Date: Fri, 28 Jun 2024 13:47:01 +0200
From: Thierry Reding <thierry.reding@...il.com>
To: "mripard@...nel.org" <mripard@...nel.org>
Cc: Christian König <christian.koenig@....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 04:40:02PM GMT, mripard@...nel.org wrote:
> 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).

Can you clarify which firmware you're talking about? Is this secure
firmware, or firmware running on the video decoding hardware?

> 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.

Doesn't userspace need to know from the start whether it's trying to do
secure playback or not? Typically this involves more than just the
decoding part. You'd typically set up things like HDCP as part of the
process, so userspace probably already does know that the buffers being
passed around are protected.

Also, the kernel shouldn't really be mapping these buffers unless
explicitly told to. In most cases you also wouldn't want the kernel to
map these kinds of buffers, right? Are there any specific cases where
you expect the kernel to need to map these?

I've been looking at this on the Tegra side recently and the way it
works on these chips is that you basically get an opaque carveout region
that has been locked down by secure firmware or early bootloaders, so
only certain hardware blocks can access it. We can allocate from that
carveout and then pass the buffers around.

It may be possible to use these protected carveout regions exclusively
from the DRM/KMS driver and share them with multimedia engines via DMA-
BUF, but I've also been looking into perhaps using DMA-BUF heaps to
expose the carveout, which would make this a bit more flexible and allow
either userspace to allocate the buffers or have multiple kernel drivers
share the carveout via the DMA-BUF heap. Though the latter would require
that there be in-kernel APIs for heaps, so not too sure about that yet.

Thierry

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