[FFmpeg-devel] [PATCH] avcodec/msp2dec: Check available space in RLE decoder

[James Almer]

On 4/7/2021 11:59 AM, Michael Niedermayer wrote:
> On Wed, Apr 07, 2021 at 12:42:50AM +0200, Andreas Rheinhardt wrote:
>> Michael Niedermayer:
>>> Fixes: out of array read
>>> Fixes: 32968/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MSP2_fuzzer-5315296027082752
>>>
>>> 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/msp2dec.c | 3 ++-
>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/libavcodec/msp2dec.c b/libavcodec/msp2dec.c
>>> index cc548d218a..87057fb5e2 100644
>>> --- a/libavcodec/msp2dec.c
>>> +++ b/libavcodec/msp2dec.c
>>> @@ -68,9 +68,10 @@ static int msp2_decode_frame(AVCodecContext *avctx,
>>>   
>>>           bytestream2_init(&gb, buf, pkt_size);
>>>           x = 0;
>>> -        while (bytestream2_get_bytes_left(&gb) && x < width) {
>>> +        while (bytestream2_get_bytes_left(&gb) > 0 && x < width) {
>>
>> This decoder uses the checked bytestream2 API, so != 0 and > 0 should be
>> equivalent for bytestream2_get_bytes_left(&gb).
> 
> I changed it to "> 0" because it felt clearer&more robust
> i will drop that as its not needed for the bugfix
> 
> 
>>
>>>               int size = bytestream2_get_byte(&gb);
>>>               if (size) {
>>> +                size = FFMIN(size, bytestream2_get_bytes_left(&gb));
>>>                   memcpy(p->data[0] + y * p->linesize[0] + x, gb.buffer, FFMIN(size, width - x));
>>
>> width can include seven bytes of the packet's padding, but it stays
>> within the padding, so I wonder where the out of array read comes from.
>> The only fishy thing in this decoder I see is that 2 * avctx->height
>> might overflow.
> 
> size is a value read from the bytestream, theres no guarntee that
> the amount that byte calls for is not beyond the end of the bytestream
> and its not checked by memcpy
> 
> bytestream2_get_buffer() would check it and it can maybe be used instead

If you knew about it, why didn't you use it? This code here basically 
duplicates the three lines in bytestream2_get_buffer().

> 
> thx
> 
> [...]
> 
> 
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