xfrm: Fix null pointer dereference when decoding sessions

On some codepaths the skb does not have a dst entry
when xfrm_decode_session() is called. So check for
a valid skb_dst() before dereferencing the device
interface index. We use 0 as the device index if
there is no valid skb_dst(), or at reverse decoding
we use skb_iif as device interface index.

Bug was introduced with git commit bafd4bd4dc
("xfrm: Decode sessions with output interface.").

Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index dd503a3535ff..5f8e128c512d 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -135,10 +135,14 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
         struct ipv6_opt_hdr *exthdr;
         const unsigned char *nh = skb_network_header(skb);
         u8 nexthdr = nh[IP6CB(skb)->nhoff];
+        int oif = 0;
+
+        if (skb_dst(skb))
+                oif = skb_dst(skb)->dev->ifindex;
 
         memset(fl6, 0, sizeof(struct flowi6));
         fl6->flowi6_mark = skb->mark;
-        fl6->flowi6_oif = skb_dst(skb)->dev->ifindex;
+        fl6->flowi6_oif = reverse ? skb->skb_iif : oif;
 
         fl6->daddr = reverse ? hdr->saddr : hdr->daddr;
         fl6->saddr = reverse ? hdr->daddr : hdr->saddr;