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Subject[PATCH] fs: handle shrinker registration failure in sget_userns
From: Michal Hocko <>

Syzbot has reported NULL ptr dereference during mntput because of
sb shrinker being NULL
CPU: 1 PID: 13231 Comm: syz-executor1 Not tainted 4.14.0-rc8+ #82
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
task: ffff8801d1dbe5c0 task.stack: ffff8801c9e38000
RIP: 0010:__list_del_entry_valid+0x7e/0x150 lib/list_debug.c:51
RSP: 0018:ffff8801c9e3f108 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff8801c53c6f98 RDI: ffff8801c53c6fa0
RBP: ffff8801c9e3f120 R08: 1ffff100393c7d55 R09: 0000000000000004
R10: ffff8801c9e3ef70 R11: 0000000000000000 R12: 0000000000000000
R13: dffffc0000000000 R14: 1ffff100393c7e45 R15: ffff8801c53c6f98
FS: 0000000000000000(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000dbc23000 CR3: 00000001c7269000 CR4: 00000000001406e0
DR0: 0000000020000000 DR1: 0000000020000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000600
Call Trace:
__list_del_entry include/linux/list.h:117 [inline]
list_del include/linux/list.h:125 [inline]
unregister_shrinker+0x79/0x300 mm/vmscan.c:301
deactivate_locked_super+0x64/0xd0 fs/super.c:308
deactivate_super+0x141/0x1b0 fs/super.c:340
cleanup_mnt+0xb2/0x150 fs/namespace.c:1173
mntput_no_expire+0x6e0/0xa90 fs/namespace.c:1237
mntput fs/namespace.c:1247 [inline]
kern_unmount+0x9c/0xd0 fs/namespace.c:2999
mq_put_mnt+0x37/0x50 ipc/mqueue.c:1609
put_ipc_ns+0x4d/0x150 ipc/namespace.c:163
free_nsproxy+0xc0/0x1f0 kernel/nsproxy.c:180
switch_task_namespaces+0x9d/0xc0 kernel/nsproxy.c:229
exit_task_namespaces+0x17/0x20 kernel/nsproxy.c:234
do_exit+0x9b0/0x1ad0 kernel/exit.c:864
do_group_exit+0x149/0x400 kernel/exit.c:968

Tetsuo has properly pointed out that the real reason is that fault
injection has caused register_shrinker to fail and the error path is not
handled in sget_userns.

Fix the issue by moving the shrinker registration up when the superblock
is allocated and fail early even before we try to register the superblock.
This should be safe wrt. parallel shrinker registration as we are holding
s_umount lock which blocks shrinker invocation.

The issue is very unlikely to trigger in the production because small
allocations do not fail usually.

Debugged-by: Tetsuo Handa <>
Signed-off-by: Michal Hocko <>

I am not really familiar with the code but something like this should
work. There are other shrinker users which need a similar treatment
of course. They can be handled separately. The failure path is rather
unlikely to lose sleep over.

fs/super.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index d4e33e8f1e6f..cfeb0e12b193 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -489,6 +489,7 @@ struct super_block *sget_userns(struct file_system_type *type,
if (user_ns != old->s_user_ns) {
+ unregister_shrinker(&s->s_shrink);
return ERR_PTR(-EBUSY);
@@ -503,12 +504,18 @@ struct super_block *sget_userns(struct file_system_type *type,
s = alloc_super(type, (flags & ~SB_SUBMOUNT), user_ns);
if (!s)
return ERR_PTR(-ENOMEM);
+ if (register_shrinker(&s->s_shrink)) {
+ spin_unlock(&sb_lock);
+ destroy_unused_super(s);
+ return ERR_PTR(-ENOMEM);
+ }
goto retry;

err = set(s, data);
if (err) {
+ unregister_shrinker(&s->s_shrink);
return ERR_PTR(err);
@@ -518,7 +525,6 @@ struct super_block *sget_userns(struct file_system_type *type,
hlist_add_head(&s->s_instances, &type->fs_supers);
- register_shrinker(&s->s_shrink);
return s;

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