netdev - [PATCH 2/2] xfrm/xfrm_policy: fix dst dev null pointer dereference in collect

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Message-Id: <20190819075327.32412-3-liuhangbin@gmail.com>
Date:   Mon, 19 Aug 2019 15:53:27 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     netdev@...r.kernel.org
Cc:     Stefano Brivio <sbrivio@...hat.com>, wenxu <wenxu@...oud.cn>,
        Alexei Starovoitov <ast@...com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Hangbin Liu <liuhangbin@...il.com>
Subject: [PATCH 2/2] xfrm/xfrm_policy: fix dst dev null pointer dereference in collect_md mode

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.

Fixes: 8d79266bc48c ("ip6_tunnel: add collect_md mode to IPv6 tunnels")
Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
---
 net/xfrm/xfrm_policy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 8ca637a72697..ec94f5795ea4 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -3269,7 +3269,7 @@ decode_session4(struct sk_buff *skb, struct flowi *fl, bool reverse)
 	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));
@@ -3387,7 +3387,7 @@ decode_session6(struct sk_buff *skb, struct flowi *fl, bool reverse)
 
 	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));
-- 
2.19.2