[FFmpeg-devel] [PATCH 1/3] avcodec/cavsdsp: Fix invalid left shifts in cavs_idct8_add_c()

[Andreas Rheinhardt]

Michael Niedermayer:
> Fixes: left shift of negative value -107
> Fixes: 20398/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CAVS_fuzzer-5725389278412800
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  libavcodec/cavsdsp.c | 52 ++++++++++++++++++++++----------------------
>  1 file changed, 26 insertions(+), 26 deletions(-)
> 
> diff --git a/libavcodec/cavsdsp.c b/libavcodec/cavsdsp.c
> index 90a67e910c..27413b9ee2 100644
> --- a/libavcodec/cavsdsp.c
> +++ b/libavcodec/cavsdsp.c
> @@ -201,20 +201,20 @@ static void cavs_idct8_add_c(uint8_t *dst, int16_t *block, ptrdiff_t stride)
>      src[0][0] += 8;
>  
>      for( i = 0; i < 8; i++ ) {
> -        const int a0 =  3*src[i][1] - (src[i][7]<<1);
> -        const int a1 =  3*src[i][3] + (src[i][5]<<1);
> -        const int a2 =  (src[i][3]<<1) - 3*src[i][5];
> -        const int a3 =  (src[i][1]<<1) + 3*src[i][7];
> +        const int a0 =  3*src[i][1] - (src[i][7]*2);
> +        const int a1 =  3*src[i][3] + (src[i][5]*2);
> +        const int a2 =  (src[i][3]*2) - 3*src[i][5];
> +        const int a3 =  (src[i][1]*2) + 3*src[i][7];
>  
> -        const int b4 = ((a0 + a1 + a3)<<1) + a1;
> -        const int b5 = ((a0 - a1 + a2)<<1) + a0;
> -        const int b6 = ((a3 - a2 - a1)<<1) + a3;
> -        const int b7 = ((a0 - a2 - a3)<<1) - a2;
> +        const int b4 = ((a0 + a1 + a3)*2) + a1;
> +        const int b5 = ((a0 - a1 + a2)*2) + a0;
> +        const int b6 = ((a3 - a2 - a1)*2) + a3;
> +        const int b7 = ((a0 - a2 - a3)*2) - a2;
>  
> -        const int a7 = (src[i][2]<<2) - 10*src[i][6];
> -        const int a6 = (src[i][6]<<2) + 10*src[i][2];
> -        const int a5 = ((src[i][0] - src[i][4]) << 3) + 4;
> -        const int a4 = ((src[i][0] + src[i][4]) << 3) + 4;
> +        const int a7 = (src[i][2]*4) - 10*src[i][6];
> +        const int a6 = (src[i][6]*4) + 10*src[i][2];
> +        const int a5 = ((src[i][0] - src[i][4]) * 8) + 4;
> +        const int a4 = ((src[i][0] + src[i][4]) * 8) + 4;
>  
>          const int b0 = a4 + a6;
>          const int b1 = a5 + a7;
> @@ -231,20 +231,20 @@ static void cavs_idct8_add_c(uint8_t *dst, int16_t *block, ptrdiff_t stride)
>          src[i][7] = (b0 - b4) >> 3;
>      }
>      for( i = 0; i < 8; i++ ) {
> -        const int a0 =  3*src[1][i] - (src[7][i]<<1);
> -        const int a1 =  3*src[3][i] + (src[5][i]<<1);
> -        const int a2 =  (src[3][i]<<1) - 3*src[5][i];
> -        const int a3 =  (src[1][i]<<1) + 3*src[7][i];
> -
> -        const int b4 = ((a0 + a1 + a3)<<1) + a1;
> -        const int b5 = ((a0 - a1 + a2)<<1) + a0;
> -        const int b6 = ((a3 - a2 - a1)<<1) + a3;
> -        const int b7 = ((a0 - a2 - a3)<<1) - a2;
> -
> -        const int a7 = (src[2][i]<<2) - 10*src[6][i];
> -        const int a6 = (src[6][i]<<2) + 10*src[2][i];
> -        const int a5 = (src[0][i] - src[4][i]) << 3;
> -        const int a4 = (src[0][i] + src[4][i]) << 3;
> +        const int a0 =  3*src[1][i] - (src[7][i]*2);
> +        const int a1 =  3*src[3][i] + (src[5][i]*2);
> +        const int a2 =  (src[3][i]*2) - 3*src[5][i];
> +        const int a3 =  (src[1][i]*2) + 3*src[7][i];
> +
> +        const int b4 = ((a0 + a1 + a3)*2) + a1;
> +        const int b5 = ((a0 - a1 + a2)*2) + a0;
> +        const int b6 = ((a3 - a2 - a1)*2) + a3;
> +        const int b7 = ((a0 - a2 - a3)*2) - a2;
> +
> +        const int a7 = (src[2][i]*4) - 10*src[6][i];
> +        const int a6 = (src[6][i]*4) + 10*src[2][i];
> +        const int a5 = (src[0][i] - src[4][i]) * 8;
> +        const int a4 = (src[0][i] + src[4][i]) * 8;
>  
>          const int b0 = a4 + a6;
>          const int b1 = a5 + a7;
> 
Duplicate of [1] which I consider more aesthetically pleasing (e.g.
when rewriting "((a0 + a1 + a3)<<1) + a1" via multiplication, one can
drop the outer parentheses, but you didn't).

- Andreas

[1]: https://ffmpeg.org/pipermail/ffmpeg-devel/2019-September/250681.html