netdev - [PATCH net] tcp: ensure skb->dev is NULL before leaving TCP stack

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Message-Id: <20200319194955.13742-1-edumazet@google.com>
Date:   Thu, 19 Mar 2020 12:49:55 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Martin Zaharinov <micron10@...il.com>,
        Florian Westphal <fw@...len.de>,
        Pablo Neira Ayuso <pablo@...filter.org>
Subject: [PATCH net] tcp: ensure skb->dev is NULL before leaving TCP stack

skb->rbnode is sharing three skb fields : next, prev, dev

When a packet is sent, TCP keeps the original skb (master)
in a rtx queue, which was converted to rbtree a while back.

__tcp_transmit_skb() is responsible to clone the master skb,
and add the TCP header to the clone before sending it
to network layer.

skb_clone() already clears skb->next and skb->prev, but copies
the master oskb->dev into the clone.

We need to clear skb->dev, otherwise lower layers could interpret
the value as a pointer to a netdev.

This old bug surfaced recently when commit 28f8bfd1ac94
("netfilter: Support iif matches in POSTROUTING") was merged.

Before this netfilter commit, skb->dev value was ignored and
changed before reaching dev_queue_xmit()

Fixes: 75c119afe14f ("tcp: implement rb-tree based retransmit queue")
Fixes: 28f8bfd1ac94 ("netfilter: Support iif matches in POSTROUTING")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Reported-by: Martin Zaharinov <micron10@...il.com>
Cc: Florian Westphal <fw@...len.de>
Cc: Pablo Neira Ayuso <pablo@...filter.org>
---
 net/ipv4/tcp_output.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 306e25d743e8de1bfe23d6e3b3a9fb0f23664912..e8cf8fde3d37dc3b455224d33d0bde0e585f989e 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1109,6 +1109,10 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 
 		if (unlikely(!skb))
 			return -ENOBUFS;
+		/* retransmit skbs might have a non zero value in skb->dev
+		 * because skb->dev is aliased with skb->rbnode.rb_left
+		 */
+		skb->dev = NULL;
 	}
 
 	inet = inet_sk(sk);
-- 
2.25.1.696.g5e7596f4ac-goog