Date: Fri, 28 Jun 2024 16:16:55 -0400
From: Nicolas Dufresne <nicolas@...fresne.ca>
To: Christian König <christian.koenig@....com>, 
 Jason-JH Lin (林睿祥)
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Subject: Re: [PATCH v5 2/9] scatterlist: Add a flag for the restricted memory

Hi Christian,

Le jeudi 27 juin 2024 à 08:57 +0200, Christian König a écrit :
> Am 27.06.24 um 05:21 schrieb Jason-JH Lin (林睿祥):
> > 
> > On Wed, 2024-06-26 at 19:56 +0200, Daniel Vetter wrote:
> > >   
> > > External email : Please do not click links or open attachments until
> > > you have verified the sender or the content.
> > >  On Wed, Jun 26, 2024 at 12:49:02PM +0200, Christian König wrote:
> > > > Am 26.06.24 um 10:05 schrieb Jason-JH Lin (林睿祥):
> > > > > > > I think I have the same problem as the ECC_FLAG mention in:
> > > > > > > > > 
> > > https://lore.kernel.org/linux-media/20240515-dma-buf-ecc-heap-v1-0-54cbbd049511@kernel.org/
> > > > > > > > > I think it would be better to have the user configurable
> > > private
> > > > > > > information in dma-buf, so all the drivers who have the same
> > > > > > > requirement can get their private information from dma-buf
> > > directly
> > > > > > > and
> > > > > > > no need to change or add the interface.
> > > > > > > > > What's your opinion in this point?
> > > > > >  > Well of hand I don't see the need for that.
> > > > > > > What happens if you get a non-secure buffer imported in your
> > > secure
> > > > > > device?
> > > > > 
> > > > > We use the same mediatek-drm driver for secure and non-secure
> > > buffer.
> > > > > If non-secure buffer imported to mediatek-drm driver, it's go to
> > > the
> > > > > normal flow with normal hardware settings.
> > > > > 
> > > > > We use different configurations to make hardware have different
> > > > > permission to access the buffer it should access.
> > > > > 
> > > > > So if we can't get the information of "the buffer is allocated
> > > from
> > > > > restricted_mtk_cma" when importing the buffer into the driver, we
> > > won't
> > > > > be able to configure the hardware correctly.
> > > > 
> > > > Why can't you get this information from userspace?
> > > 
> > > Same reason amd and i915/xe also pass this around internally in the
> > > kernel, it's just that for those gpus the render and kms node are the
> > > same
> > > driver so this is easy.
> > > 
> 
> The reason I ask is that encryption here looks just like another 
> parameter for the buffer, e.g. like format, stride, tilling etc..

I'm mostly a reader of the thread here, but I'd like to avoid basic mistakes.
The buffer in question are "protected", meaning that the CPU HW does not have
access to the underlying pages (or zone in the case of Meditatek).

This is different from encrypted buffers, which don't need this level of
protection, as without the security key to decrypt them, their content is close
to random data.

> 
> So instead of this during buffer import:
> 
> mtk_gem->secure = (!strncmp(attach->dmabuf->exp_name, "restricted", 10));
> mtk_gem->dma_addr = sg_dma_address(sg->sgl);
> mtk_gem->size = attach->dmabuf->size;
> mtk_gem->sg = sg;
> 
> You can trivially say during use hey this buffer is encrypted.
> 
> At least that's my 10 mile high view, maybe I'm missing some extensive 
> key exchange or something like that.

If we take secure video path as an example, in the context of digital right
management, the handling of user session, retrieval of the device specific "key"
is entirely something for userspace to handle and the kernel have no business
into that. As long as the data is encrypted, its safe to carry around like any
other buffers.

It is only once decryption (usally done by a TF-A) that restricted memory start
being used. Initially in the form of a compressed video stream, and eventually
in the format of raw images.

> >   
> > > But on arm you have split designs everywhere and dma-buf
> > > import/export, so
> > > something else is needed. And neither current kms uapi nor
> > > protocols/extensions have provisions for this (afaik) because it
> > > works on
> > > the big gpus, and on android it's just hacked up with backchannels.
> > > 
> > > So yeah essentially I think we probably need something like this, as
> > > much
> > > as it sucks. I see it somewhat similar to handling pcip2pdma
> > > limitations
> > > in the kernel too.
> > > 
> > > Not sure where/how it should be handled though, and maybe I've missed
> > > something around protocols, in which case I guess we should add some
> > > secure buffer flags to the ADDFB2 ioctl.
> > 
> > Thanks for your hint, I'll try to add the secure flag to the ADDFB2
> > ioctl. If it works, I'll send the patch.
> 
> Yeah, exactly what I would suggest as well.
> 
> I'm not an expert for that part, but as far as I know we already have 
> bunch of device specific tilling flags in there.
> 
> Adding an MTK_ENCRYPTED flag should be trivial.

Just to be clear, my comment was just a concept correction, I also think its
nice to give a ADDFB2 flag a try, from my userspace experience, this is an easy
place to provide this type of information. Also, the V4L2 proposal for the same
endup with a flag at during buffer queue configuration, which is pretty close.

Nicolas

> 
> Regards,
> Christian.
> 
> > 
> > Regards,
> > Jason-JH.Lin
> > 
> > > -Sima
> > 
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