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

[James Almer <jamrial@gmail.com>]

On 4/22/2024 8:49 PM, Michael Niedermayer wrote:
> On Mon, Apr 22, 2024 at 06:07:42PM -0300, James Almer wrote:
>> On 4/22/2024 6:01 PM, Michael Niedermayer wrote:
>>> On Mon, Apr 22, 2024 at 05:46:10PM -0300, James Almer wrote:
>>>> 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.
>>>
>>> The patch adds the same kind of check as are used in the AV_CODEC_ID_HEVC
>>> case earlier in the file already
>>>
>>> I think what is done should approximately stay in sync
>>
>> fwiw, better not add an initial length check to ff_h2645_packet_split() like
>> i suggested, as it could potentially break samples that would otherwise
>> decode just fine. It setting nb_nals to 0 should be enough.
> 
> iam not 100% i parsed that text correctly

I suggested to add an initial length check to ff_h2645_packet_split(), 
since it will do nothing (Other than set nb_nals to 0) if the buffer is 
less than 4 bytes long. If we do that, hypothetical bitstreams that for 
whatever reason pass a small buffer to ff_h2645_packet_split() and 
currently decode just fine will stop working. Just that.

> 
> 
>>
>> So this patch is fine. Further bytestream2_get_bytes_left() checks can be
>> added too, namely one that checks the buffer is at least the smallest size a
>> vvcC box can be.
> 
> ok will apply patch
> 
> thx
> 
> [...]
> 
> 
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