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>

diff --git a/slirp/ip_input.c b/slirp/ip_input.c
index 6e291eabf00b031982ad8eebb3a52e5fa5ac330b..01b3f9d4e0eeea95aed6eddcbc73677e2563a1ad 100644 (file)
--- a/slirp/ip_input.c
+++ b/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: