[FFmpeg-devel] [PATCH 1/2] avcodec: add mvdv video decoder

[James Almer]

On 11/26/2019 4:29 PM, Paul B Mahol wrote:
> On 11/26/19, James Almer <jamrial at gmail.com> wrote:
>> On 11/26/2019 6:47 AM, Paul B Mahol wrote:
>>> On 11/25/19, Tomas Härdin <tjoppen at acc.umu.se> wrote:
>>>> mån 2019-11-25 klockan 22:09 +0100 skrev Paul B Mahol:
>>>>> Signed-off-by: Paul B Mahol <onemda at gmail.com>
>>>>> +static int decode_mvdv(MidiVidContext *s, AVCodecContext *avctx,
>>>>> AVFrame
>>>>> *frame)
>>>>> +{
>>>>> +    GetByteContext *gb = &s->gb;
>>>>> +    GetBitContext mask;
>>>>> +    GetByteContext idx9;
>>>>> +    uint16_t nb_vectors, intra_flag;
>>>>> +    const uint8_t *vec;
>>>>> +    const uint8_t *mask_start;
>>>>> +    uint8_t *skip;
>>>>> +    int mask_size;
>>>>> +    int idx9bits = 0;
>>>>> +    int idx9val = 0;
>>>>> +    int num_blocks;
>>>>> +
>>>>> +    nb_vectors = bytestream2_get_le16(gb);
>>>>> +    intra_flag = bytestream2_get_le16(gb);
>>>>> +    if (intra_flag) {
>>>>> +        num_blocks = (avctx->width / 2) * (avctx->height / 2);
>>>>
>>>> Will UB if width*height/4 > INT_MAX
>>>>
>>>>> +    } else {
>>>>> +        int skip_linesize;
>>>>> +
>>>>> +        num_blocks = bytestream2_get_le32(gb);
>>>>
>>>> Might want to use uint32_t so this doesn't lead to weirdness on 32-bit
>>>>
>>>>> +        skip_linesize = avctx->width >> 1;
>>>>> +        mask_start = gb->buffer_start + bytestream2_tell(gb);
>>>>> +        mask_size = (avctx->width >> 5) * (avctx->height >> 2);
>>>>
>>>> This can also UB
>>>>
>>>> /Tomas
>>>>
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>>>
>>> Nothing of this can actually happen.
>>
>> It can and i'm fairly sure it will happen as soon as the fuzzer starts
>> testing this decoder using big dimensions.
> 
> I'm not that guy doing such work. Please stop bikesheding those
> patches for once.
> 
>>
>> You don't need asserts here, you just need to check the calculations
>> will not overflow. Do something like "if ((int64_t)avctx->width *
>> avctx->height / 4 > INT_MAX) return AVERROR_INVALIDDATA" and call it a day.
>> Also, maybe num_blocks should be unsigned, seeing you set it using
>> bytestream2_get_le32() for P-frames.
> 
> No decoder does this.

Most decoders call av_image_check_size2() directly or indirectly to set
dimensions, which does w*h overflow checks similar to the one above.

> 
>>
>>>
>>> Applied.
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>>
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