[PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Qian Cai]

Long time ago, there fixed a similar deadlock in show_slab_objects()
[1]. However, it is apparently due to the commits like 01fb58bcba63
("slab: remove synchronous synchronize_sched() from memcg cache
deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
just reading files in /sys/kernel/slab which will generate a lockdep
splat below.

Since the "mem_hotplug_lock" here is only to obtain a stable online node
mask while racing with NUMA node hotplug, in the worst case, the results
may me miscalculated while doing NUMA node hotplug, but they shall be
corrected by later reads of the same files.

WARNING: possible circular locking dependency detected
------------------------------------------------------
cat/5224 is trying to acquire lock:
ffff900012ac3120 (mem_hotplug_lock.rw_sem){++++}, at:
show_slab_objects+0x94/0x3a8

but task is already holding lock:
b8ff009693eee398 (kn->count#45){++++}, at: kernfs_seq_start+0x44/0xf0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (kn->count#45){++++}:
       lock_acquire+0x31c/0x360
       __kernfs_remove+0x290/0x490
       kernfs_remove+0x30/0x44
       sysfs_remove_dir+0x70/0x88
       kobject_del+0x50/0xb0
       sysfs_slab_unlink+0x2c/0x38
       shutdown_cache+0xa0/0xf0
       kmemcg_cache_shutdown_fn+0x1c/0x34
       kmemcg_workfn+0x44/0x64
       process_one_work+0x4f4/0x950
       worker_thread+0x390/0x4bc
       kthread+0x1cc/0x1e8
       ret_from_fork+0x10/0x18

-> #1 (slab_mutex){+.+.}:
       lock_acquire+0x31c/0x360
       __mutex_lock_common+0x16c/0xf78
       mutex_lock_nested+0x40/0x50
       memcg_create_kmem_cache+0x38/0x16c
       memcg_kmem_cache_create_func+0x3c/0x70
       process_one_work+0x4f4/0x950
       worker_thread+0x390/0x4bc
       kthread+0x1cc/0x1e8
       ret_from_fork+0x10/0x18

-> #0 (mem_hotplug_lock.rw_sem){++++}:
       validate_chain+0xd10/0x2bcc
       __lock_acquire+0x7f4/0xb8c
       lock_acquire+0x31c/0x360
       get_online_mems+0x54/0x150
       show_slab_objects+0x94/0x3a8
       total_objects_show+0x28/0x34
       slab_attr_show+0x38/0x54
       sysfs_kf_seq_show+0x198/0x2d4
       kernfs_seq_show+0xa4/0xcc
       seq_read+0x30c/0x8a8
       kernfs_fop_read+0xa8/0x314
       __vfs_read+0x88/0x20c
       vfs_read+0xd8/0x10c
       ksys_read+0xb0/0x120
       __arm64_sys_read+0x54/0x88
       el0_svc_handler+0x170/0x240
       el0_svc+0x8/0xc

other info that might help us debug this:

Chain exists of:
  mem_hotplug_lock.rw_sem --> slab_mutex --> kn->count#45

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(kn->count#45);
                               lock(slab_mutex);
                               lock(kn->count#45);
  lock(mem_hotplug_lock.rw_sem);

 *** DEADLOCK ***

3 locks held by cat/5224:
 #0: 9eff00095b14b2a0 (&p->lock){+.+.}, at: seq_read+0x4c/0x8a8
 #1: 0eff008997041480 (&of->mutex){+.+.}, at: kernfs_seq_start+0x34/0xf0
 #2: b8ff009693eee398 (kn->count#45){++++}, at:
kernfs_seq_start+0x44/0xf0

stack backtrace:
Call trace:
 dump_backtrace+0x0/0x248
 show_stack+0x20/0x2c
 dump_stack+0xd0/0x140
 print_circular_bug+0x368/0x380
 check_noncircular+0x248/0x250
 validate_chain+0xd10/0x2bcc
 __lock_acquire+0x7f4/0xb8c
 lock_acquire+0x31c/0x360
 get_online_mems+0x54/0x150
 show_slab_objects+0x94/0x3a8
 total_objects_show+0x28/0x34
 slab_attr_show+0x38/0x54
 sysfs_kf_seq_show+0x198/0x2d4
 kernfs_seq_show+0xa4/0xcc
 seq_read+0x30c/0x8a8
 kernfs_fop_read+0xa8/0x314
 __vfs_read+0x88/0x20c
 vfs_read+0xd8/0x10c
 ksys_read+0xb0/0x120
 __arm64_sys_read+0x54/0x88
 el0_svc_handler+0x170/0x240
 el0_svc+0x8/0xc

[1] http://lkml.iu.edu/hypermail/linux/kernel/1101.0/02850.html

Fixes: 01fb58bcba63 ("slab: remove synchronous synchronize_sched() from memcg cache deactivation path")
Fixes: 03afc0e25f7f ("slab: get_online_mems for kmem_cache_{create,destroy,shrink}")
Cc: stable@vger.kernel.org
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Qian Cai <cai@lca.pw>
---

v2: fix the comment alignment and improve the changelog.

 mm/slub.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 42c1b3af3c98..86bfd9d98af5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4838,7 +4838,13 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 		}
 	}
 
-	get_online_mems();
+	/*
+	 * It is impossible to take "mem_hotplug_lock" here with "kernfs_mutex"
+	 * already held which will conflict with an existing lock order:
+	 *
+	 * mem_hotplug_lock->slab_mutex->kernfs_mutex
+	 */
+
 #ifdef CONFIG_SLUB_DEBUG
 	if (flags & SO_ALL) {
 		struct kmem_cache_node *n;
@@ -4879,7 +4885,6 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
 			x += sprintf(buf + x, " N%d=%lu",
 					node, nodes[node]);
 #endif
-	put_online_mems();
 	kfree(nodes);
 	return x + sprintf(buf + x, "\n");
 }
-- 
1.8.3.1

Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Michal Hocko]

On Fri 04-10-19 08:31:49, Qian Cai wrote:
> Long time ago, there fixed a similar deadlock in show_slab_objects()
> [1]. However, it is apparently due to the commits like 01fb58bcba63
> ("slab: remove synchronous synchronize_sched() from memcg cache
> deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> just reading files in /sys/kernel/slab which will generate a lockdep
> splat below.
> 
> Since the "mem_hotplug_lock" here is only to obtain a stable online node
> mask while racing with NUMA node hotplug, in the worst case, the results
> may me miscalculated while doing NUMA node hotplug, but they shall be
> corrected by later reads of the same files.

I think it is important to mention that this doesn't expose the
show_slab_objects to use-after-free. There is only a single path that
might really race here and that is the slab hotplug notifier callback
__kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
doesn't really destroy kmem_cache_node data structures.

Thanks!
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Michal Hocko]

On Fri 04-10-19 14:57:01, Michal Hocko wrote:
> On Fri 04-10-19 08:31:49, Qian Cai wrote:
> > Long time ago, there fixed a similar deadlock in show_slab_objects()
> > [1]. However, it is apparently due to the commits like 01fb58bcba63
> > ("slab: remove synchronous synchronize_sched() from memcg cache
> > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> > just reading files in /sys/kernel/slab which will generate a lockdep
> > splat below.
> > 
> > Since the "mem_hotplug_lock" here is only to obtain a stable online node
> > mask while racing with NUMA node hotplug, in the worst case, the results
> > may me miscalculated while doing NUMA node hotplug, but they shall be
> > corrected by later reads of the same files.
> 
> I think it is important to mention that this doesn't expose the
> show_slab_objects to use-after-free. There is only a single path that
> might really race here and that is the slab hotplug notifier callback
> __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
> doesn't really destroy kmem_cache_node data structures.

Andrew, please add this to the changelog so that we do not have to
scratch heads again when looking into that code.

