[PATCH net] tcp: clear saved_syn in tcp_disconnect()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

In the (very unlikely) case a passive socket becomes a listener,
we do not want to duplicate its saved SYN headers.

This would lead to double frees, use after free, and please hackers and
various fuzzers 

Tested:
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 setsockopt(3, IPPROTO_TCP, TCP_SAVE_SYN, [1], 4) = 0
   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0

   +0 bind(3, ..., ...) = 0
   +0 listen(3, 5) = 0

   +0 < S 0:0(0) win 32972 <mss 1460,nop,wscale 7>
   +0 > S. 0:0(0) ack 1 <...>
  +.1 < . 1:1(0) ack 1 win 257
   +0 accept(3, ..., ...) = 4

   +0 connect(4, AF_UNSPEC, ...) = 0
   +0 close(3) = 0
   +0 bind(4, ..., ...) = 0
   +0 listen(4, 5) = 0

   +0 < S 0:0(0) win 32972 <mss 1460,nop,wscale 7>
   +0 > S. 0:0(0) ack 1 <...>
  +.1 < . 1:1(0) ack 1 win 257


Fixes: cd8ae85299d5 ("tcp: provide SYN headers for passive connections")
Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/ipv4/tcp.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 
94f0b5b50e0d728c3edab175aee9d769cd80907f..04843ae77b9ecacb3e4f2e81096f11d35ae1915e
 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2322,6 +2322,7 @@ int tcp_disconnect(struct sock *sk, int flags)
        tcp_init_send_head(sk);
        memset(&tp->rx_opt, 0, sizeof(tp->rx_opt));
        __sk_dst_reset(sk);
+       tcp_saved_syn_free(tp);
 
        /* Clean up fastopen related fields */
        tcp_free_fastopen_req(tp);