[FFmpeg-devel] [PATCH 1/2] MxPEG decoder

[Michael Niedermayer]

On Sat, Nov 06, 2010 at 07:37:40AM +0300, Anatoly Nenashev wrote:
> On 06.11.2010 06:01, Michael Niedermayer wrote:
>> On Thu, Nov 04, 2010 at 05:39:40PM +0300, Anatoly Nenashev wrote:
>>    
>>> On 04.11.2010 03:16, Michael Niedermayer wrote:
>>>      
>>>> On Tue, Nov 02, 2010 at 01:58:21PM +0300, Anatoly Nenashev wrote:
>>>>
>>>>        
>>>>>    int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
>>>>>    {
>>>>>        int len, nb_components, i, width, height, pix_fmt_id;
>>>>> @@ -342,14 +376,12 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
>>>>>                s->avctx->pix_fmt = PIX_FMT_GRAY16;
>>>>>        }
>>>>>
>>>>> -    if(s->picture.data[0])
>>>>> -        s->avctx->release_buffer(s->avctx,&s->picture);
>>>>> -
>>>>> -    s->picture.reference= 0;
>>>>> -    if(s->avctx->get_buffer(s->avctx,&s->picture)<   0){
>>>>> -        av_log(s->avctx, AV_LOG_ERROR, "get_buffer() failed\n");
>>>>> -        return -1;
>>>>> +    if (s->avctx->codec_id == CODEC_ID_MXPEG&&   !s->got_picture) {
>>>>> +        if (mxpeg_allocate_picture(s)<   0) return -1;
>>>>> +    } else {
>>>>> +        if (mjpeg_allocate_picture(s)<   0) return -1;
>>>>>        }
>>>>>
>>>>>          
>>>> looks like s->picture.reference will b wrong for the first pic
>>>>
>>>>
>>>>
>>>>        
>>> No, s->picture.reference will be always 1 for MxPEG frames. But I've
>>> reimplemented this code to be more clean. See attachment.
>>>      
>>>> [...]
>>>>
>>>>        
>>>>> +static int mxpeg_decode_mxm(MJpegDecodeContext *s, char *buf, int len)
>>>>> +{
>>>>> +    int32_t  mb_width, mb_height, bitmask_size, i;
>>>>> +    uint32_t mb_count;
>>>>> +
>>>>> +    mb_width  = AV_RL16(&buf[4]);
>>>>> +    mb_height = AV_RL16(&buf[6]);
>>>>> +    mb_count = (uint32_t)mb_width*mb_height;
>>>>> +
>>>>> +    if (!mb_count || mb_count>   0x7FFFFFF8) {
>>>>> +        av_log(s->avctx, AV_LOG_ERROR, "MXM wrong macroblocks count");
>>>>> +        return AVERROR(EINVAL);
>>>>> +    }
>>>>> +
>>>>> +    bitmask_size = (int32_t)(mb_count + 7)>>   3;
>>>>> +
>>>>> +    if (bitmask_size>   (len - 12)) {
>>>>> +        av_log(s->avctx, AV_LOG_ERROR, "MXM bitmask is not complete");
>>>>> +        return AVERROR(EINVAL);
>>>>> +    }
>>>>> +
>>>>> +    av_freep(&s->mxctx.comment_buffer);
>>>>> +    s->mxctx.comment_buffer = buf;
>>>>> +    s->mxctx.mxm_bitmask = buf + 12;
>>>>> +
>>>>> +    if (!s->got_picture&&   s->picture.data[0]) {
>>>>> +        if (mxpeg_allocate_picture(s)<   0) return -1;
>>>>> +        s->got_picture = 1;
>>>>> +    }
>>>>>
>>>>>          
>>>> allocating pictures in the comment parsing code is not a good idea IMHO
>>>>
>>>>        
>>> I don't agree.
>>> There are 3 types of frames in MxPEG stream:
>>> 1) clear JPEG which contains SOF, DHT, DQT, SOS  - may be the first
>>> frame in stream or (in new cameras firmware) may be periodically
>>> available.
>>> 2) Pseudo-I frame which contains SOF, DHT, DQT, MXM bitmask (in COM),
>>> SOS - must be periodically available but doesn't need to contain full
>>> parts of image.
>>> 3) P-frame which contains MXM bitmask, SOS and optional DHT, DQT - just
>>> usual frame.
>>>
>>> So where I must allocate picture for the last frame type? I've decided
>>> to do it in MXM parser.
>>>
>>>
>>>      
>>>> also maybe it would be easier to use reget_buffer() ?
>>>>
>>>>
>>>>
>>>>        
>>> No, I think reget_buffer is a bad idea. We don't need to copy all
>>> reference frame but only non-changed part of it.
>>> And we had talking about it in August. It was your idea to remove
>>> reget_buffer from this code to reject full image copying.
>>>      
>> both methods have their advantages and disadvantages.
>> i dont care which is implemented but it should be done cleanly
>> allocating the frame in the comment parsing code is not ok.
>> Allocating a picture currently implicates that a SOF has been successfully
>> parsed as the later code checks for this.
>> Simply allocating a picture without successfully running the SOF parsing but
>> maybe having it run just to the middle where it failed could lead to
>> exploitable security issues
>>
>>
>>
>>    
>
> As I have specified before there are frames which don't contain SOF  
> marker. I've named them P-frames.
> If we have got Pseudo-I frame before then new picture for P-frame will  
> be allocated according to previously parsed params.

you dont know if the previous params are valid. Its possible that frame
1 had a correct SOF and frame 2 failed in the middle of SOF parsing and so
you have a mix a values now frame 3 has no SOF and uses this mix, this
looks bad to me

Iam not sure how to best solve it but i dont think we can just pick a random
solution and hope it wont be exploitable

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Breaking DRM is a little like attempting to break through a door even
though the window is wide open and the only thing in the house is a bunch
of things you dont want and which you would get tomorrow for free anyway
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