From: Eric Dumazet <edumazet@google.com>
Date: Thu, 19 Mar 2020 12:49:55 -0700
Subject: tcp: ensure skb->dev is NULL before leaving TCP stack
Patch-mainline: v5.6
Git-commit: b738a185beaab8728943acdb3e67371b8a88185e
References: bsc#1176447

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@google.com>
Reported-by: Martin Zaharinov <micron10@gmail.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/ipv4/tcp_output.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1048,6 +1048,10 @@ static int __tcp_transmit_skb(struct soc
 
 		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);