[PATCH net 4/9] netfilter: nf_conntrack_sip: validate skb_dst() before accessing it

[Florian Westphal <fw@strlen.de>]

From: Pablo Neira Ayuso <pablo@netfilter.org>

tc ingress and openvswitch do not guarantee routing information to be
available. These subsystems use the conntrack helper infrastructure, and
the SIP helper relies on the skb_dst() to be present if
sip_external_media is set to 1 (which is disabled by default as a module
parameter).

This effectively disables the sip_external_media toggle for these
subsystems without resulting in a crash.

Fixes: cae3a2627520 ("openvswitch: Allow attaching helpers to ct action")
Fixes: b57dc7c13ea9 ("net/sched: Introduce action ct")
Cc: stable@vger.kernel.org
Reported-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/netfilter/nf_conntrack_sip.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index 5ec3a4a4bbd7..f3f90a866338 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -956,7 +956,6 @@ static int set_expected_rtp_rtcp(struct sk_buff *skb, unsigned int protoff,
 			return NF_ACCEPT;
 		saddr = &ct->tuplehash[!dir].tuple.src.u3;
 	} else if (sip_external_media) {
-		struct net_device *dev = skb_dst(skb)->dev;
 		struct dst_entry *dst = NULL;
 		struct flowi fl;
 
@@ -978,7 +977,11 @@ static int set_expected_rtp_rtcp(struct sk_buff *skb, unsigned int protoff,
 		 * through the same interface as the signalling peer.
 		 */
 		if (dst) {
-			bool external_media = (dst->dev == dev);
+			const struct dst_entry *this_dst = skb_dst(skb);
+			bool external_media = false;
+
+			if (this_dst && dst->dev == this_dst->dev)
+				external_media = true;
 
 			dst_release(dst);
 			if (external_media)
-- 
2.53.0