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

[HouLei]

在 2021/4/11 上午5:21, Michael Niedermayer 写道:
> On Fri, Apr 09, 2021 at 10:59:44PM -0300, James Almer wrote:
>> 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().
> why i didnt use it, well ultimatly probable because i had a bugfix that
> i tested and that solved the issue and that was easy to backport. And
> noone asked me to include the simplification.
> Of course no question when looking at it now, its prettier to replace
> these 3 lines by bytestream2_get_buffer().
> Thats what review is for, to spot these things. Which it now did.
> Feel free to replace it or ill do it tomorrow if i dont forget
>
> thx
>
> [...]
>
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