packet: fix leakage of tx_ring memory

When releasing a packet socket, the routine packet_set_ring() is reused
to free rings instead of allocating them. But when calling it for the
first time, it fills req->tp_block_nr with the value of rb->pg_vec_len
which in the second invocation makes it bail out since req->tp_block_nr
is greater zero but req->tp_block_size is zero.

This patch solves the problem by passing a zeroed auto-variable to
packet_set_ring() upon each invocation from packet_release().

As far as I can tell, this issue exists even since 69e3c75f (net: TX_RING
and packet mmap), i.e. the original inclusion of TX ring support into
af_packet, but applies only to sockets with both RX and TX ring
allocated, which is probably why this was unnoticed all the time.
Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Cc: Johann Baudy <johann.baudy@gnu-log.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2361,13 +2361,15 @@ static int packet_release(struct socket *sock)
 
 	packet_flush_mclist(sk);
 
-	memset(&req_u, 0, sizeof(req_u));
-
-	if (po->rx_ring.pg_vec)
+	if (po->rx_ring.pg_vec) {
+		memset(&req_u, 0, sizeof(req_u));
 		packet_set_ring(sk, &req_u, 1, 0);
+	}
 
-	if (po->tx_ring.pg_vec)
+	if (po->tx_ring.pg_vec) {
+		memset(&req_u, 0, sizeof(req_u));
 		packet_set_ring(sk, &req_u, 1, 1);
+	}
 
 	fanout_release(sk);