Thanks!
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Andrew Morton]

On Mon, 7 Oct 2019 10:16:21 +0200 Michal Hocko <mhocko@kernel.org> wrote:

> On Fri 04-10-19 14:57:01, Michal Hocko wrote:
> > On Fri 04-10-19 08:31:49, Qian Cai wrote:
> > > Long time ago, there fixed a similar deadlock in show_slab_objects()
> > > [1]. However, it is apparently due to the commits like 01fb58bcba63
> > > ("slab: remove synchronous synchronize_sched() from memcg cache
> > > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> > > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> > > just reading files in /sys/kernel/slab which will generate a lockdep
> > > splat below.
> > > 
> > > Since the "mem_hotplug_lock" here is only to obtain a stable online node
> > > mask while racing with NUMA node hotplug, in the worst case, the results
> > > may me miscalculated while doing NUMA node hotplug, but they shall be
> > > corrected by later reads of the same files.
> > 
> > I think it is important to mention that this doesn't expose the
> > show_slab_objects to use-after-free. There is only a single path that
> > might really race here and that is the slab hotplug notifier callback
> > __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
> > doesn't really destroy kmem_cache_node data structures.

Yes, I noted this during review.  It's a bit subtle and is worthy of
more than a changelog note, I think.  How about this?

--- a/mm/slub.c~mm-slub-fix-a-deadlock-in-show_slab_objects-fix
+++ a/mm/slub.c
@@ -4851,6 +4851,10 @@ static ssize_t show_slab_objects(struct
 	 * already held which will conflict with an existing lock order:
 	 *
 	 * mem_hotplug_lock->slab_mutex->kernfs_mutex
+	 *
+	 * We don't really need mem_hotplug_lock (to hold off
+	 * slab_mem_going_offline_callback()) here because slab's memory hot
+	 * unplug code doesn't destroy the kmem_cache->node[] data.
 	 */
 
 #ifdef CONFIG_SLUB_DEBUG
_

> Andrew, please add this to the changelog so that we do not have to
> scratch heads again when looking into that code.

I did that as well.

Re: [PATCH v2] mm/slub: fix a deadlock in show_slab_objects()

[Michal Hocko]

On Mon 07-10-19 14:59:02, Andrew Morton wrote:
> On Mon, 7 Oct 2019 10:16:21 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Fri 04-10-19 14:57:01, Michal Hocko wrote:
> > > On Fri 04-10-19 08:31:49, Qian Cai wrote:
> > > > Long time ago, there fixed a similar deadlock in show_slab_objects()
> > > > [1]. However, it is apparently due to the commits like 01fb58bcba63
> > > > ("slab: remove synchronous synchronize_sched() from memcg cache
> > > > deactivation path") and 03afc0e25f7f ("slab: get_online_mems for
> > > > kmem_cache_{create,destroy,shrink}"), this kind of deadlock is back by
> > > > just reading files in /sys/kernel/slab which will generate a lockdep
> > > > splat below.
> > > > 
> > > > Since the "mem_hotplug_lock" here is only to obtain a stable online node
> > > > mask while racing with NUMA node hotplug, in the worst case, the results
> > > > may me miscalculated while doing NUMA node hotplug, but they shall be
> > > > corrected by later reads of the same files.
> > > 
> > > I think it is important to mention that this doesn't expose the
> > > show_slab_objects to use-after-free. There is only a single path that
> > > might really race here and that is the slab hotplug notifier callback
> > > __kmem_cache_shrink (via slab_mem_going_offline_callback) but that path
> > > doesn't really destroy kmem_cache_node data structures.
> 
> Yes, I noted this during review.  It's a bit subtle and is worthy of
> more than a changelog note, I think.  How about this?
> 
> --- a/mm/slub.c~mm-slub-fix-a-deadlock-in-show_slab_objects-fix
> +++ a/mm/slub.c
> @@ -4851,6 +4851,10 @@ static ssize_t show_slab_objects(struct
>  	 * already held which will conflict with an existing lock order:
>  	 *
>  	 * mem_hotplug_lock->slab_mutex->kernfs_mutex
> +	 *
> +	 * We don't really need mem_hotplug_lock (to hold off
> +	 * slab_mem_going_offline_callback()) here because slab's memory hot
> +	 * unplug code doesn't destroy the kmem_cache->node[] data.
>  	 */

Yes please! 

>  #ifdef CONFIG_SLUB_DEBUG
> _
> 
> > Andrew, please add this to the changelog so that we do not have to
> > scratch heads again when looking into that code.
> 
> I did that as well.

Thanks!
-- 
Michal Hocko
SUSE Labs