linux-kernel - Re: [PATCH v5 2/9] scatterlist: Add a flag for the restricted memory

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Date: Thu, 27 Jun 2024 16:40:02 +0200
From: "mripard@...nel.org" <mripard@...nel.org>
To: Christian König <christian.koenig@....com>
Cc: Jason-JH Lin (林睿祥) <Jason-JH.Lin@...iatek.com>, 
	"daniel@...ll.ch" <daniel@...ll.ch>, "quic_vjitta@...cinc.com" <quic_vjitta@...cinc.com>, 
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Subject: Re: [PATCH v5 2/9] scatterlist: Add a flag for the restricted memory

On Thu, Jun 27, 2024 at 08:57:40AM GMT, Christian König wrote:
> 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..
> 
> 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.

That doesn't work in all cases, unfortunately.

If you're doing secure video playback, the firmware is typically in
charge of the frame decryption/decoding, and you'd get dma-buf back that
aren't accessible by the CPU (or at least, not at the execution level
Linux runs with).

So nobody can map that buffer, and the firmware driver is the one who
knows that this buffer cannot be accessed by anyone. Putting this on the
userspace to know would be pretty weird, and wouldn't solve the case
where the kernel would try to map it.

Maxime

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