xfrm: Fix NULL pointer dereference when skb_dst_force clears the dst_entry.

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-list@genoetigt.de>
Reported-by: Kristian Evensen <kristian.evensen@gmail.com>
Reported-by: Wolfgang Walter <linux@stwm.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

1 files changed, 4 insertions, 0 deletions

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);