[PATCH 5/6] xfrm: Fix NULL pointer dereference when skb_dst_force clears the dst_entry.

[Steffen Klassert]

Since commit 222d7dbd258d ("net: prevent dst uses after free")
skb_dst_force() might clear the dst_entry attached to the skb.
The xfrm code don't expect this to happen, so we crash with
a NULL pointer dereference in this case. Fix it by checking
skb_dst(skb) for NULL after skb_dst_force() and drop the packet
in cast the dst_entry was cleared.

Fixes: 222d7dbd258d ("net: prevent dst uses after free")
Reported-by: Tobias Hommel <netdev-l...@genoetigt.de>
Reported-by: Kristian Evensen <kristian.even...@gmail.com>
Reported-by: Wolfgang Walter <li...@stwm.de>
Signed-off-by: Steffen Klassert <steffen.klass...@secunet.com>
---
 net/xfrm/xfrm_output.c | 4 ++++
 net/xfrm/xfrm_policy.c | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/net/xfrm/xfrm_output.c b/net/xfrm/xfrm_output.c
index 89b178a78dc7..36d15a38ce5e 100644
--- a/net/xfrm/xfrm_output.c
+++ b/net/xfrm/xfrm_output.c
@@ -101,6 +101,10 @@ static int xfrm_output_one(struct sk_buff *skb, int err)
                spin_unlock_bh(&x->lock);
 
                skb_dst_force(skb);
+               if (!skb_dst(skb)) {
+                       XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTERROR);
+                       goto error_nolock;
+               }
 
                if (xfrm_offload(skb)) {
                        x->type_offload->encap(x, skb);
diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 7c5e8978aeaa..626e0f4d1749 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2548,6 +2548,10 @@ int __xfrm_route_forward(struct sk_buff *skb, unsigned 
short family)
        }
 
        skb_dst_force(skb);
+       if (!skb_dst(skb)) {
+               XFRM_INC_STATS(net, LINUX_MIB_XFRMFWDHDRERROR);
+               return 0;
+       }
 
        dst = xfrm_lookup(net, skb_dst(skb), &fl, NULL, XFRM_LOOKUP_QUEUE);
        if (IS_ERR(dst)) {
-- 
2.17.1