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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Tobias Hommel <netdev-list@genoetigt.de>,
        Kristian Evensen <kristian.evensen@gmail.com>,
        Wolfgang Walter <linux@stwm.de>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.18 011/150] xfrm: Fix NULL pointer dereference when skb_dst_force clears the dst_entry.
Date:   Fri,  2 Nov 2018 19:32:53 +0100
Message-Id: <20181102182903.432633549@linuxfoundation.org>
In-Reply-To: <20181102182902.250560510@linuxfoundation.org>
References: <20181102182902.250560510@linuxfoundation.org>
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4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

[ Upstream commit 9e1437937807b0122e8da1ca8765be2adca9aee6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 a94983e03a8b..526e6814ed4b 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2551,6 +2551,10 @@ 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