[FFmpeg-devel] [PATCH 2/4] avcodec/hevc_ps: Fix integer overflow with num_tile_rows

[James Almer]

On 6/19/2019 3:59 PM, James Almer wrote:
> On 6/19/2019 3:13 PM, Michael Niedermayer wrote:
>> On Wed, Jun 19, 2019 at 12:54:25PM -0300, James Almer wrote:
>>> On 6/19/2019 6:22 AM, Michael Niedermayer wrote:
>>>> On Mon, Jun 17, 2019 at 07:55:45PM -0300, James Almer wrote:
>>>>> On 6/17/2019 6:54 PM, Michael Niedermayer wrote:
>>>>>> On Sun, Jun 16, 2019 at 11:10:43PM -0300, James Almer wrote:
>>>>>>> On 6/13/2019 3:32 PM, Michael Niedermayer wrote:
>>>>>>>> Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
>>>>>>>> Fixes: 14880/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5130977304641536
>>>>>>>>
>>>>>>>> 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/hevc_ps.c | 2 +-
>>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/libavcodec/hevc_ps.c b/libavcodec/hevc_ps.c
>>>>>>>> index 80df417e4f..0ed6682bb4 100644
>>>>>>>> --- a/libavcodec/hevc_ps.c
>>>>>>>> +++ b/libavcodec/hevc_ps.c
>>>>>>>> @@ -1596,7 +1596,7 @@ int ff_hevc_decode_nal_pps(GetBitContext *gb, AVCodecContext *avctx,
>>>>>>>>          if (pps->num_tile_rows <= 0 ||
>>>>>>>>              pps->num_tile_rows >= sps->height) {
>>>>>>>>              av_log(avctx, AV_LOG_ERROR, "num_tile_rows_minus1 out of range: %d\n",
>>>>>>>> -                   pps->num_tile_rows - 1);
>>>>>>>> +                   pps->num_tile_rows - 1U);
>>>>>>>
>>>>>>> The proper fix for this is making pps->num_tile_rows/cols unsigned. 
>>>> [...]
>>>>>>
>>>>>> is this here ok if num_tile_rows is 0 ?
>>>>>> for (i = 0; i < pps->num_tile_rows - 1; i++) { (example line from ffmpeg git)
>>>>>>
>>>>>> i would guess nearly everyone wold say yes without having seen the
>>>>>> discussion about the type. but of course if this is unsigned its not
>>>>>> going to be safe with it being 0. 
>>>>>
>>>>> pps->num_tile_rows is set to a value returned by "get_ue_golomb_long() +
>>>>> 1", which will always be in the 1..UINT32_MAX range. It can't be 0, as
>>>>> it would be a bug. Int is definitely not the right type for it.
>>>>
>>>> i dont think num_tile_rows of more than 2^31 (that is the negative when signed range)
>>>> makes much sense but sure i can change it to unsigned if preferred.
>>>
>>> Of course it doesn't. In pretty much any sample it will be at least 1
>>> and at most 22, which is the max value allowed by hevc level 6.2 in
>>> table A.6. Only if the stream reports an undefined level it could go up
>>> to, if i'm reading this right, sps->ctb_height and not sps->height as
>>> the decoder is currently checking. This means you can even replace
>>> get_ue_golomb_long() for a get_ue_golomb(). That would also fix this.
>>>
>>> In any case, the bitstream value is unsigned, so the struct field should
>>> be unsigned as well. Having it be signed and assigning it a value using
>>> a function that potentially returns huge unsigned values introduced this
>>> issue to being with, so instead of working around it using type
>>> promotion, just make it follow the spec.
>>
>> what would be your feeling/oppinon about making the field uint16_t ?
>> this would make it unsigned in the struct but avoid the problems with
>> unsigned int ?
> 
> That's fine. Could even make it uint8_t, like cbs_h265.h does. I'd also
> change it to use get_ue_golomb() while at it. And please do it for both
> rows and columns. Can be in separate patches if you want the rows one to
> explicitly address the fuzzing testcase.
> 
> Can you also confirm my suspicion that the checks should be comparing
> the values with sps->ctb_height/weight and not with sps->height/weight?

Ok, so yes, it is. Should be changed in a separate patch as well.

> 
>>
>> Thanks
>>
>> [...]
>>
>>
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