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

From: Jiri Slaby
Date: Mon Apr 27 2015 - 17:52:18 EST

From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
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))) {

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