Re: [PATCH v4 6/8] tee: Support kernel shm registration without dma-buf backing

[Sumit Garg <sumit.garg@linaro.org>]

On Fri, 11 Jun 2021 at 18:46, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
>
> On 2021-06-11 08:10:01, Tyler Hicks wrote:
> > On 2021-06-11 10:46:20, Sumit Garg wrote:
> > > On Fri, 11 Jun 2021 at 02:39, Tyler Hicks <tyhicks@linux.microsoft.com> wrote:
> > > >
> > > > Uncouple the registration of kernel shared memory buffers from the
> > > > TEE_SHM_DMA_BUF flag. Drivers may wish to allocate multi-page contiguous
> > > > shared memory regions but do not need them to be backed by a dma-buf
> > > > when the memory region is only used by the driver.
> > > >
> > > > If the TEE implementation does not require 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.
> > > >
> > >
> > > That's the main point that no other TEE implementation would honour
> > > TEE_SHM_REGISTER and I think it's just the incorrect usage of
> > > TEE_SHM_REGISTER flag to suffice OP-TEE underlying implementation.
> > >
> > > > Allow callers of tee_shm_alloc_kernel_buf() to allocate and register a
> > > > shared memory region without the backing of dma-buf.
> > > >
> > > > Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> > > > ---
> > > >  drivers/tee/optee/shm_pool.c |  5 ++---
> > > >  drivers/tee/tee_shm.c        | 13 +++++++++++--
> > > >  2 files changed, 13 insertions(+), 5 deletions(-)
> > > >
> > >
> > > This patch is just mixing two separate approaches to TEE shared
> > > memory. Have a look at alternative suggestions below.
> > >
> > > > 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) {
> > >
> > > Here you can just do following check instead:
> > >
> > >        if (!(shm->flags & TEE_SHM_PRIV)) {
> >
> > This is a bug fix series that's intended to fix the current and older
> > kernels. tee_shm_alloc_anon_kernel_buf()/TEE_SHM_PRIV is not present in
> > older kernels and isn't required to fix these kexec/kdump bugs. Your
> > suggestion feels like something that should be done in the allocator
> > rewrite that Jens is working on to clean all of this up going forward.
>
> I want to add that I do fully agree with you that TEE_SHM_REGISTER is an
> OP-TEE thing and not a TEE thing. Ideally, it wouldn't be defined in
> tee_drv.h and would be completely private to the OP-TEE driver.
> Likewise, I don't think that tee_shm_register() should exist (certainly
> not at the TEE level) because it only works with OP-TEE.

I think there is some confusion going on. tee_shm_register() is a
standard interface that is listed in TEE client API specification as
an alternative approach to tee_shm_alloc(). As I earlier mentioned,
please read through "3.2.4. Shared Memory" in TEE Client API
Specification.

In the initial times, OP-TEE only supported tee_shm_alloc() approach
but with the addition of dynamic shared memory feature it became
possible to support tee_shm_register() as well but we had to add new
capability as TEE_GEN_CAP_REG_MEM in order to maintain OP-TEE
backwards compatibility. It can very well be the same case for AMD-TEE
which currently only supports tee_shm_alloc() approach.

The reason for confusion here seems to be that OP-TEE driver is
providing a way to leverage dynamic shared memory approach in order to
implement tee_shm_alloc() but that doesn't mean at TEE level we should
intermix both approaches via using TEE_SHM_REGISTER to implement
tee_shm_alloc().

>
> That said, I think the first step is to fix the kexec/kdump bugs and the
> second step is to clean up the code to remove the layering violation of
> exposing shm registration from the TEE interfaces.
>

Doesn't the following patch sound suitable to be backported to a
stable kernel? It has even less changes compared to your patch as well
:).

-Sumit

-------------------------------------------
Subject: [PATCH] tee: Correct inappropriate usage of TEE_SHM_DMA_BUF flag

Currently TEE_SHM_DMA_BUF flag has been inappropriately used to not
register shared memory allocated for private usage by underlying TEE
driver: OP-TEE in this case. So rather add a new flag as TEE_SHM_PRIV
that can be utilized by underlying TEE drivers for private allocation
and usage of shared memory.

