[PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[Thomas Gleixner]

memcg_propagate_slab_attrs() abuses the sysfs attribute file functions to
propagate settings from the root kmem_cache to a newly created
kmem_cache. It does that with:

     attr->show(root, buf);
     attr->store(new, buf, strlen(bug);

Aside of being a lazy and absurd hackery this is broken because it does not
check the return value of the show() function.

Some of the show() functions return 0 w/o touching the buffer. That means in
such a case the store function is called with the stale content of the
previous show(). That causes nonsense like invoking kmem_cache_shrink() on
a newly created kmem_cache. In the worst case it would cause handing in an
uninitialized buffer.

This should be rewritten proper by adding a propagate() callback to those
slub_attributes which must be propagated and avoid that insane conversion
to and from ASCII, but that's too large for a hot fix.

Check at least the return value of the show() function, so calling store()
with stale content is prevented.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org

---
 mm/slub.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5512,6 +5512,7 @@ static void memcg_propagate_slab_attrs(s
 		char mbuf[64];
 		char *buf;
 		struct slab_attribute *attr = to_slab_attr(slab_attrs[i]);
+		ssize_t len;
 
 		if (!attr || !attr->store || !attr->show)
 			continue;
@@ -5536,8 +5537,9 @@ static void memcg_propagate_slab_attrs(s
 			buf = buffer;
 		}
 
-		attr->show(root_cache, buf);
-		attr->store(s, buf, strlen(buf));
+		len = attr->show(root_cache, buf);
+		if (len > 0)
+			attr->store(s, buf, len);
 	}
 
 	if (buffer)

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Re: [PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[Christoph Hellwig]

On Sat, May 20, 2017 at 12:52:03PM +0200, Thomas Gleixner wrote:
> This should be rewritten proper by adding a propagate() callback to those
> slub_attributes which must be propagated and avoid that insane conversion
> to and from ASCII

Exactly..

>, but that's too large for a hot fix.

What made this such a hot fix?  Looks like this crap has been in
for quite a while.

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Re: [PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[Steven Rostedt]

On Sat, 20 May 2017 06:16:45 -0700
Christoph Hellwig <hch@infradead.org> wrote:

> On Sat, May 20, 2017 at 12:52:03PM +0200, Thomas Gleixner wrote:
> > This should be rewritten proper by adding a propagate() callback to those
> > slub_attributes which must be propagated and avoid that insane conversion
> > to and from ASCII
> 
> Exactly..
> 
> >, but that's too large for a hot fix.
> 
> What made this such a hot fix?  Looks like this crap has been in
> for quite a while.

It can cause a deadlock with get_online_cpus() that has been uncovered
by recent cpu hotplug and lockdep changes that Thomas and Peter have
been doing.

[  102.567308]  Possible unsafe locking scenario:
[  102.567308] 
[  102.574846]        CPU0                    CPU1
[  102.580148]        ----                    ----
[  102.585421]   lock(cpu_hotplug.lock);
[  102.589808]                                lock(slab_mutex);
[  102.596166]                                lock(cpu_hotplug.lock);
[  102.603028]   lock(slab_mutex);
[  102.606846] 
[  102.606846]  *** DEADLOCK ***

-- Steve

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Re: [PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[Thomas Gleixner]

On Sat, 20 May 2017, Christoph Hellwig wrote:

> On Sat, May 20, 2017 at 12:52:03PM +0200, Thomas Gleixner wrote:
> > This should be rewritten proper by adding a propagate() callback to those
> > slub_attributes which must be propagated and avoid that insane conversion
> > to and from ASCII
> 
> Exactly..
> 
> >, but that's too large for a hot fix.
> 
> What made this such a hot fix?  Looks like this crap has been in
> for quite a while.
 
Well, having something in tree which uses stale or uninitialized buffers
does justify a hot fix which can be easily backported to stable.

Thanks,

	tglx

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Re: [PATCH] slub/memcg: Cure the brainless abuse of sysfs attributes

[David Rientjes]

On Sat, 20 May 2017, Thomas Gleixner wrote:

> memcg_propagate_slab_attrs() abuses the sysfs attribute file functions to
> propagate settings from the root kmem_cache to a newly created
> kmem_cache. It does that with:
> 
>      attr->show(root, buf);
>      attr->store(new, buf, strlen(bug);
> 
> Aside of being a lazy and absurd hackery this is broken because it does not
> check the return value of the show() function.
> 
> Some of the show() functions return 0 w/o touching the buffer. That means in
> such a case the store function is called with the stale content of the
> previous show(). That causes nonsense like invoking kmem_cache_shrink() on
> a newly created kmem_cache. In the worst case it would cause handing in an
> uninitialized buffer.
> 
> This should be rewritten proper by adding a propagate() callback to those
> slub_attributes which must be propagated and avoid that insane conversion
> to and from ASCII, but that's too large for a hot fix.
> 
> Check at least the return value of the show() function, so calling store()
> with stale content is prevented.
> 
> Reported-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: stable@vger.kernel.org

Acked-by: David Rientjes <rientjes@google.com>

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