linux-kernel - Re: [PATCH] mm/slab_common: fix possiable double free of kmem

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Date:   Mon, 19 Sep 2022 11:12:38 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Feng Tang <feng.tang@...el.com>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com
Subject: Re: [PATCH] mm/slab_common: fix possiable double free of kmem_cache

On 9/19/22 05:12, Feng Tang wrote:
> When doing slub_debug test, kfence's 'test_memcache_typesafe_by_rcu'
> kunit test case cause a use-after-free error:
> 
>   BUG: KASAN: use-after-free in kobject_del+0x14/0x30
>   Read of size 8 at addr ffff888007679090 by task kunit_try_catch/261
> 
>   CPU: 1 PID: 261 Comm: kunit_try_catch Tainted: G    B            N 6.0.0-rc5-next-20220916 #17
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x34/0x48
>    print_address_description.constprop.0+0x87/0x2a5
>    print_report+0x103/0x1ed
>    kasan_report+0xb7/0x140
>    kobject_del+0x14/0x30
>    kmem_cache_destroy+0x130/0x170
>    test_exit+0x1a/0x30
>    kunit_try_run_case+0xad/0xc0
>    kunit_generic_run_threadfn_adapter+0x26/0x50
>    kthread+0x17b/0x1b0
>    </TASK>
> 
> The cause is inside kmem_cache_destroy():
> 
> kmem_cache_destroy
>     acquire lock/mutex
>     shutdown_cache
>         schedule_work(kmem_cache_release) (if RCU flag set)
>     release lock/mutex
>     kmem_cache_release (if RCU flag set)

				      ^ not set

I've fixed that up.

> 
> in some certain timing, the scheduled work could be run before
> the next RCU flag checking which will get a wrong state.
> 
> Fix it by caching the RCU flag inside protected area, just like 'refcnt'
> 
> Signed-off-by: Feng Tang <feng.tang@...el.com>

Thanks!

> ---
> 
> note:
> 
> The error only happens on linux-next tree, and not in Linus' tree,
> which already has Waiman's commit:
> 0495e337b703 ("mm/slab_common: Deleting kobject in kmem_cache_destroy()
> without holding slab_mutex/cpu_hotplug_lock")

Actually that commit is already in Linus' rc5 too, so I will send your fix
this week too. Added a Fixes: 0495e337b703 (...) too.

>  mm/slab_common.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 07b948288f84..ccc02573588f 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -475,6 +475,7 @@ void slab_kmem_cache_release(struct kmem_cache *s)
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
>  	int refcnt;
> +	bool rcu_set;
>  
>  	if (unlikely(!s) || !kasan_check_byte(s))
>  		return;
> @@ -482,6 +483,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	cpus_read_lock();
>  	mutex_lock(&slab_mutex);
>  
> +	rcu_set = s->flags & SLAB_TYPESAFE_BY_RCU;
> +
>  	refcnt = --s->refcount;
>  	if (refcnt)
>  		goto out_unlock;
> @@ -492,7 +495,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  out_unlock:
>  	mutex_unlock(&slab_mutex);
>  	cpus_read_unlock();
> -	if (!refcnt && !(s->flags & SLAB_TYPESAFE_BY_RCU))
> +	if (!refcnt && !rcu_set)
>  		kmem_cache_release(s);
>  }
>  EXPORT_SYMBOL(kmem_cache_destroy);