With this corrected, allow tee_shm_alloc_kernel_buf() to allocate a
shared memory region without the backing of dma-buf.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
---
 drivers/tee/optee/call.c     | 2 +-
 drivers/tee/optee/core.c     | 3 ++-
 drivers/tee/optee/shm_pool.c | 8 ++++++--
 drivers/tee/tee_shm.c        | 2 +-
 include/linux/tee_drv.h      | 1 +
 5 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/tee/optee/call.c b/drivers/tee/optee/call.c
index 6132cc8d014c..faaa13c9172b 100644
--- a/drivers/tee/optee/call.c
+++ b/drivers/tee/optee/call.c
@@ -184,7 +184,7 @@ static struct tee_shm *get_msg_arg(struct
tee_context *ctx, size_t num_params,
        struct optee_msg_arg *ma;

        shm = tee_shm_alloc(ctx, OPTEE_MSG_GET_ARG_SIZE(num_params),
-                           TEE_SHM_MAPPED);
+                           TEE_SHM_MAPPED | TEE_SHM_PRIV);
        if (IS_ERR(shm))
                return shm;

diff --git a/drivers/tee/optee/core.c b/drivers/tee/optee/core.c
index ddb8f9ecf307..eac0e91ec559 100644
--- a/drivers/tee/optee/core.c
+++ b/drivers/tee/optee/core.c
@@ -277,7 +277,8 @@ static void optee_release(struct tee_context *ctx)
        if (!ctxdata)
                return;

-       shm = tee_shm_alloc(ctx, sizeof(struct optee_msg_arg), TEE_SHM_MAPPED);
+       shm = tee_shm_alloc(ctx, sizeof(struct optee_msg_arg),
+                           TEE_SHM_MAPPED | TEE_SHM_PRIV);
        if (!IS_ERR(shm)) {
                arg = tee_shm_get_va(shm, 0);
                /*
diff --git a/drivers/tee/optee/shm_pool.c b/drivers/tee/optee/shm_pool.c
index d767eebf30bd..3b4a3853a10f 100644
--- a/drivers/tee/optee/shm_pool.c
+++ b/drivers/tee/optee/shm_pool.c
@@ -27,7 +27,11 @@ 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) {
+       /*
+        * Shared memory private to the OP-TEE driver doesn't need
+        * to be registered with OP-TEE.
+        */
+       if (!(shm->flags & TEE_SHM_PRIV)) {
                unsigned int nr_pages = 1 << order, i;
                struct page **pages;

@@ -52,7 +56,7 @@ 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_PRIV))
                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 c425ad80d6a6..f8b73e734094 100644
--- a/drivers/tee/tee_shm.c
+++ b/drivers/tee/tee_shm.c
@@ -207,7 +207,7 @@ EXPORT_SYMBOL_GPL(tee_shm_alloc);
  */
 struct tee_shm *tee_shm_alloc_kernel_buf(struct tee_context *ctx, size_t size)
 {
-       return tee_shm_alloc(ctx, size, TEE_SHM_MAPPED | TEE_SHM_DMA_BUF);
+       return tee_shm_alloc(ctx, size, TEE_SHM_MAPPED);
 }
 EXPORT_SYMBOL_GPL(tee_shm_alloc_kernel_buf);
diff --git a/include/linux/tee_drv.h b/include/linux/tee_drv.h
index 8990f7628387..3ebfea0781f1 100644
--- a/include/linux/tee_drv.h
+++ b/include/linux/tee_drv.h
@@ -27,6 +27,7 @@
 #define TEE_SHM_USER_MAPPED    BIT(4)  /* Memory mapped in user space */
 #define TEE_SHM_POOL           BIT(5)  /* Memory allocated from pool */
 #define TEE_SHM_KERNEL_MAPPED  BIT(6)  /* Memory mapped in kernel space */
+#define TEE_SHM_PRIV           BIT(7)  /* Memory private to TEE driver */

 struct device;
 struct tee_device;