Date: Tue, 9 Jun 2015 09:38:27 +0900
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] zsmalloc: fix a null pointer dereference in
 destroy_handle_cache()
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 <20150608135532.ac913746b6394217e92a229a@linux-foundation.org>
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On Mon, Jun 08, 2015 at 01:55:32PM -0700, Andrew Morton wrote:
> On Fri,  5 Jun 2015 20:11:30 +0900 Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:
> 
> > zs_destroy_pool()->destroy_handle_cache() invoked from
> > zs_create_pool() can pass a NULL ->handle_cachep pointer
> > to kmem_cache_destroy(), which will dereference it.
> >
> 
> That's slightly lacking in details (under what circumstances will it
> crash) so I changed it to
> 
> : If zs_create_pool()->create_handle_cache()->kmem_cache_create() fails,
> : zs_create_pool()->destroy_handle_cache() will dereference the NULL
> : pool->handle_cachep.
> :
> : Modify destroy_handle_cache() to avoid this.
> 
> 
> > ...
> >
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -285,7 +285,8 @@ static int create_handle_cache(struct zs_pool *pool)
> >  
> >  static void destroy_handle_cache(struct zs_pool *pool)
> >  {
> > -	kmem_cache_destroy(pool->handle_cachep);
> > +	if (pool->handle_cachep)
> > +		kmem_cache_destroy(pool->handle_cachep);
> >  }
> >  
> >  static unsigned long alloc_handle(struct zs_pool *pool)
> 
> I'll apply this, but...  from a bit of grepping I'm estimating that we
> have approximately 200 instances of
> 
> 	if (foo)
> 		kmem_cache_destroy(foo);
> 
> so obviously kmem_cache_destroy() should be doing the check.

Hello, Andrew.

I'm not sure if doing the check in kmem_cache_destroy() is better.
My quick grep for other pool based allocators(ex. mempool, zpool) also
says that they don't check whether passed pool pointer is NULL or not
in destroy function. I think that it's general convention that proper
pool pointer should be passed to pool based function APIs.

Thanks.
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