Re: [FFmpeg-devel] [PATCH] avcodec/cbs_h2645: Check NAL space

[James Almer <jamrial@gmail.com>]

On 4/22/2024 5:40 PM, Mark Thompson wrote:
> On 22/04/2024 02:31, Michael Niedermayer wrote:
>> Found-by-reviewing: CID1419833 Untrusted loop bound
>>
>> Sponsored-by: Sovereign Tech Fund
>> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
>> ---
>>   libavcodec/cbs_h2645.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/libavcodec/cbs_h2645.c b/libavcodec/cbs_h2645.c
>> index fe2e383ff33..1a45d424bae 100644
>> --- a/libavcodec/cbs_h2645.c
>> +++ b/libavcodec/cbs_h2645.c
>> @@ -709,7 +709,11 @@ static int cbs_h2645_split_fragment(CodedBitstreamContext *ctx,
>>   
>>               start = bytestream2_tell(&gbc);
>>               for(i = 0; i < num_nalus; i++) {
>> +                if (bytestream2_get_bytes_left(&gbc) < 2)
>> +                    return AVERROR_INVALIDDATA;
>>                   size = bytestream2_get_be16(&gbc);
>> +                if (bytestream2_get_bytes_left(&gbc) < size)
>> +                    return AVERROR_INVALIDDATA;
>>                   bytestream2_skip(&gbc, size);
>>               }
>>               end = bytestream2_tell(&gbc);
> 
> Seems fair.
> 
> The problem looks more general with missing bounds checks in all the H.266 code around this, though?  Compare with H.26[45], which have checks on all the reads - seems like H.266 should be doing that.
> 
> Thanks,

Not against this approach, but since the bytestream2_get_* functions 
return 0, never overread the buffer or move the internal pointer, 
wouldn't it be enough to just ensure end > start?
Particularly in ff_h2645_packet_split(), we can return an error if 
length (in this case being set to end - start) is < 4.
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