Re: [PATCH 12/14] media: medkatek: vcodec: set secure mode to decoder driver

[Hans Verkuil]

On 26/09/2023 22:59, Jeffrey Kardatzke wrote:
> Hans,
> 
> I've been looking through the v4l2/vbuf2 code to get an idea of the
> details for implementing a new memory type for secure buffers.  What
> it comes down to essentially is that it would behave just like
> V4L2_MEMORY_DMABUF, but then there would be an extra check in
> __prepare_dmabuf (in videobuf2-core.c) when the memory type is SECURE
> to ensure that it is actually from a secure dma-buf allocation.  So
> I'm thinking an alternate solution might be cleaner so we don't have
> two memory types that are handled nearly identically in most of the
> code. What do you think about a new memory flag like
> V4L2_MEMORY_FLAG_SECURE?  This would be set in vb2_queue struct like
> the other existing memory flag. Then when it gets into
> __prepare_dmabuf and invokes attach_dmabuf on each buffer...that call
> could then check for the existence of that flag, and if it's there it
> could validate it is actually secure memory.  Then in various other
> dmabuf vb2_mem_ops (maybe alloc, get_userptr, vaddr and mmap) those
> could also check for the secure flag, and if present return an
> error/null.  Then also in the driver specific vb2_ops for queue_setup,
> the MTK driver could recognize the flag there and then configure
> itself for secure mode.
> 
> How does that sound as an overall strategy?

Yes, I actually had the same thought.

You would also need a new capability: V4L2_BUF_CAP_SUPPORTS_SECURE_MEMORY

It makes more sense than creating a new V4L2_MEMORY_ type, and it still
is handled at the right place (creating the buffers).

