linux-kernel - Re: [PATCH] slub: fix slab_pad_check() and SLAB_DESTROY_BY

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Message-ID: <84144f020909022331x2b275aa5n428f88670e0ae8bc@mail.gmail.com>
Date:	Thu, 3 Sep 2009 09:31:01 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Zdenek Kabelac <zdenek.kabelac@...il.com>,
	Patrick McHardy <kaber@...sh.net>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Robin Holt <holt@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jesper Dangaard Brouer <hawk@...x.dk>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] slub: fix slab_pad_check() and SLAB_DESTROY_BY_RCU

On Thu, Sep 3, 2009 at 4:04 AM, Eric Dumazet<eric.dumazet@...il.com> wrote:
> Zdenek Kabelac a écrit :
>>
>> Well I'm not noticing any ill behavior - also note - rcu_barrier() is
>> there before the cache is destroyed.
>> But as I said - it's just my shot into the dark - which seems to work for me...
>>
>
> Reading again your traces, I do believe there are two bugs in slub
>
> Maybe not explaining your problem, but worth to fix !
>
> Thank you
>
> [PATCH] slub: fix slab_pad_check() and SLAB_DESTROY_BY_RCU
>
> When SLAB_POISON is used and slab_pad_check() finds an overwrite of the
> slab padding, we call restore_bytes() on the whole slab, not only
> on the padding.
>
> kmem_cache_destroy() should call rcu_barrier() *after* kmem_cache_close()
> and *before* sysfs_slab_remove() or risk rcu_free_slab()
> being called after kmem_cache is deleted (kfreed).
>
> rmmod nf_conntrack can crash the machine because it has to
> kmem_cache_destroy() a SLAB_DESTROY_BY_RCU enabled cache.
>
> Reported-by: Zdenek Kabelac <zdenek.kabelac@...il.com>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> ---
> diff --git a/mm/slub.c b/mm/slub.c
> index b9f1491..0ac839f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -646,7 +646,7 @@ static int slab_pad_check(struct kmem_cache *s, struct page *page)
>        slab_err(s, page, "Padding overwritten. 0x%p-0x%p", fault, end - 1);
>        print_section("Padding", end - remainder, remainder);
>
> -       restore_bytes(s, "slab padding", POISON_INUSE, start, end);
> +       restore_bytes(s, "slab padding", POISON_INUSE, end - remainder, end);

OK, makes sense.

>        return 0;
>  }
>
> @@ -2594,8 +2594,6 @@ static inline int kmem_cache_close(struct kmem_cache *s)
>  */
>  void kmem_cache_destroy(struct kmem_cache *s)
>  {
> -       if (s->flags & SLAB_DESTROY_BY_RCU)
> -               rcu_barrier();
>        down_write(&slub_lock);
>        s->refcount--;
>        if (!s->refcount) {
> @@ -2606,6 +2604,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
>                                "still has objects.\n", s->name, __func__);
>                        dump_stack();
>                }
> +               if (s->flags & SLAB_DESTROY_BY_RCU)
> +                       rcu_barrier();
>                sysfs_slab_remove(s);
>        } else
>                up_write(&slub_lock);

The rcu_barrier() call was added by this commit:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=7ed9f7e5db58c6e8c2b4b738a75d5dcd8e17aad5

I guess we should CC Paul as well.
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