linux-kernel - [PATCH 3.12 19/83] tcp: Fix crash in TCP Fast Open

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Date:	Mon, 27 Apr 2015 23:31:50 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Ben Hutchings <ben@...adent.org.uk>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 19/83] tcp: Fix crash in TCP Fast Open

From: Ben Hutchings <ben@...adent.org.uk>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

Commit 355a901e6cf1 ("tcp: make connect() mem charging friendly")
changed tcp_send_syn_data() to perform an open-coded copy of the 'syn'
skb rather than using skb_copy_expand().

The open-coded copy does not cover the skb_shared_info::gso_segs
field, so in the new skb it is left set to 0.  When this commit was
backported into stable branches between 3.10.y and 3.16.7-ckty
inclusive, it triggered the BUG() in tcp_transmit_skb().

Since Linux 3.18 the GSO segment count is kept in the
tcp_skb_cb::tcp_gso_segs field and tcp_send_syn_data() does copy the
tcp_skb_cb structure to the new skb, so mainline and newer stable
branches are not affected.

Set skb_shared_info::gso_segs to the correct value of 1.

Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 net/ipv4/tcp_output.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e07ccba040be..56db79dbc633 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2919,6 +2919,7 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 		goto fallback;
 	syn_data->ip_summed = CHECKSUM_PARTIAL;
 	memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
+	skb_shinfo(syn_data)->gso_segs = 1;
 	if (unlikely(memcpy_fromiovecend(skb_put(syn_data, space),
 					 fo->data->msg_iov, 0, space))) {
 		kfree_skb(syn_data);
-- 
2.3.5

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