udp: check upd_input buffer size

Fixes: CVE-2021-3594
Fixes: https://gitlab.freedesktop.org/slirp/libslirp/-/issues/47

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/udp.c b/src/udp.c
index 767ca85..06b7b7d 100644
--- a/src/udp.c
+++ b/src/udp.c
@@ -96,7 +96,10 @@ void udp_input(register struct mbuf *m, int iphlen)
     /*
      * Get IP and UDP header together in first mbuf.
      */
-    ip = mtod(m, struct ip *);
+    ip = mtod_check(m, iphlen + sizeof(struct udphdr));
+    if (ip == NULL) {
+        goto bad;
+    }
     uh = (struct udphdr *)((char *)ip + iphlen);
 
     /*