netdev - Re: [PATCH] net: nf_conntrack

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Message-ID: <4A5F176E.7010508@trash.net>
Date:	Thu, 16 Jul 2009 14:05:02 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] net: nf_conntrack_alloc() fixes

Eric Dumazet wrote:
> [PATCH] net: nf_conntrack_alloc() fixes
> 
> When a slab cache uses SLAB_DESTROY_BY_RCU, we must be careful when allocating
> objects, since slab allocator could give a freed object still used by lockless
> readers.
> 
> In particular, nf_conntrack RCU lookups rely on ct->tuplehash[xxx].hnnode.next
> being always valid (ie containing a valid 'nulls' value, or a valid pointer to next
> object in hash chain.)
> 
> kmem_cache_zalloc() setups object with NULL values, but a NULL value is not valid
> for ct->tuplehash[xxx].hnnode.next.
> 
> Fix is to call kmem_cache_alloc() and do the zeroing ourself.
> 
> As spotted by Patrick, we also need to make sure lookup keys are committed to
> memory before setting refcount to 1, or a lockless reader could get a reference
> on the old version of the object. Its key re-check could then pass the barrier. 

Looks good to me. Applied, thanks Eric. I'll push it to -stable
with the other fixes in a couple of days.
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