[Qemu-devel] [PATCH v1 1/2] slirp: correct size computation while concatenating mbuf

[P J P <ppandit@redhat.com>]

From: Prasad J Pandit <pjp@fedoraproject.org>

While reassembling incoming fragmented datagrams, 'm_cat' routine
extends the 'mbuf' buffer, if it has insufficient room. It computes
a wrong buffer size, which leads to overwriting adjacent heap buffer
area. Correct this size computation in m_cat.

Reported-by: ZDI Disclosures <zdi-disclosures@trendmicro.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 slirp/mbuf.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

Update v1: fixed indentation
    https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg01121.html

diff --git a/slirp/mbuf.c b/slirp/mbuf.c
index 5ff24559fd..6650981fc7 100644
--- a/slirp/mbuf.c
+++ b/slirp/mbuf.c
@@ -137,8 +137,9 @@ m_cat(struct mbuf *m, struct mbuf *n)
 	/*
 	 * If there's no room, realloc
 	 */
-	if (M_FREEROOM(m) < n->m_len)
-		m_inc(m,m->m_size+MINCSIZE);
+    if (M_FREEROOM(m) < n->m_len) {
+        m_inc(m, m->m_len + n->m_len);
+    }
 
 	memcpy(m->m_data+m->m_len, n->m_data, n->m_len);
 	m->m_len += n->m_len;
@@ -158,12 +159,12 @@ m_inc(struct mbuf *m, int size)
 
         if (m->m_flags & M_EXT) {
 	  datasize = m->m_data - m->m_ext;
-          m->m_ext = g_realloc(m->m_ext, size);
+          m->m_ext = g_realloc(m->m_ext, size + datasize);
 	  m->m_data = m->m_ext + datasize;
         } else {
 	  char *dat;
 	  datasize = m->m_data - m->m_dat;
-          dat = g_malloc(size);
+      dat = g_malloc(size + datasize);
 	  memcpy(dat, m->m_dat, m->m_size);
 
 	  m->m_ext = dat;
@@ -171,7 +172,7 @@ m_inc(struct mbuf *m, int size)
 	  m->m_flags |= M_EXT;
         }
 
-        m->m_size = size;
+        m->m_size = size + datasize;
 
 }
 
-- 
2.17.1