[FFmpeg-devel] [PATCH] avutil/hwcontext: check the null pointer input value before use it

[James Almer]

On 2/10/2022 10:43 PM, Steven Liu wrote:
> 
> 
>> 2022年2月10日 下午8:27，James Almer <jamrial at gmail.com> 写道：
>>
>> On 2/10/2022 9:20 AM, Steven Liu wrote:
>>> because the src, src->hw_frames_ctx and src->hw_frames_ctx->data can be
>>> set to null when the user calling av_hwframe_transfer_data, this will
>>> get crash if they are null.
>>
>> src can not be NULL. The doxy doesn't allow it.
> 
> Hi James,
> 
> User call av_hwframe_transfer_data like this:
> 
> av_hwframe_transfer_data(dst, NULL, 0);
> 
> It will crash when dst->buf[0] is null.
> Because dst->buf[0] is null and src is null, it will call transfer_data_alloc, but the first line is ctx = (AVHWFramesContext*)src->hw_frames_ctx->data; in transfer_data_alloc,
> It using src->hw_frames_ctx.
> 
> av_hwframe_transfer_data is av_*, it is API to user.
> Maybe this is not logic problem, looks like a security problem.

I know what happens when you pass NULL as src argument. My point is that 
it's not a security problem because that's an API violation and an 
explicitly forbidden scenario: Neither src or dst can be NULL, and at 
least one of them must have an AVHWFramesContext attached. Any 
application not following that is faulty and buggy, and needs to be fixed.

And you can get crashes by passing NULL arguments to lots of public 
libav* functions, not just this one.

>>
>> And if transfer_data_alloc() is called, it's because dst is "clean", and src must then have a hw_frames_ctx (The doxy explicitly states "At least one of dst/src must have an AVHWFramesContext attached").
>>
>>> Signed-off-by: Steven Liu <lq at chinaffmpeg.org>
>>> ---
>>>   libavutil/hwcontext.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>> diff --git a/libavutil/hwcontext.c b/libavutil/hwcontext.c
>>> index 31c7840dba..b42a3a6d4d 100644
>>> --- a/libavutil/hwcontext.c
>>> +++ b/libavutil/hwcontext.c
>>> @@ -396,10 +396,13 @@ int av_hwframe_transfer_get_formats(AVBufferRef *hwframe_ref,
>>>     static int transfer_data_alloc(AVFrame *dst, const AVFrame *src, int flags)
>>>   {
>>> -    AVHWFramesContext *ctx = (AVHWFramesContext*)src->hw_frames_ctx->data;
>>> +    AVHWFramesContext *ctx = NULL;
>>>       AVFrame *frame_tmp;
>>>       int ret = 0;
>>>   +    if (!src || !src->hw_frames_ctx || !src->hw_frames_ctx->data)
>>> +        return AVERROR(EINVAL);
>>> +    ctx = (AVHWFramesContext*)src->hw_frames_ctx->data;
>>>       frame_tmp = av_frame_alloc();
>>>       if (!frame_tmp)
>>>           return AVERROR(ENOMEM);
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> 
> Thanks
> 
> Steven Liu
> 
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