[PATCH 5.2 058/143] xfrm/xfrm_policy: fix dst dev null pointer dereference in collect_md mode

[Greg Kroah-Hartman]

From: Hangbin Liu <liuhangbin@xxxxxxxxx>

[ Upstream commit c3b4c3a47e05d5fecf7354d75824a9d1b37f3e84 ]

In decode_session{4,6} there is a possibility that the skb dst dev is NULL,
e,g, with tunnel collect_md mode, which will cause kernel crash.
Here is what the code path looks like, for GRE:

- ip6gre_tunnel_xmit
  - ip6gre_xmit_ipv6
    - __gre6_xmit
      - ip6_tnl_xmit
        - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
    - icmpv6_send
      - icmpv6_route_lookup
        - xfrm_decode_session_reverse
          - decode_session4
            - oif = skb_dst(skb)->dev->ifindex; <-- here
          - decode_session6
            - oif = skb_dst(skb)->dev->ifindex; <-- here

The reason is __metadata_dst_init() init dst->dev to NULL by default.
We could not fix it in __metadata_dst_init() as there is no dev supplied.
On the other hand, the skb_dst(skb)->dev is actually not needed as we
called decode_session{4,6} via xfrm_decode_session_reverse(), so oif is not
used by: fl4->flowi4_oif = reverse ? skb->skb_iif : oif;

So make a dst dev check here should be clean and safe.

v4: No changes.

v3: No changes.

v2: fix the issue in decode_session{4,6} instead of updating shared dst dev
in {ip_md, ip6}_tunnel_xmit.

Fixes: 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
Signed-off-by: Hangbin Liu <liuhangbin@xxxxxxxxx>
Tested-by: Jonathan Lemon <jonathan.lemon@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/xfrm/xfrm_policy.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -3272,7 +3272,7 @@ decode_session4(struct sk_buff *skb, str
 	struct flowi4 *fl4 = &fl->u.ip4;
 	int oif = 0;
 
-	if (skb_dst(skb))
+	if (skb_dst(skb) && skb_dst(skb)->dev)
 		oif = skb_dst(skb)->dev->ifindex;
 
 	memset(fl4, 0, sizeof(struct flowi4));
@@ -3390,7 +3390,7 @@ decode_session6(struct sk_buff *skb, str
 
 	nexthdr = nh[nhoff];
 
-	if (skb_dst(skb))
+	if (skb_dst(skb) && skb_dst(skb)->dev)
 		oif = skb_dst(skb)->dev->ifindex;
 
 	memset(fl6, 0, sizeof(struct flowi6));