fs: handle shrinker registration failure in sget_userns

From: Michal Hocko <[email protected]>

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 <[email protected]>

Signed-off-by Michal Hocko <[email protected]>

---

Hi,
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(-)

fs/super.c +0 -0

index d4e33e8f1e6f..cfeb0e12b193 100644

fs/super.c +6 -1

 				destroy_unused_super(s);
 				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) {
 		spin_unlock(&sb_lock);
+		unregister_shrinker(&s->s_shrink);
 		destroy_unused_super(s);
 		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);
 	spin_unlock(&sb_lock);
 	get_filesystem(type);
-	register_shrinker(&s->s_shrink);
 	return s;
 }
 

Signature · 1 line

2.15.0

On Thu 23-11-17 12:52:47, Michal Hocko wrote:

Review struct super_block *sget_userns(struct file_system_type *type, · line 492

Quoted 5 lines

(quoted diff) +1 -0

@@ -489,6 +489,7 @@ struct super_block *sget_userns(struct file_system_type *type,
 				continue;
 			if (user_ns != old->s_user_ns) {
 				spin_unlock(&sb_lock);
+				unregister_shrinker(&s->s_shrink);

This is wrong as 's' can be NULL at this point.

I think the right fix is to
move unregister_shrinker() into destroy_unused_super(). But for that we need a reliable way to detect whether the shrinker has been already registered - possibly by initializing sb->shrinker.list in alloc_super() and then checking for list_empty() in destroy_unused_super().

Also I'd note that early shrinker registration breaks assumption of destroy_unused_super() that nobody could have seen the superblock - shrinkers could have - but since shrinker code doesn't use RCU to access the superblock, we are fine. But still comment before destroy_unused_super() should be probably updated.

From cffea62e7f8605c370c8115afae530a1831e75f3 Mon Sep 17 00:00:00 2001

What about the following?

From 467be16ca5165613daf292a68592e3b5bc7252c5 Mon Sep 17 00:00:00 2001

On Thu 23-11-17 13:45:41, Michal Hocko wrote:
[...]

Quoted 1 line

What about the following?

Dohh, a rebase artifact sneaked in. So let me try again. Sorry about spamming :/
---
From 467be16ca5165613daf292a68592e3b5bc7252c5 Mon Sep 17 00:00:00 2001
From: Michal Hocko <[email protected]>
Date: Thu, 23 Nov 2017 12:28:35 +0100
Subject: [PATCH] fs: handle shrinker registration failure in sget_userns

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 invocation 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 <[email protected]>

Signed-off-by Michal Hocko <[email protected]>

---
 fs/super.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

fs/super.c +14 -2

index d4e33e8f1e6f..80b118cc4bb6 100644
@@ -155,11 +155,19 @@ static void destroy_super_rcu(struct rcu_head *head)
 	schedule_work(&s->destroy_work);
 }
 
-/* Free a superblock that has never been seen by anyone */
+/*
+ * Free a superblock that has never been seen by anyone. Note that shrinkers
+ * could have been invoked already but we rely on s_umount to not actually
+ * touch it.
+ */
 static void destroy_unused_super(struct super_block *s)
 {
 	if (!s)
 		return;
+
+	if (!list_empty(&s->s_shrink.list))
+		unregister_shrinker(&s->s_shrink);
+
 	up_write(&s->s_umount);
 	list_lru_destroy(&s->s_dentry_lru);
 	list_lru_destroy(&s->s_inode_lru);

@@ -252,6 +260,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_shrink.count_objects = super_cache_count;
 	s->s_shrink.batch = 1024;
 	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
+	INIT_LIST_HEAD(&s->s_shrink.list);
 	return s;
 
 fail:

@@ -503,6 +512,10 @@ 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)) {
+			destroy_unused_super(s);
+			return ERR_PTR(-ENOMEM);
+		}
 		goto retry;
 	}
 

@@ -518,7 +531,6 @@ struct super_block *sget_userns(struct file_system_type *type,
 	hlist_add_head(&s->s_instances, &type->fs_supers);
 	spin_unlock(&sb_lock);
 	get_filesystem(type);
-	register_shrinker(&s->s_shrink);
 	return s;
 }
 

Signature · 5 lines

2.15.0

-- 
Michal Hocko
SUSE Labs

Looks good to me now. You can add:

Reviewed-by Jan Kara <[email protected]>

Quoted 4 lines · Jan Kara

Looks good to me now. You can add:

Reviewed-by Jan Kara <[email protected]>

It does not look good to me, for "goto fail" can call destroy_unused_super() before s->s_shrink.list is initialized. Also, the comment block saying "this object isn't exposed yet" wants to be updated?

