From 63b07dfe20a0d4971b0929d27359f478ba2d816b Mon Sep 17 00:00:00 2001
From: Samuel Thibault <samuel.thibault@ens-lyon.org>
Date: Fri, 22 May 2020 10:52:55 +0800
Subject: [PATCH] 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's avatarSamuel Thibault <samuel.thibault@ens-lyon.org>

diff --git a/slirp/src/ip_input.c b/slirp/src/ip_input.c
index 8c75d91..c07d7d4 100644
--- a/slirp/src/ip_input.c
+++ b/slirp/src/ip_input.c
@@ -292,6 +292,7 @@ static struct ip *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;
@@ -299,9 +300,10 @@ static struct ip *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:
-- 
1.8.3.1