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

Sumit Garg
2021-06-09T10:54:16Z

+ Rijo

On Wed, 9 Jun 2021 at 11:16, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
>
> On 2021-06-09 09:59:04, Sumit Garg wrote:
> > Hi Tyler,
>
> Hey Sumit - Thanks for the review.
>
> >
> > On Wed, 9 Jun 2021 at 05:55, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
> > >
> > > 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.

> - 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.

> - 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.

> 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()

-Sumit

> > > Allow callers of tee_shm_alloc() to specify the TEE_SHM_REGISTER flag to.
> > > request registration. If the TEE implementation does not require dynamic
> > > shared memory to be registered, clear the flag prior to calling the
> > > corresponding pool alloc function. Update the OP-TEE driver to respect
> > > TEE_SHM_REGISTER, rather than TEE_SHM_DMA_BUF, when deciding whether to
> > > (un)register on alloc/free operations. The AMD-TEE driver continues to
> > > ignore the TEE_SHM_REGISTER flag.
> > >
> > > Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> > > ---
> > > drivers/tee/optee/shm_pool.c | 5 ++---
> > > drivers/tee/tee_shm.c | 11 ++++++++++-
> > > 2 files changed, 12 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/tee/optee/shm_pool.c b/drivers/tee/optee/shm_pool.c
> > > index da06ce9b9313..6054343a29fb 100644
> > > --- a/drivers/tee/optee/shm_pool.c
> > > +++ b/drivers/tee/optee/shm_pool.c
> > > @@ -27,7 +27,7 @@ static int pool_op_alloc(struct tee_shm_pool_mgr *poolm,
> > > shm->paddr = page_to_phys(page);
> > > shm->size = PAGE_SIZE << order;
> > >
> > > - if (shm->flags & TEE_SHM_DMA_BUF) {
> > > + if (shm->flags & TEE_SHM_REGISTER) {
> > > unsigned int nr_pages = 1 << order, i;
> > > struct page **pages;
> > >
> > > @@ -42,7 +42,6 @@ static int pool_op_alloc(struct tee_shm_pool_mgr *poolm,
> > > page++;
> > > }
> > >
> > > - shm->flags |= TEE_SHM_REGISTER;
> > > rc = optee_shm_register(shm->ctx, shm, pages, nr_pages,
> > > (unsigned long)shm->kaddr);
> > > kfree(pages);
> > > @@ -60,7 +59,7 @@ static int pool_op_alloc(struct tee_shm_pool_mgr *poolm,
> > > static void pool_op_free(struct tee_shm_pool_mgr *poolm,
> > > struct tee_shm *shm)
> > > {
> > > - if (shm->flags & TEE_SHM_DMA_BUF)
> > > + if (shm->flags & TEE_SHM_REGISTER)
> > > optee_shm_unregister(shm->ctx, shm);
> > >
> > > free_pages((unsigned long)shm->kaddr, get_order(shm->size));
> > > diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> > > index 00472f5ce22e..1c0176550b9c 100644
> > > --- a/drivers/tee/tee_shm.c
> > > +++ b/drivers/tee/tee_shm.c
> > > @@ -117,7 +117,7 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > return ERR_PTR(-EINVAL);
> > > }
> > >
> > > - if ((flags & ~(TEE_SHM_MAPPED | TEE_SHM_DMA_BUF))) {
> > > + if ((flags & ~(TEE_SHM_MAPPED | TEE_SHM_DMA_BUF | TEE_SHM_REGISTER))) {
> > > dev_err(teedev->dev.parent, "invalid shm flags 0x%x", flags);
> > > return ERR_PTR(-EINVAL);
> > > }
> > > @@ -137,6 +137,15 @@ struct tee_shm *tee_shm_alloc(struct tee_context *ctx, size_t size, u32 flags)
> > > goto err_dev_put;
> > > }
> > >
> > > + if (!teedev->desc->ops->shm_register ||
> > > + !teedev->desc->ops->shm_unregister) {
> > > + /* registration is not required by the TEE implementation */
> > > + flags &= ~TEE_SHM_REGISTER;
> > > + } else if (flags & TEE_SHM_DMA_BUF) {
> > > + /* all dma-buf backed shm allocations are registered */
> > > + flags |= TEE_SHM_REGISTER;
> > > + }
> > > +
> > > shm->flags = flags | TEE_SHM_POOL;
> > > shm->ctx = ctx;
> > > if (flags & TEE_SHM_DMA_BUF)
> > > --
> > > 2.25.1
> > >
> >