[SRU][Xenial][Bionic][Cosmic][PATCH] xfrm: Fix NULL pointer dereference when skb_dst_force clears the dst_entry.

Gavin Guo gavin.guo at canonical.com
Tue Nov 6 14:46:34 UTC 2018

From: Steffen Klassert <steffen.klassert at secunet.com>

BugLink: https://bugs.launchpad.net/bugs/1801878

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 at genoetigt.de>
Reported-by: Kristian Evensen <kristian.evensen at gmail.com>
Reported-by: Wolfgang Walter <linux at stwm.de>
Signed-off-by: Steffen Klassert <steffen.klassert at secunet.com>
(cherry picked from commit 9e1437937807b0122e8da1ca8765be2adca9aee6)
Signed-off-by: Gavin Guo <gavin.guo at canonical.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 35610cc881a9..c47660fba498 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)
+		if (!skb_dst(skb)) {
+			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 7bcd6f1b86a0..1690efebba8a 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2544,6 +2544,10 @@ int __xfrm_route_forward(struct sk_buff *skb, unsigned short family)
+	if (!skb_dst(skb)) {
+		return 0;
+	}
 	dst = xfrm_lookup(net, skb_dst(skb), &fl, NULL, XFRM_LOOKUP_QUEUE);
 	if (IS_ERR(dst)) {

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