[FFmpeg-cvslog] h264: prevent theoretical infinite loop in SEI parsing

[Vittorio Giovara]

ffmpeg | branch: release/2.2 | Vittorio Giovara <vittorio.giovara at gmail.com> | Wed Jul 30 19:33:36 2014 +0100| [2273e5ed992661e0c4b37208e792e2253d5a0b5b] | committer: Vittorio Giovara

h264: prevent theoretical infinite loop in SEI parsing

Properly address CVE-2011-3946 and parse bitstream as described in the spec.

CC: libav-stable at libav.org
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=2273e5ed992661e0c4b37208e792e2253d5a0b5b
---

 libavcodec/h264_sei.c |   18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/libavcodec/h264_sei.c b/libavcodec/h264_sei.c
index 6fca2c3..1e5a134 100644
--- a/libavcodec/h264_sei.c
+++ b/libavcodec/h264_sei.c
@@ -205,14 +205,20 @@ int ff_h264_decode_sei(H264Context *h)
         int size = 0;
         int type = 0;
         int ret  = 0;
+        int last = 0;
 
-        do
-            type += show_bits(&h->gb, 8);
-        while (get_bits(&h->gb, 8) == 255);
+        while (get_bits_left(&h->gb) >= 8 &&
+               (last = get_bits(&h->gb, 8)) == 255) {
+            type += 255;
+        }
+        type += last;
 
-        do
-            size += show_bits(&h->gb, 8);
-        while (get_bits(&h->gb, 8) == 255);
+        last = 0;
+        while (get_bits_left(&h->gb) >= 8 &&
+               (last = get_bits(&h->gb, 8)) == 255) {
+            size += 255;
+        }
+        size += last;
 
         if (size > get_bits_left(&h->gb) / 8) {
             av_log(h->avctx, AV_LOG_ERROR, "SEI type %d truncated at %d\n",