slirp: check data length while emulating ident function

While emulating identification protocol, tcp_emu() does not check
available space in the 'sc_rcv->sb_data' buffer. It could lead to
heap buffer overflow issue. Add check to avoid it.
Reported-by: NKira <864786842@qq.com>
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>
(cherry picked from commit a7104eda)
*CVE-2019-6778
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

slirp/tcp_subr.c

@@ -640,6 +640,11 @@ tcp_emu(struct socket *so, struct mbuf *m)
 			socklen_t addrlen = sizeof(struct sockaddr_in);
 			struct sbuf *so_rcv = &so->so_rcv;
 
+			if (m->m_len > so_rcv->sb_datalen
+					- (so_rcv->sb_wptr - so_rcv->sb_data)) {
+			    return 1;
+			}
+
 			memcpy(so_rcv->sb_wptr, m->m_data, m->m_len);
 			so_rcv->sb_wptr += m->m_len;
 			so_rcv->sb_rptr += m->m_len;