From 477c7aea5f2f9090c016c0a9813dc5901bd1b66a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
Date: Fri, 24 Apr 2020 11:36:41 +0800
Subject: [PATCH] Fix use-afte-free in ip_reass() (CVE-2020-1983)
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The q pointer is updated when the mbuf data is moved from m_dat to
m_ext.

m_ext buffer may also be realloc()'ed and moved during m_cat():
q should also be updated in this case.

Reported-by: Aviv Sasson <asasson@paloaltonetworks.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

diff --git a/slirp/src/ip_input.c b/slirp/src/ip_input.c
index 89ae04e0..7fdde631 100644
--- a/slirp/src/ip_input.c
+++ b/slirp/src/ip_input.c
@@ -333,7 +333,7 @@ insert:
     q = fp->frag_link.next;
 	m = dtom(slirp, q);
 
-	int was_ext = m->m_flags & M_EXT;
+	int delta = (char *)q - (m->m_flags & M_EXT ? m->m_ext : m->m_dat);
 
 	q = (struct ipasfrag *) q->ipf_next;
 	while (q != (struct ipasfrag*)&fp->frag_link) {
@@ -356,8 +356,7 @@ insert:
 	 * then an m_ext buffer was alloced. But fp->ipq_next points to the old
 	 * buffer (in the mbuf), so we must point ip into the new buffer.
 	 */
-	if (!was_ext && m->m_flags & M_EXT) {
-	  int delta = (char *)q - m->m_dat;
+	if (m->m_flags & M_EXT) {
 	  q = (struct ipasfrag *)(m->m_ext + delta);
 	}
 
-- 
2.23.0