netdev - [PATCHv4 net 1/2] ipv4/icmp: fix rt dst dev null pointer dereference

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Message-Id: <20190822141949.29561-2-liuhangbin@gmail.com>
Date:   Thu, 22 Aug 2019 22:19:48 +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>,
        Julian Anastasov <ja@....bg>,
        Hangbin Liu <liuhangbin@...il.com>
Subject: [PATCHv4 net 1/2] ipv4/icmp: fix rt dst dev null pointer dereference

In __icmp_send() there is a possibility that the rt->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_ipv4
    - __gre6_xmit
      - ip6_tnl_xmit
        - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
    - icmp_send
      - net = dev_net(rt->dst.dev); <-- 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 reason we need rt->dst.dev is to get the net.
So we can just try get it from skb->dev when rt->dst.dev is NULL.

v4: Julian Anastasov remind skb->dev also could be NULL. We'd better
still use dst.dev and do a check to avoid crash.

v3: No changes.

v2: fix the issue in __icmp_send() instead of updating shared dst dev
in {ip_md, ip6}_tunnel_xmit.

Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
Signed-off-by: Hangbin Liu <liuhangbin@...il.com>
---
 net/ipv4/icmp.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 1510e951f451..001f03f76bc4 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -582,7 +582,13 @@ void __icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info,

 	if (!rt)
 		goto out;
-	net = dev_net(rt->dst.dev);
+
+	if (rt->dst.dev)
+		net = dev_net(rt->dst.dev);
+	else if (skb_in->dev)
+		net = dev_net(skb_in->dev);
+	else
+		goto out;

 	/*
 	 *	Find the original header. It is expected to be valid, of course.
-- 
2.19.2