Re: [PATCH v3 5/7] tee: Support shm registration without dma-buf backing

Sumit Garg
2021-06-10T12:15:11Z

Hi Jens,

On Thu, 10 Jun 2021 at 12:48, Jens Wiklander <jens.wiklander@linaro.org> wrote:
>
> On Wed, Jun 09, 2021 at 04:22:49PM +0530, Sumit Garg wrote:
> > + Rijo
> >
> > On Wed, 9 Jun 2021 at 11:16, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
> [snip]
> >
> > > - tee_shm_alloc() performs allocations using contiguous pages
> > > from alloc_pages() while tee_shm_register() performs non-contiguous
> > > allocations with kcalloc(). I suspect this would be fine but I don't
> > > know the secure world side of these things well enough to assess the
> > > risk involved with such a change on the kernel side.
> > >
> >
> > I don't think that would make any difference.
>
> Agree.
>
> >
> > > I should have mentioned this in the cover letter but my hope was that
> > > these minimal changes would be accepted and then additional work could
> > > be done to merge tee_shm_alloc() and tee_shm_register() in a way that
> > > would allow the caller to request contiguous or non-contiguous pages,
> > > fix up the additional issues mentioned above, and then adjust the
> > > call sites in ftpm and tee_bnxt_fw as appropriate.
> > >
> > > I think that's a bigger set of changes because there are several things
> > > that still confuse/concern me:
> > >
> > > - Why does tee_shm_alloc() use TEE_SHM_MAPPED while tee_shm_register()
> > > uses TEE_SHM_KERNEL_MAPPED or TEE_SHM_USER_MAPPED? Why do all three
> > > exist?
> >
> > AFAIK, its due the the inherent nature of tee_shm_alloc() and
> > tee_shm_register() where tee_shm_alloc() doesn't need to know whether
> > its a kernel or user-space memory since it is the one that allocates
> > whereas tee_shm_register() need to know that since it has to register
> > pre-allocated client memory.
> >
> > > - Why does tee_shm_register() unconditionally use non-contiguous
> > > allocations without ever taking into account whether or not
> > > OPTEE_SMC_SEC_CAP_DYNAMIC_SHM was set? It sounds like that's required
> > > from my reading of https://optee.readthedocs.io/en/latest/architecture/core.html#noncontiguous-shared-buffers.
> >
> > Yeah, but do we have platforms in OP-TEE that don't support dynamic
> > shared memory? I guess it has become the sane default which is a
> > mandatory requirement when it comes to OP-TEE driver in u-boot.
> >
> > > - Why is TEE_SHM_REGISTER implemented at the TEE driver level when it is
> > > specific to OP-TEE? How to better abstract that away?
> > >
> >
> > I would like you to go through Section "3.2.4. Shared Memory" in TEE
> > Client API Specification. There are two standard ways for shared
> > memory approach with TEE:
> >
> > 1. A Shared Memory block can either be existing Client Application
> > memory (kernel driver in our case) which is subsequently registered
> > with the TEE Client API (using tee_shm_register() in our case).
> >
> > 2. Or memory which is allocated on behalf of the Client Application
> > using the TEE
> > Client API (using tee_shm_alloc() in our case).
> >
> > > Let me know if you agree with the more minimal approach that I took for
> > > these bug fix series or still feel like tee_shm_register() should be
> > > fixed up so that it is usable. Thanks!
> >
> > From drivers perspective I think the change should be:
> >
> > tee_shm_alloc()
> >
> > to
> >
> > kcalloc()
> > tee_shm_register()
>
> I had another approach in mind in "[PATCH 0/7] tee: shared memory updates",
> https://lore.kernel.org/lkml/20210609102324.2222332-1-jens.wiklander@linaro.org/
>
> The flags needed by tee_shm_alloc() and tee_shm_register() aren't
> very intuitive and in fact only accept quite few combinations. So my
> idea was to hide those flags from callers outside of the TEE subsystem
> with tee_shm_alloc_kernel_buf().
>

That looks like a good idea to hide flags from users. BTW, my only
objection earlier with Tyler's and your patch-set is the usage of
TEE_SHM_REGISTER flag in generic TEE methods: tee_shm_alloc*. AFAIU,
the only reason for such an additional flag is in case of OP-TEE only
because the OP-TEE driver could implement allocated shared memory via
re-using dynamic shared memory approach as well. And that additional
flag is only needed to differentiate that OP-TEE driver's private
memory shouldn't be registered with OP-TEE. If this understanding is
correct then we should introduce a separate flag as TEE_SHM_PRIV that
should only be set inside tee_shm_alloc_anon_kernel_buf().

As otherwise passing TEE_SHM_REGISTER flag for shared memory alloc API
for other TEEs like AMD-TEE etc. would be useless.

> The approach with tee_shm_register() you suggest above has the drawback
> that the TEE driver is forced to be able to handle any kernel memory.

That's the value-add in the problem that Tyler is trying to resolve
that driver should be able to free up the memory as needed as a
private buffer.

> This is OK with OP-TEE and dynamic shared memory enabled, but there are
> platforms where dynamic shared memory isn't enabled. In those case must
> the memory be allocated from a special pool.

Is there any limitation for those platforms to not support dynamic
shared memory in OP-TEE? If there isn't then we should able to handle
this via match for TEE_GEN_CAP_REG_MEM in the ftpm_tee_match() and
optee_ctx_match() APIs.

>
> Do you see any problem with instead replacing tee_shm_alloc()
> with tee_shm_alloc_kernel_buf()?

I don't see any problems apart from one mentioned above.

-Sumit

>
> Cheers,
> Jens