---
 fs/super.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

fs/super.c +0 -0

index 80b118c..44f0c6b 100644

Quoted 7 lines

fs/super.c +0 -3

@@ -260,9 +261,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_shrink.count_objects = super_cache_count;
 	s->s_shrink.batch = 1024;
 	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
-	INIT_LIST_HEAD(&s->s_shrink.list);
-	return s;
-

fs/super.c static struct super_block *alloc_super(struct file_system_type *type, int flags, · line 268

Quoted 5 lines

fs/super.c +2 -0

+	if (register_shrinker(&s->s_shrink) == 0)
+		return s;
 fail:
 	destroy_unused_super(s);
 	return NULL;

But I am not sure this is correct. So what protects shrinker invocation while the object is not initialized yet?

Quoted 7 lines · Michal Hocko

Quoted 7 lines

fs/super.c +0 -3

@@ -260,9 +261,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_shrink.count_objects = super_cache_count;
 	s->s_shrink.batch = 1024;
 	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
-	INIT_LIST_HEAD(&s->s_shrink.list);
-	return s;
-

Quoted 8 lines

fs/super.c static struct super_block *alloc_super(struct file_system_type *type, int flags, · line 268

Quoted 5 lines

fs/super.c +2 -0

+	if (register_shrinker(&s->s_shrink) == 0)
+		return s;
 fail:
 	destroy_unused_super(s);
 	return NULL;

But I am not sure this is correct. So what protects shrinker invocation while the object is not initialized yet?

Then, what protects shrinker invocation in your patch?

Quoted 17 lines · Tetsuo Handa

Quoted 15 lines · Michal Hocko

fs/super.c static struct super_block *alloc_super(struct file_system_type *type, int flags, · line 268

Quoted 12 lines

fs/super.c +2 -3

@@ -260,9 +261,8 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_shrink.count_objects = super_cache_count;
 	s->s_shrink.batch = 1024;
 	s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
-	INIT_LIST_HEAD(&s->s_shrink.list);
-	return s;
-
+	if (register_shrinker(&s->s_shrink) == 0)
+		return s;
 fail:
 	destroy_unused_super(s);
 	return NULL;

But I am not sure this is correct. So what protects shrinker invocation while the object is not initialized yet?

Quoted 1 line

Then, what protects shrinker invocation in your patch?

It is s_umount lock but that one is alreay held at the point where you suggested register_shrinker. My bad, I could have noticed that. Feel free to take over and send a patch. Considering I've screwed several times already I do not feel I am the right one to send the fix.

From f2a86a9dc45853149d4f29a5ecff77ec4c827b9f Mon Sep 17 00:00:00 2001

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 6 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

Quoted 8 lines

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Who is going to unregister that shrinker on other failure paths?

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 6 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

Quoted 9 lines

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Doesn't deactivate_locked_super() call unregister_shrinker() ?

Quoted 17 lines · Tetsuo Handa

Quoted 15 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 1 line

Doesn't deactivate_locked_super() call unregister_shrinker() ?

And? unregister_shrinker() will do list_del() on empty list_head and kfree(NULL); where's the problem with that?

Quoted 18 lines · Al Viro

Quoted 17 lines · Tetsuo Handa

Quoted 15 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 5 lines

Quoted 1 line

Doesn't deactivate_locked_super() call unregister_shrinker() ?

And? unregister_shrinker() will do list_del() on empty list_head and kfree(NULL); where's the problem with that?

The problem is that calling unregister_shrinker() without successful register_shrinker() causes crash due to s_shrink.list.{prev,next} == NULL.

Quoted 19 lines · Tetsuo Handa

Quoted 18 lines · Al Viro

Quoted 17 lines · Tetsuo Handa

Quoted 15 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 7 lines

Quoted 5 lines

Quoted 1 line

Doesn't deactivate_locked_super() call unregister_shrinker() ?

And? unregister_shrinker() will do list_del() on empty list_head and kfree(NULL); where's the problem with that?

The problem is that calling unregister_shrinker() without successful register_shrinker() causes crash due to s_shrink.list.{prev,next} == NULL.

