slrip: ip_reass: Fix use after free

Using ip_deq after m_free might read pointers from an allocation reuse.

This would be difficult to exploit, but that is still related with
CVE-2019-14378 which generates fragmented IP packets that would trigger this
issue and at least produce a DoS.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
(from libslirp.git commit c59279437eda91841b9d26079c70b8a540d41204)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

slirp/ip_input.c

@@ -300,6 +300,7 @@ ip_reass(Slirp *slirp, struct ip *ip, struct ipq *fp)
 	 */
 	while (q != (struct ipasfrag*)&fp->frag_link &&
             ip->ip_off + ip->ip_len > q->ipf_off) {
+		struct ipasfrag *prev;
 		i = (ip->ip_off + ip->ip_len) - q->ipf_off;
 		if (i < q->ipf_len) {
 			q->ipf_len -= i;
@@ -307,9 +308,10 @@ ip_reass(Slirp *slirp, struct ip *ip, struct ipq *fp)
 			m_adj(dtom(slirp, q), i);
 			break;
 		}
+		prev = q;
 		q = q->ipf_next;
-		m_free(dtom(slirp, q->ipf_prev));
-		ip_deq(q->ipf_prev);
+		ip_deq(prev);
+		m_free(dtom(slirp, prev));
 	}
 
 insert: