Hi Jens,

On Thu, 10 Jun 2021 at 12:48, Jens Wiklander [off-list ref] wrote:

On Wed, Jun 09, 2021 at 04:22:49PM +0530, Sumit Garg wrote:

quoted

+ Rijo

On Wed, 9 Jun 2021 at 11:16, Tyler Hicks [off-list ref] wrote:

[snip]

quoted

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- 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.

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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.

quoted

- 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.

quoted

- 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).

quoted

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/ (local)

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