[FFmpeg-devel,02/11] avcodec/mpeg12dec: Sanitize start codes earlier

Submitted by Andreas Rheinhardt on Sept. 19, 2019, 10:16 p.m.

Details

Message ID 20190919221706.16529-3-andreas.rheinhardt@gmail.com
State Accepted
Commit 093b6894bfc2a8f0804f1c774bfe7827e44ffdb2
Headers show

Commit Message

Andreas Rheinhardt Sept. 19, 2019, 10:16 p.m.
The MPEG-1/2 decoder uses avpriv_find_start_code to search for start
codes and worked with the resulting start code before checking that it
is really a start code of a slice. In particular, if the picture is so
big that a slice_vertical_position_extension is present, it added the
slice_vertical_position_extension as if it had a slice. Then a left
shift is performed, without making sure that the value to be shifted is
nonnegative.
Afterwards the end result is checked, but even if a start code of a
non-slice has been found, it might pass these checks: If
slice_vertical_position_extension is present a start code <
SLICE_MIN_START_CODE can lead to a macroblock-row index that appears
valid. Furthermore, the left shift might make an invalid start code
appear valid by discarding the highest bit.
This has been fixed by checking directly after avpriv_find_start_code
has returned.

Fixes ticket #8162 (which is about the undefined left shifts).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
---
 libavcodec/mpeg12dec.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Comments

Paul B Mahol Sept. 26, 2019, 12:19 p.m.
applied

On 9/20/19, Andreas Rheinhardt <andreas.rheinhardt@gmail.com> wrote:
> The MPEG-1/2 decoder uses avpriv_find_start_code to search for start
> codes and worked with the resulting start code before checking that it
> is really a start code of a slice. In particular, if the picture is so
> big that a slice_vertical_position_extension is present, it added the
> slice_vertical_position_extension as if it had a slice. Then a left
> shift is performed, without making sure that the value to be shifted is
> nonnegative.
> Afterwards the end result is checked, but even if a start code of a
> non-slice has been found, it might pass these checks: If
> slice_vertical_position_extension is present a start code <
> SLICE_MIN_START_CODE can lead to a macroblock-row index that appears
> valid. Furthermore, the left shift might make an invalid start code
> appear valid by discarding the highest bit.
> This has been fixed by checking directly after avpriv_find_start_code
> has returned.
>
> Fixes ticket #8162 (which is about the undefined left shifts).
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
> ---
>  libavcodec/mpeg12dec.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
> index 83e537884b..1904b75213 100644
> --- a/libavcodec/mpeg12dec.c
> +++ b/libavcodec/mpeg12dec.c
> @@ -2011,13 +2011,15 @@ static int slice_decode_thread(AVCodecContext *c,
> void *arg)
>
>          start_code = -1;
>          buf        = avpriv_find_start_code(buf, s->gb.buffer_end,
> &start_code);
> +        if (start_code < SLICE_MIN_START_CODE || start_code >
> SLICE_MAX_START_CODE)
> +            return AVERROR_INVALIDDATA;
>          mb_y       = start_code - SLICE_MIN_START_CODE;
>          if (s->codec_id != AV_CODEC_ID_MPEG1VIDEO && s->mb_height >
> 2800/16)
>              mb_y += (*buf&0xE0)<<2;
>          mb_y <<= field_pic;
>          if (s->picture_structure == PICT_BOTTOM_FIELD)
>              mb_y++;
> -        if (mb_y < 0 || mb_y >= s->end_mb_y)
> +        if (mb_y >= s->end_mb_y)
>              return AVERROR_INVALIDDATA;
>      }
>  }
> --
> 2.20.1
>
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diff --git a/libavcodec/mpeg12dec.c b/libavcodec/mpeg12dec.c
index 83e537884b..1904b75213 100644
--- a/libavcodec/mpeg12dec.c
+++ b/libavcodec/mpeg12dec.c
@@ -2011,13 +2011,15 @@  static int slice_decode_thread(AVCodecContext *c, void *arg)
 
         start_code = -1;
         buf        = avpriv_find_start_code(buf, s->gb.buffer_end, &start_code);
+        if (start_code < SLICE_MIN_START_CODE || start_code > SLICE_MAX_START_CODE)
+            return AVERROR_INVALIDDATA;
         mb_y       = start_code - SLICE_MIN_START_CODE;
         if (s->codec_id != AV_CODEC_ID_MPEG1VIDEO && s->mb_height > 2800/16)
             mb_y += (*buf&0xE0)<<2;
         mb_y <<= field_pic;
         if (s->picture_structure == PICT_BOTTOM_FIELD)
             mb_y++;
-        if (mb_y < 0 || mb_y >= s->end_mb_y)
+        if (mb_y >= s->end_mb_y)
             return AVERROR_INVALIDDATA;
     }
 }