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: Eric Dumazet <edumazet@google.com>
Reported-by: Dormando <dormando@rydia.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 097b3e7..54b6731 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -73,12 +73,7 @@
{
struct dst_entry *old_dst;
- if (dst) {
- if (dst->flags & DST_NOCACHE)
- dst = NULL;
- else
- dst_clone(dst);
- }
+ dst_clone(dst);
old_dst = xchg((__force struct dst_entry **)&idst->dst, dst);
dst_release(old_dst);
}
@@ -108,13 +103,14 @@
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;