){+.+.}-{3:3}:
check_prev_add kernel/locking/lockdep.c:3133 [inline]
check_prevs_add kernel/locking/lockdep.c:3252 [inline]
validate_chain+0x18e0/0x5900 kernel/locking/lockdep.c:3868
__lock_acquire+0x137a/0x2040 kernel/locking/lockdep.c:5142
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5759
__mutex_lock_common kernel/locking/mutex.c:608 [inline]
__mutex_lock+0x136/0xd70 kernel/locking/mutex.c:752
fsnotify_group_lock include/linux/fsnotify_backend.h:270 [inline]
fsnotify_destroy_mark+0x38/0x3c0 fs/notify/mark.c:578
fsnotify_destroy_marks+0x14a/0x660 fs/notify/mark.c:934
fsnotify_inoderemove include/linux/fsnotify.h:264 [inline]
dentry_unlink_inode+0x2e0/0x430 fs/dcache.c:403
__dentry_kill+0x20d/0x630 fs/dcache.c:610
shrink_kill+0xa9/0x2c0 fs/dcache.c:1055
shrink_dentry_list+0x2c0/0x5b0 fs/dcache.c:1082
prune_dcache_sb+0x10f/0x180 fs/dcache.c:1163
super_cache_scan+0x34f/0x4b0 fs/super.c:221
do_shrink_slab+0x701/0x1160 mm/shrinker.c:435
shrink_slab+0x1093/0x14d0 mm/shrinker.c:662
shrink_one+0x43b/0x850 mm/vmscan.c:4815
shrink_many mm/vmscan.c:4876 [inline]
lru_gen_shrink_node mm/vmscan.c:4954 [inline]
shrink_node+0x3799/0x3de0 mm/vmscan.c:5934
kswapd_shrink_node mm/vmscan.c:6762 [inline]
balance_pgdat mm/vmscan.c:6954 [inline]
kswapd+0x1bcd/0x35a0 mm/vmscan.c:7223
kthread+0x2f0/0x390 kernel/kthread.c:389
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(fs_reclaim);
lock(&group->mark_mutex);
lock(fs_reclaim);
lock(&group->mark_mutex);

*** DEADLOCK ***

[Analysis]
The inotify_new_watch() call passes through GFP_KERNEL, use memalloc_nofs_save/
memalloc_nofs_restore to make sure we don't end up with the fs reclaim dependency.

That any notification group needs to use NOFS allocations to be safe
against this race so we can just remove FSNOTIFY_GROUP_NOFS and
unconditionally do memalloc_nofs_save() in fsnotify_group_lock().

Reported-and-tested-by: syzbot+c679f13773f295d2da53@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c679f13773f295d2da53
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
V1 -> V2: remove FSNOTIFY_GROUP_NOFS in fsnotify_group_lock and unlock

fs/notify/inotify/inotify_user.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 8be029bc50b1..7b0a2809fc2d 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -268,14 +268,12 @@ struct fsnotify_group {
static inline void fsnotify_group_lock(struct fsnotify_group *group)
{
mutex_lock(&group->mark_mutex);
- if (group->flags & FSNOTIFY_GROUP_NOFS)
- group->owner_flags = memalloc_nofs_save();
+ group->owner_flags = memalloc_nofs_save();
}

static inline void fsnotify_group_unlock(struct fsnotify_group *group)
{
- if (group->flags & FSNOTIFY_GROUP_NOFS)
- memalloc_nofs_restore(group->owner_flags);
+ memalloc_nofs_restore(group->owner_flags);
mutex_unlock(&group->mark_mutex);
}

--
2.43.0