*shrug*

        shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
        if (!shrinker->nr_deferred) {
		/* make sure it's in consistent state */
		INIT_LIST_HEAD(&shrinker->list);
                return -ENOMEM;
	}

Quoted 28 lines · Al Viro

Quoted 19 lines · Tetsuo Handa

Quoted 18 lines · Al Viro

Quoted 17 lines · Tetsuo Handa

Quoted 15 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 7 lines

Quoted 5 lines

Quoted 1 line

Doesn't deactivate_locked_super() call unregister_shrinker() ?

And? unregister_shrinker() will do list_del() on empty list_head and kfree(NULL); where's the problem with that?

The problem is that calling unregister_shrinker() without successful register_shrinker() causes crash due to s_shrink.list.{prev,next} == NULL.

Quoted 9 lines

*shrug*

        shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
        if (!shrinker->nr_deferred) {
		/* make sure it's in consistent state */
		INIT_LIST_HEAD(&shrinker->list);
                return -ENOMEM;
	}

Yes, that will work.

Michal, like Al thinks, making unregister_shrinker() no-op if register_shrinker() failed simplifies things. Can we go with
http://lkml.kernel.org/r/1511265853-15654-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
with patch description updated to include Syzbot report?

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 6 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

Quoted 8 lines

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Al, do you plan to push this fix? Tetsuo's unregister_shrinker fortification is already in the mmotm tree.

Quoted 16 lines · Michal Hocko

Quoted 14 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 2 lines

Al, do you plan to push this fix? Tetsuo's unregister_shrinker fortification is already in the mmotm tree.

Is it in any git branch I could pull from? Or I could just throw it into vfs.git#for-linus before the fix above - up to you, folks...

Quoted 17 lines · Al Viro

Quoted 16 lines · Michal Hocko

Quoted 14 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

Quoted 5 lines

Quoted 2 lines

Al, do you plan to push this fix? Tetsuo's unregister_shrinker fortification is already in the mmotm tree.

Is it in any git branch I could pull from? Or I could just throw it into vfs.git#for-linus before the fix above - up to you, folks...

Actually, looking at mmotm... I don't see it there. Which patch is it in?

Quoted 18 lines · Al Viro

Quoted 17 lines · Al Viro

Quoted 16 lines · Michal Hocko

Quoted 14 lines · Al Viro

Quoted 4 lines · Michal Hocko

Hopefully less screwed version. But as I've said I am not really familiar with the code and do not feel competent to change it so please be very careful here. I've moved the shrinker registration to alloc_super which turned out to be simpler.

I don't get it. Why the hell do we need all that PITA in the first place? Just make sget_userns() end with

	if (unlikely(regsiter_shrinker(&s->s_shrink) != 0)) {
		deactivate_locked_super(s);
		s = ERR_PTR(-ENOMEM);
	}
	return s;

and be done with that. All there is to it...

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Quoted 2 lines

Al, do you plan to push this fix? Tetsuo's unregister_shrinker fortification is already in the mmotm tree.

Is it in any git branch I could pull from? Or I could just throw it into vfs.git#for-linus before the fix above - up to you, folks...

Actually, looking at mmotm... I don't see it there. Which patch is it in?

My unregister_shrinker() fortification patch
( http://lkml.kernel.org/r/1511523385-6433-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp ) is not yet in the mmotm tree due to disagreement between Michal and I, but
you can throw your sget_userns() patch into vfs.git#for-linus anyway.
We will eventually apply unregister_shrinker() fortification patch.

Quoted 5 lines

My unregister_shrinker() fortification patch
( http://lkml.kernel.org/r/1511523385-6433-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp ) is not yet in the mmotm tree due to disagreement between Michal and I, but
you can throw your sget_userns() patch into vfs.git#for-linus anyway.
We will eventually apply unregister_shrinker() fortification patch.

I've acked the patch http://lkml.kernel.org/r/[email protected]
I disagreed with your must_check patch which has nothing to do with the
patch disussed here. I've also suggested some changelog clarifications. Please repost and I am pretty sure Andew will pick it up.

fs/super.c struct super_block *sget_userns(struct file_system_type *type, · line 510

Quoted 8 lines · Michal Hocko

fs/super.c +4 -0

@@ -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);

Have you even tested that? What the hell is that spin_unlock() doing there?