Regards,

	Hans

> 
> Cheers,
> Jeff
> 
> On Mon, Sep 25, 2023 at 9:51 AM Jeffrey Kardatzke <jkardatzke@xxxxxxxxxx> wrote:
>>
>> On Mon, Sep 25, 2023 at 2:00 AM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
>>>
>>> On 22/09/2023 21:17, Jeffrey Kardatzke wrote:
>>>> On Fri, Sep 22, 2023 at 1:44 AM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
>>>>>
>>>>> On 22/09/2023 05:28, Yunfei Dong (董云飞) wrote:
>>>>>> Hi Hans,
>>>>>>
>>>>>> Thanks for your help to give some good advice.
>>>>>> On Wed, 2023-09-20 at 09:20 +0200, Hans Verkuil wrote:
>>>>>>>
>>>>>>>>>>> In any case, using a control to switch to secure mode and using
>>>>>>> a control
>>>>>>>>>>> to convert a dmabuf fd to a secure handle seems a poor choice to
>>>>>>> me.
>>>>>>>>>>>
>>>>>>>>>>> I was wondering if it wouldn't be better to create a new
>>>>>>> V4L2_MEMORY_ type,
>>>>>>>>>>> e.g. V4L2_MEMORY_DMABUF_SECURE (or perhaps _DMABUF_OPTEE). That
>>>>>>> ensures that
>>>>>>>>>>> once you create buffers for the first time, the driver can
>>>>>>> switch into secure
>>>>>>>>>>> mode, and until all buffers are released again you know that the
>>>>>>> driver will
>>>>>>>>>>> stay in secure mode.
>>>>>>>>>>
>>>>>>>>>> Why do you think the control for setting secure mode is a poor
>>>>>>> choice?
>>>>>>>>>> There's various places in the driver code where functionality
>>>>>>> changes
>>>>>>>>>> based on being secure/non-secure mode, so this is very much a
>>>>>>> 'global'
>>>>>>>>>> setting for the driver. It could be inferred based off a new
>>>>>>> memory
>>>>>>>>>> type for the queues...which then sets that flag in the driver;
>>>>>>> but
>>>>>>>>>> that seems like it would be more fragile and would require
>>>>>>> checking
>>>>>>>>>> for incompatible output/capture memory types. I'm not against
>>>>>>> another
>>>>>>>>>> way of doing this; but didn't see why you think the proposed
>>>>>>> method is
>>>>>>>>>> a poor choice.
>>>>>>>>>
>>>>>>>>> I assume you are either decoding to secure memory all the time, or
>>>>>>> not
>>>>>>>>> at all. That's something you would want to select the moment you
>>>>>>> allocate
>>>>>>>>> the first buffer. Using the V4L2_MEMORY_ value would be the
>>>>>>> natural place
>>>>>>>>> for that. A control can typically be toggled at any time, and it
>>>>>>> makes
>>>>>>>>> no sense to do that for secure streaming.
>>>>>>>>>
>>>>>>>>> Related to that: if you pass a dmabuf fd you will need to check
>>>>>>> somewhere
>>>>>>>>> if the fd points to secure memory or not. You don't want to mix
>>>>>>> the two
>>>>>>>>> but you want to check that at VIDIOC_QBUF time.
>>>>>>>>>
>>>>>>>>> Note that the V4L2_MEMORY_ value is already checked in the v4l2
>>>>>>> core,
>>>>>>>>> drivers do not need to do that.
>>>>>>>>
>>>>>>>> Just to clarify a bit, and make sure I understand this too. You are
>>>>>>> proposing to
>>>>>>>> introduce something like:
>>>>>>>>
>>>>>>>>    V4L2_MEMORY_SECURE_DMABUF
>>>>>>>>
>>>>>>>> Which like V4L2_MEMORY_DMABUF is meant to import dmabuf, while
>>>>>>> telling the
>>>>>>>> driver that the memory is secure according to the definition of
>>>>>>> "secure" for the
>>>>>>>> platform its running on.
>>>>>>>>
>>>>>>>> This drivers also allocate secure SHM (a standard tee concept) and
>>>>>>> have internal
>>>>>>>> allocation for reconstruction buffer and some hw specific reference
>>>>>>> metadata. So
>>>>>>>> the idea would be that it would keep allocation using the dmabuf
>>>>>>> heap internal
>>>>>>>> APIs ? And decide which type of memory based on the memory type
>>>>>>> found in the
>>>>>>>> queue?
>>>>>>>
>>>>>>> Yes. Once you request the first buffer you basically tell the driver
>>>>>>> whether it
>>>>>>> will operate in secure or non-secure mode, and that stays that way
>>>>>>> until all
>>>>>>> buffers are freed. I think that makes sense.
>>>>>>>
>>>>>>
>>>>>> According to iommu's information, the dma operation for secure and non-
>>>>>> secure are the same, whether just need to add one memory type in v4l2
>>>>>> framework the same as V4L2_MEMORY_DMABUF? The dma operation in
>>>>>> videobuf2-dma-contig.c can use the same functions.
>>>>>
>>>>> So if I pass a non-secure dma fd to the capture queue of the codec, who
>>>>> will check that it can't write the data to that fd? Since doing so would
>>>>> expose the video. Presumably at some point the tee code will prevent that?
>>>>> (I sincerely hope so!)
>>>>
>>>> It is entirely the job of the TEE to prevent this. Nothing in the
>>>> kernel should allow exploitation of what happens in the TEE no matter
>>>> what goes on in the kernel
>>>>
>>>>>
>>>>> Having a separate V4L2_MEMORY_DMABUF_SECURE type is to indicate to the
>>>>> driver that 1) it can expect secure dmabuf fds, 2) it can configure itself
>>>>> for that (that avoids using a control to toggle between normal and secure mode),
>>>>> and at VIDIOC_QBUF time it is easy for the V4L2 core to verify that the
>>>>> fd that is passed in is for secure memory. This means that mistakes by
>>>>> userspace are caught at QBUF time.
>>>>>
>>>>> Of course, this will not protect you (people can disable this check by
>>>>> recompiling the kernel), that still has to be done by the firmware, but
>>>>> it catches userspace errors early on.
>>>>>
>>>>> Also, while for this hardware the DMA operation is the same, that might
>>>>> not be the case for other hardware.
>>>>
>>>> That's a really good point. So one of the other models that is used
>>>> for secure video decoding is to send the encrypted buffer into the
>>>> video decoder directly (i.e. V4L2_MEMORY_MMAP) and then also send in
>>>> all the corresponding crypto parameters (i.e. algorithm, IV,
>>>> encryption pattern, etc.). Then the video driver internally does the
>>>> decryption and decode in one operation.  That's not what we want to
>>>> use here for Mediatek; but I've done other integrations that work that
>>>> way (that was for VAAPI [1], not V4L2...but there are other ARM
>>>> implementations that do operate that way).  So if we end up requiring
>>>> V4L2_MEMORY_DMABUF_SECURE to indicate secure mode and enforce it on
>>>> output+capture, that'll close off other potential solutions in the
>>>> future.
>>>>
>>>> Expanding on your point about DMA operations being different on
>>>> various hardware, that also makes me think a general check for this in
>>>> v4l2 code may also be limiting. There are various ways secure video
>>>> pipelines are done, so leaving these checks up to the individual
>>>> drivers that implement secure video decode may be more pragmatic. If
>>>> there's a generic V4L2 _CID_SECURE_MODE control, that makes it more
>>>> general for how drivers can handle secure video decode.
>>>
>>> No, using a control for this is really wrong.
>>>
>>> The reason why I want it as a separate memory type is that that is
>>> what you use when you call VIDIOC_REQBUFS, and that ioctl is also
>>> when things are locked down in a driver. As long as no buffers have
>>> been allocated, you can still change formats, parameters, etc. But
>>> once buffers are allocated, most of that can't be changed, since
>>> changing e.g. the format would also change the buffer sizes.
>>>
>>> It also locks down who owns the buffers by storing the file descriptor.
>>> This prevents other processes from hijacking the I/O streaming, only
>>> the owner can stream buffers.
>>>
>>> So it is a natural point in the sequence for selecting secure
>>> buffers.
>>>
>>> If you request V4L2_MEMORY_DMABUF_SECURE for the output, then the
>>> capture side must also use DMABUF_SECURE. Whether or not you can
>>> use regular DMABUF for the output side and select DMABUF_SECURE
>>> on the capture side is a driver decision. It can be useful to
>>> support this for testing the secure capture using regular video
>>> streams (something Nicolas discussed as well), but it depends on
>>> the hardware whether you can use that technique.
>>
>> OK, that does work for the additional cases I mentioned.  And for
>> testing...we would still want to use DMABUF_SECURE on both ends for
>> Mediatek at least (that's the only way they support it). But rather
>> than having to bother with a clearkey implementation...we can just do
>> something that directly copies compressed video into the secure
>> dmabufs and then exercises the whole pipeline from there. This same
>> thing happens with the 'clear lead' that is sometimes there with
>> encrypted video (where the first X seconds are unencrypted and then it
>> switches to encrypted...but you're still using the secure video
>> pipeline on the unencrypted frames in that case).
>>
>>
>>>
>>> Regards,
>>>
>>>         Hans
>>>
>>>>
>>>> [1] - https://github.com/intel/libva/blob/master/va/va.h#L2177
>>>>
>>>>>
>>>>> Regards,
>>>>>
>>>>>         Hans
>>>>>
>>>>>>
>>>>>> Best Regards,
>>>>>> Yunfei Dong
>>>>>>
>>>>>
>>>