From: Patrick McHardy <kaber@trash•net>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: David Miller <davem@davemloft•net>,
netdev@vger•kernel.org, paulmck@linux•vnet.ibm.com
Subject: Re: [PATCH] net: nf_conntrack_alloc() should not use kmem_cache_zalloc()
Date: Wed, 15 Jul 2009 17:28:52 +0200 [thread overview]
Message-ID: <4A5DF5B4.5090809@trash.net> (raw)
In-Reply-To: <4A5DCB7C.9000502@gmail.com>
Eric Dumazet wrote:
> [PATCH] net: nf_conntrack_alloc() should not use kmem_cache_zalloc()
>
> 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.
I think this is still racy, please see below:
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 7508f11..23feafa 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -561,17 +561,28 @@ struct nf_conn *nf_conntrack_alloc(struct net *net,
> }
> }
>
> - ct = kmem_cache_zalloc(nf_conntrack_cachep, gfp);
> + /*
> + * Do not use kmem_cache_zalloc(), as this cache uses
> + * SLAB_DESTROY_BY_RCU.
> + */
> + ct = kmem_cache_alloc(nf_conntrack_cachep, gfp);
> if (ct == NULL) {
> pr_debug("nf_conntrack_alloc: Can't alloc conntrack.\n");
> atomic_dec(&net->ct.count);
> return ERR_PTR(-ENOMEM);
> }
> -
__nf_conntrack_find() on another CPU finds the entry at this point.
> + /*
> + * Let ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode.next
> + * and ct->tuplehash[IP_CT_DIR_REPLY].hnnode.next unchanged.
> + */
> + memset(&ct->tuplehash[IP_CT_DIR_MAX], 0,
> + sizeof(*ct) - offsetof(struct nf_conn, tuplehash[IP_CT_DIR_MAX]));
> spin_lock_init(&ct->lock);
> atomic_set(&ct->ct_general.use, 1);
nf_conntrack_find_get() successfully tries to atomic_inc_not_zero()
at this point, following by another tuple comparison which is also
successful.
Am I missing something? I think we need to make sure the reference
count is not increased until the new tuples are visible.
> ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple = *orig;
> + ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode.pprev = NULL;
> ct->tuplehash[IP_CT_DIR_REPLY].tuple = *repl;
> + ct->tuplehash[IP_CT_DIR_REPLY].hnnode.pprev = NULL;
> /* Don't set timer yet: wait for confirmation */
> setup_timer(&ct->timeout, death_by_timeout, (unsigned long)ct);
> #ifdef CONFIG_NET_NS
>
next prev parent reply other threads:[~2009-07-15 15:29 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-07-01 18:08 WARNING: at include/net/sock.h:417 udp_lib_unhash Tantilov, Emil S
2009-07-02 6:10 ` Eric Dumazet
2009-07-07 0:54 ` Emil S Tantilov
2009-07-07 7:21 ` Eric Dumazet
2009-07-07 7:40 ` Eric Dumazet
2009-07-07 16:14 ` [PATCH] net: sk_alloc() should not blindly overwrite memory Eric Dumazet
2009-07-07 18:33 ` Tantilov, Emil S
2009-07-07 22:33 ` [PATCH] net: sk_prot_alloc() " Eric Dumazet
2009-07-08 2:14 ` David Miller
2009-07-08 6:50 ` Eric Dumazet
2009-07-09 5:36 ` Eric Dumazet
2009-07-09 17:13 ` Paul E. McKenney
2009-07-09 20:50 ` Eric Dumazet
2009-07-12 3:27 ` David Miller
2009-07-12 7:07 ` Eric Dumazet
2009-07-15 12:28 ` [PATCH] net: nf_conntrack_alloc() should not use kmem_cache_zalloc() Eric Dumazet
2009-07-15 15:28 ` Patrick McHardy [this message]
2009-07-15 19:54 ` [PATCH] net: nf_conntrack_alloc() fixes Eric Dumazet
2009-07-16 9:13 ` [PATCH] net: sock_copy() fixes Eric Dumazet
2009-07-17 1:09 ` David Miller
2009-07-16 12:05 ` [PATCH] net: nf_conntrack_alloc() fixes Patrick McHardy
2009-07-08 17:02 ` [PATCH] net: sk_prot_alloc() should not blindly overwrite memory Tantilov, Emil S
2009-07-08 17:45 ` David Miller
2009-07-08 23:21 ` Eric Dumazet
2009-07-08 23:35 ` Tantilov, Emil S
2009-07-09 0:20 ` [PATCH] net: ip_push_pending_frames() fix Eric Dumazet
2009-07-09 14:32 ` Tantilov, Emil S
2009-07-09 14:38 ` Eric Dumazet
2009-07-12 3:27 ` David Miller
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