[PATCH 94/97] slirp: ip_reass: Fix use after free

[Michael Roth]

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 <address@hidden>
(from libslirp.git commit c59279437eda91841b9d26079c70b8a540d41204)
Signed-off-by: Michael Roth <address@hidden>
---
 slirp/src/ip_input.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/slirp/src/ip_input.c b/slirp/src/ip_input.c
index 68a99de5b5..89ae04e0c1 100644
--- a/slirp/src/ip_input.c
+++ b/slirp/src/ip_input.c
@@ -297,6 +297,7 @@ 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;
@@ -304,9 +305,10 @@ 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:
-- 
2.17.1