[FFmpeg-devel] [PATCH] Fix 'while' loop condition to prevent movtext crashes by tracking packet size

[Niklesh Lalwani]

From: Niklesh <niklesh.lalwani at iitb.ac.in>

Hi all,
This patch fixes some movtext crashes caused due to incorrect 'while' loop condition. I will post several other patches to improve upon the code and null pointer dereferences once this is applied.

Signed-off-by: Niklesh <niklesh.lalwani at iitb.ac.in>
---
 libavcodec/movtextdec.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/libavcodec/movtextdec.c b/libavcodec/movtextdec.c
index 3059599..d03a188 100644
--- a/libavcodec/movtextdec.c
+++ b/libavcodec/movtextdec.c
@@ -96,7 +96,7 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
     char *ptr = avpkt->data;
     char *end;
     //char *ptr_temp;
-    int text_length, tsmb_type, style_entries, tsmb_size;
+    int text_length, tsmb_type, style_entries, tsmb_size, tracksize;
     int **style_start = {0,};
     int **style_end = {0,};
     int **style_flags = {0,};
@@ -135,11 +135,12 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
                             (AVRational){1,100});
 
     tsmb_size = 0;
+    tracksize = 2 + text_length;
     // Note that the spec recommends lines be no longer than 2048 characters.
     av_bprint_init(&buf, 0, AV_BPRINT_SIZE_UNLIMITED);
     if (text_length + 2 != avpkt->size) {
-        while (text_length + 2 + tsmb_size < avpkt->size)  {
-            tsmb = ptr + text_length + tsmb_size;
+        while (tracksize < avpkt->size) {
+            tsmb = ptr + tracksize -2;
             tsmb_size = AV_RB32(tsmb);
             tsmb += 4;
             tsmb_type = AV_RB32(tsmb);
@@ -176,6 +177,7 @@ static int mov_text_decode_frame(AVCodecContext *avctx,
                 av_freep(&style_end);
                 av_freep(&style_flags);
             }
+            tracksize = tracksize + tsmb_size;
         }
     } else
         text_to_ass(&buf, ptr, end, NULL, NULL, 0, 0);
-- 
1.9.1