On Thu, Jul 23, 2009 at 04:15:34PM +0200, Patrick McHardy wrote:
> commit 2b12e9634d9676c55f576195a7f950956179d881
> Author: Patrick McHardy <kaber@xxxxxxxxx>
> Date: Thu Jul 23 16:02:18 2009 +0200
>
> netfilter: nf_conntrack: nf_conntrack_alloc() fixes
>
> Upstream commit 941297f4:
>
> 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.
The atomic_inc() works, but only if ->refcnt is set to zero on the
initial allocation. That said, this approach seems safer, so:
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Signed-off-by: Patrick McHardy <kaber@xxxxxxxxx>
>
> diff --git a/Documentation/RCU/rculist_nulls.txt b/Documentation/RCU/rculist_nulls.txt
> index 6389dec..d0c017e 100644
> --- a/Documentation/RCU/rculist_nulls.txt
> +++ b/Documentation/RCU/rculist_nulls.txt
> @@ -83,11 +83,12 @@ not detect it missed following items in original chain.
> obj = kmem_cache_alloc(...);
> lock_chain(); // typically a spin_lock()
> obj->key = key;
> -atomic_inc(&obj->refcnt);
> /*
> * we need to make sure obj->key is updated before obj->next
> + * or obj->refcnt
> */
> smp_wmb();
> +atomic_set(&obj->refcnt, 1);
> hlist_add_head_rcu(&obj->obj_node, list);
> unlock_chain(); // typically a spin_unlock()
>
> @@ -159,6 +160,10 @@ out:
> obj = kmem_cache_alloc(cachep);
> lock_chain(); // typically a spin_lock()
> obj->key = key;
> +/*
> + * changes to obj->key must be visible before refcnt one
> + */
> +smp_wmb();
> atomic_set(&obj->refcnt, 1);
> /*
> * insert obj in RCU way (readers might be traversing chain)
> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> index 2d3584f..0d961ee 100644
> --- a/net/netfilter/nf_conntrack_core.c
> +++ b/net/netfilter/nf_conntrack_core.c
> @@ -525,22 +525,37 @@ 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);
> }
> -
> - atomic_set(&ct->ct_general.use, 1);
> + /*
> + * 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]));
> 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
> ct->ct_net = net;
> #endif
>
> + /*
> + * changes to lookup keys must be done before setting refcnt to 1
> + */
> + smp_wmb();
> + atomic_set(&ct->ct_general.use, 1);
> return ct;
> }
> EXPORT_SYMBOL_GPL(nf_conntrack_alloc);
> --
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