[FFmpeg-devel] [PATCH 2/2] avcodec/dvdsubdec: reject some broken packets

[wm4]

If cmd_pos is broken, this would just keep accumulating packets in the
reassembly buffer, until it fails and flushes the buffer on overflow.
Since packets are usually rather small, this will take a lot of subtitle
packets. The perceived effect is that subtitles are not displayed
anymore after the faulty packet was passed to the decoder.

I'm not terribly sure about this, but on the other hand this code is
active only when fragmented packets need to be reassembled.

Fixes sample file in trac issue #4872.
---
Oh, by the way, the sample was supposedly created by mencoder. Playing
directly from DVD doesn't show the issue.
---
 libavcodec/dvdsubdec.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/libavcodec/dvdsubdec.c b/libavcodec/dvdsubdec.c
index 57eafbf..1264e86 100644
--- a/libavcodec/dvdsubdec.c
+++ b/libavcodec/dvdsubdec.c
@@ -227,6 +227,7 @@ static int decode_dvd_subtitles(DVDSubContext *ctx, AVSubtitle *sub_header,
     int date;
     int i;
     int is_menu = 0;
+    uint32_t size;
 
     if (buf_size < 10)
         return -1;
@@ -241,10 +242,16 @@ static int decode_dvd_subtitles(DVDSubContext *ctx, AVSubtitle *sub_header,
         cmd_pos = 2;
     }
 
+    size = READ_OFFSET(buf);
     cmd_pos = READ_OFFSET(buf + cmd_pos);
 
-    if (cmd_pos < 0 || cmd_pos > buf_size - 2 - offset_size)
+    if (cmd_pos < 0 || cmd_pos > buf_size - 2 - offset_size) {
+        if (cmd_pos > size) {
+            av_log(NULL, AV_LOG_ERROR, "Discarding invalid packet\n");
+            return 0;
+        }
         return AVERROR(EAGAIN);
+    }
 
     while (cmd_pos > 0 && cmd_pos < buf_size - 2 - offset_size) {
         date = AV_RB16(buf + cmd_pos);
-- 
2.5.1