tcp: Fix for race due to temporary drop of the socket lock in skb_splice_bits.

skb_splice_bits temporary drops the socket lock while iterating over
the socket queue in order to break a reverse locking condition which
happens with sendfile. This, however, opens a window of opportunity
for tcp_collapse() to aggregate skbs and thus potentially free the
current skb used in skb_splice_bits and tcp_read_sock.

This patch fixes the problem by (re-)getting the same "logical skb"
after the lock has been temporary dropped.

Based on idea and initial patch from Evgeniy Polyakov.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 1 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f88653138621..ab66683b8043 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1227,7 +1227,14 @@ int tcp_read_sock(struct sock *sk, read_descriptor_t *desc,
                                 copied += used;
                                 offset += used;
                         }
-                        if (offset != skb->len)
+                        /*
+                         * If recv_actor drops the lock (e.g. TCP splice
+                         * receive) the skb pointer might be invalid when
+                         * getting here: tcp_collapse might have deleted it
+                         * while aggregating skbs from the socket queue.
+                         */
+                        skb = tcp_recv_skb(sk, seq-1, &offset);
+                        if (!skb || (offset+1 != skb->len))
                                 break;
                 }
                 if (tcp_hdr(skb)->fin) {