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Commit 941297f4 authored by Eric Dumazet's avatar Eric Dumazet Committed by Patrick McHardy
Browse files

netfilter: nf_conntrack: 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.

Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: default avatarPatrick McHardy <kaber@trash.net>
parent aa6a03eb

Documentation/RCU/rculist_nulls.txt

+6 −1
Original line number Original line Diff line number Diff line
@@ -83,11 +83,12 @@ not detect it missed following items in original chain. 
obj = kmem_cache_alloc(...);

obj = kmem_cache_alloc(...);

lock_chain(); // typically a spin_lock()

lock_chain(); // typically a spin_lock()

obj->key = key;

obj->key = key;

atomic_inc(&obj->refcnt);

/*

/*

 * we need to make sure obj->key is updated before obj->next

 * we need to make sure obj->key is updated before obj->next

 * or obj->refcnt

 */

 */

smp_wmb();

smp_wmb();

atomic_set(&obj->refcnt, 1);

hlist_add_head_rcu(&obj->obj_node, list);

hlist_add_head_rcu(&obj->obj_node, list);

unlock_chain(); // typically a spin_unlock()

unlock_chain(); // typically a spin_unlock()
@@ -159,6 +160,10 @@ out: 
obj = kmem_cache_alloc(cachep);

obj = kmem_cache_alloc(cachep);

lock_chain(); // typically a spin_lock()

lock_chain(); // typically a spin_lock()

obj->key = key;

obj->key = key;

/*

 * changes to obj->key must be visible before refcnt one

 */

smp_wmb();

atomic_set(&obj->refcnt, 1);

atomic_set(&obj->refcnt, 1);

/*

/*

 * insert obj in RCU way (readers might be traversing chain)

 * insert obj in RCU way (readers might be traversing chain)

net/netfilter/nf_conntrack_core.c

+18 −3
Original line number Original line Diff line number Diff line
@@ -561,23 +561,38 @@ 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) {

	if (ct == NULL) {

		pr_debug("nf_conntrack_alloc: Can't alloc conntrack.\n");

		pr_debug("nf_conntrack_alloc: Can't alloc conntrack.\n");

		atomic_dec(&net->ct.count);

		atomic_dec(&net->ct.count);

		return ERR_PTR(-ENOMEM);

		return ERR_PTR(-ENOMEM);

	}

	}



	/*

	 * 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);

	spin_lock_init(&ct->lock);

	atomic_set(&ct->ct_general.use, 1);

	ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple = *orig;

	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].tuple = *repl;

	ct->tuplehash[IP_CT_DIR_REPLY].hnnode.pprev = NULL;

	/* Don't set timer yet: wait for confirmation */

	/* Don't set timer yet: wait for confirmation */

	setup_timer(&ct->timeout, death_by_timeout, (unsigned long)ct);

	setup_timer(&ct->timeout, death_by_timeout, (unsigned long)ct);

#ifdef CONFIG_NET_NS

#ifdef CONFIG_NET_NS

	ct->ct_net = net;

	ct->ct_net = net;

#endif

#endif





	/*

	 * changes to lookup keys must be done before setting refcnt to 1

	 */

	smp_wmb();

	atomic_set(&ct->ct_general.use, 1);

	return ct;

	return ct;

}

}

EXPORT_SYMBOL_GPL(nf_conntrack_alloc);

EXPORT_SYMBOL_GPL(nf_conntrack_alloc);