[PATCH net] tcp: fix potential double free issue for fastopen_req

[Wei Wang]

From: Wei Wang <wei...@google.com>

tp->fastopen_req could potentially be double freed if a malicious
user does the following:
1. Enable TCP_FASTOPEN_CONNECT sockopt and do a connect() on the socket.
2. Call connect() with AF_UNSPEC to disconnect the socket.
3. Make this socket a listening socket by calling listen().
4. Accept incoming connections and generate child sockets. All child
   sockets will get a copy of the pointer of fastopen_req.
5. Call close() on all sockets. fastopen_req will get freed multiple
   times.

Fixes: 19f6d3f3c842 ("net/tcp-fastopen: Add new API support")
Reported-by: Andrey Konovalov <andreyk...@google.com>
Signed-off-by: Wei Wang <wei...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
---
 net/ipv4/tcp.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index da385ae997a3..cf4555581282 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1110,9 +1110,14 @@ static int tcp_sendmsg_fastopen(struct sock *sk, struct 
msghdr *msg,
        flags = (msg->msg_flags & MSG_DONTWAIT) ? O_NONBLOCK : 0;
        err = __inet_stream_connect(sk->sk_socket, msg->msg_name,
                                    msg->msg_namelen, flags, 1);
-       inet->defer_connect = 0;
-       *copied = tp->fastopen_req->copied;
-       tcp_free_fastopen_req(tp);
+       /* fastopen_req could already be freed in __inet_stream_connect
+        * if the connection times out or gets rst
+        */
+       if (tp->fastopen_req) {
+               *copied = tp->fastopen_req->copied;
+               tcp_free_fastopen_req(tp);
+               inet->defer_connect = 0;
+       }
        return err;
 }
 
@@ -2318,6 +2323,10 @@ int tcp_disconnect(struct sock *sk, int flags)
        memset(&tp->rx_opt, 0, sizeof(tp->rx_opt));
        __sk_dst_reset(sk);
 
+       /* Clean up fastopen related fields */
+       tcp_free_fastopen_req(tp);
+       inet->defer_connect = 0;
+
        WARN_ON(inet->inet_num && !icsk->icsk_bind_hash);
 
        sk->sk_error_report(sk);
-- 
2.12.0.rc1.440.g5b76565f74-goog