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

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

[Eric Dumazet]

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>
---
 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

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

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Thu, 19 Mar 2020 12:49:55 -0700

> 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>

Applied and queued up for -stable, thanks Eric.