Commit f8864972 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

ipv4: fix dst race in sk_dst_get() 



When IP route cache had been removed in linux-3.6, we broke assumption
that dst entries were all freed after rcu grace period. DST_NOCACHE
dst were supposed to be freed from dst_release(). But it appears
we want to keep such dst around, either in UDP sockets or tunnels.

In sk_dst_get() we need to make sure dst refcount is not 0
before incrementing it, or else we might end up freeing a dst
twice.

DST_NOCACHE set on a dst does not mean this dst can not be attached
to a socket or a tunnel.

Then, before actual freeing, we need to observe a rcu grace period
to make sure all other cpus can catch the fact the dst is no longer
usable.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reported-by: default avatarDormando <dormando@rydia.net>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 99e72a0f

include/net/sock.h

+2 −2
Original line number Diff line number Diff line
@@ -1730,8 +1730,8 @@ sk_dst_get(struct sock *sk) 


	rcu_read_lock();

	dst = rcu_dereference(sk->sk_dst_cache);

	if (dst)

		dst_hold(dst);

	if (dst && !atomic_inc_not_zero(&dst->__refcnt))

		dst = NULL;

	rcu_read_unlock();

	return dst;

}

net/core/dst.c

+11 −5
Original line number Diff line number Diff line
@@ -269,6 +269,15 @@ again: 
}

EXPORT_SYMBOL(dst_destroy);



static void dst_destroy_rcu(struct rcu_head *head)

{

	struct dst_entry *dst = container_of(head, struct dst_entry, rcu_head);



	dst = dst_destroy(dst);

	if (dst)

		__dst_free(dst);

}



void dst_release(struct dst_entry *dst)

{

	if (dst) {
@@ -276,11 +285,8 @@ void dst_release(struct dst_entry *dst) 


		newrefcnt = atomic_dec_return(&dst->__refcnt);

		WARN_ON(newrefcnt < 0);

		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {

			dst = dst_destroy(dst);

			if (dst)

				__dst_free(dst);

		}

		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)

			call_rcu(&dst->rcu_head, dst_destroy_rcu);

	}

}

EXPORT_SYMBOL(dst_release);

net/ipv4/ip_tunnel.c

+5 −9
Original line number Diff line number Diff line
@@ -73,12 +73,7 @@ static void __tunnel_dst_set(struct ip_tunnel_dst *idst, 
{

	struct dst_entry *old_dst;



	if (dst) {

		if (dst->flags & DST_NOCACHE)

			dst = NULL;

		else

	dst_clone(dst);

	}

	old_dst = xchg((__force struct dst_entry **)&idst->dst, dst);

	dst_release(old_dst);

}
@@ -108,13 +103,14 @@ static struct rtable *tunnel_rtable_get(struct ip_tunnel *t, u32 cookie) 


	rcu_read_lock();

	dst = rcu_dereference(this_cpu_ptr(t->dst_cache)->dst);

	if (dst && !atomic_inc_not_zero(&dst->__refcnt))

		dst = NULL;

	if (dst) {

		if (dst->obsolete && dst->ops->check(dst, cookie) == NULL) {

			rcu_read_unlock();

			tunnel_dst_reset(t);

			return NULL;

			dst_release(dst);

			dst = NULL;

		}

		dst_hold(dst);

	}

	rcu_read_unlock();

	return (struct rtable *)dst;

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