[Qemu-devel] [PATCH-for-3.1.1 1/1] Fix heap overflow in ip_reass on big packet input

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

From: Samuel Thibault <samuel.thibault@ens-lyon.org>

When the first fragment does not fit in the preallocated buffer, q will
already be pointing to the ext buffer, so we mustn't try to update it.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
(cherry picked from libslirp commit 126c04acbabd7ad32c2b018fe10dfac2a3bc1210)
Fixes: CVE-2019-14378
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 slirp/ip_input.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/slirp/ip_input.c b/slirp/ip_input.c
index 348e1dca5a..07d8808671 100644
--- a/slirp/ip_input.c
+++ b/slirp/ip_input.c
@@ -334,6 +334,8 @@ insert:
     q = fp->frag_link.next;
 	m = dtom(slirp, q);
 
+	int was_ext = m->m_flags & M_EXT;
+
 	q = (struct ipasfrag *) q->ipf_next;
 	while (q != (struct ipasfrag*)&fp->frag_link) {
 	  struct mbuf *t = dtom(slirp, q);
@@ -356,7 +358,7 @@ insert:
 	 * the old buffer (in the mbuf), so we must point ip
 	 * into the new buffer.
 	 */
-	if (m->m_flags & M_EXT) {
+	if (!was_ext && m->m_flags & M_EXT) {
 	  int delta = (char *)q - m->m_dat;
 	  q = (struct ipasfrag *)(m->m_ext + delta);
 	}
-- 
2.20.1