On 2021-06-09 14:15:33, Jens Wiklander wrote:

Hi,

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:

quoted

On 2021-06-09 09:59:04, Sumit Garg wrote:

quoted

Hi Tyler,

Hey Sumit - Thanks for the review.

quoted

On Wed, 9 Jun 2021 at 05:55, Tyler Hicks [off-list ref] wrote:

quoted

Uncouple the registration of dynamic shared memory buffers from the
TEE_SHM_DMA_BUF flag. Drivers may wish to allocate dynamic shared memory
regions but do not need them to be backed by a dma-buf when the memory
region is private to the driver.

In this case drivers should use tee_shm_register() instead where the
memory allocated is actually private to the driver. However, you need
to remove TEE_SHM_DMA_BUF as a mandatory flag for tee_shm_register().
Have a look at an example here [1]. So modifying tee_shm_alloc() for
this purpose doesn't look appropriate to me.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/keys/trusted-keys/trusted_tee.c#n73

I noticed what you did in commit 2a6ba3f794e8 ("tee: enable support to
register kernel memory") and considered moving ftpm and tee_bnxt_fw over
to tee_shm_register(). I think that's likely the right long term
approach but I decided against it since this series is a minimal set of
bug fixes that will hopefully go to stable (I'm affected by these bugs
in 5.4). Here are my reasons for feeling like moving to
tee_shm_register() isn't minimal in terms of a stable-focused fix:

- tee_shm_alloc() looks like it should work fine with AMD-TEE today.
  tee_shm_register() definitely does not since AMD-TEE doesn't provide a
  .shm_register or .shm_unregister hook. This may break existing users
  of AMD-TEE?

AFAIK, ftpm and tee_bnxt_fw drivers only support OP-TEE at this point.
See ftpm_tee_match() and optee_ctx_match() APIs in corresponding
drivers.

quoted

- tee_shm_register() has not historically been used for kernel
  allocations and is not fixed wrt the bug that Jens fixed in commit
  f1bbacedb0af ("tee: don't assign shm id for private shms").

Yes, that's what I meant earlier to make the TEE_SHM_DMA_BUF flag optional.

quoted

- 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.

quoted

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've just posted "[PATCH 0/7] tee: shared memory updates",
https://lore.kernel.org/lkml/20210609102324.2222332-1-jens.wiklander@linaro.org/ (local)

Where tee_shm_alloc() is replaced by among other functions
tee_shm_alloc_kernel_buf(). tee_shm_alloc_kernel_buf() takes care of the
problem with TEE_SHM_DMA_BUF.

Thanks! At first glance, that series would take care of the last three
patches in my kexec/kdump series.

I'm a bit worried that it is a rewrite of the shm allocator. Do you plan
to send all of that to stable? (I mentioned earlier in this thread that
I'm affected by these bugs in linux-5.4.y.)

Also, you and Sumit don't seem to have the same opinion on kernel
drivers making use of tee_shm_register() for allocations that are only
used internally. Can you comment on that?

I'm not clear on the next steps for fixing these kexec/kdump bugs in
older releases. I appreciate any guidance here.

Tyler

Cheers,
Jens