[PATCH net] tcp: avoid infinite loop in tcp_splice_read()

[Eric Dumazet]

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

Splicing from TCP socket is vulnerable when a packet with URG flag is
received and stored into receive queue.

__tcp_splice_read() returns 0, and sk_wait_data() immediately
returns since there is the problematic skb in queue.

This is a nice way to burn cpu (aka infinite loop) and trigger
soft lockups.

Again, this gem was found by syzkaller tool.

Fixes: 9c55e01c0cc8 ("[TCP]: Splice receive support.")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: Dmitry Vyukov  <dvyu...@google.com>
Cc: Willy Tarreau <w...@1wt.eu>
---
 net/ipv4/tcp.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 
4a044964da6670829e5c47fef52d2cd76360b59f..0efb4c7f6704f662b6c762e48698a41564add2a4
 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -770,6 +770,12 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
                                ret = -EAGAIN;
                                break;
                        }
+                       /* if __tcp_splice_read() got nothing while we have
+                        * an skb in receive queue, we do not want to loop.
+                        * This might happen with URG data.
+                        */
+                       if (!skb_queue_empty(&sk->sk_receive_queue))
+                               break;
                        sk_wait_data(sk, &timeo, NULL);
                        if (signal_pending(current)) {
                                ret = sock_intr_errno(timeo);