Re: [PATCH v6 09/10] optee: FF-A: dynamic restricted memory allocation

Sumit Garg
2025-04-08T09:20:28Z

On Tue, Apr 01, 2025 at 02:26:59PM +0200, Jens Wiklander wrote:
> On Tue, Apr 1, 2025 at 12:13 PM Sumit Garg <sumit.garg@kernel.org> wrote:
> >
> > + MM folks to seek guidance here.
> >
> > On Thu, Mar 27, 2025 at 09:07:34AM +0100, Jens Wiklander wrote:
> > > Hi Sumit,
> > >
> > > On Tue, Mar 25, 2025 at 8:42 AM Sumit Garg <sumit.garg@kernel.org> wrote:
> > > >
> > > > On Wed, Mar 05, 2025 at 02:04:15PM +0100, Jens Wiklander wrote:
> > > > > Add support in the OP-TEE backend driver dynamic restricted memory
> > > > > allocation with FF-A.
> > > > >
> > > > > The restricted memory pools for dynamically allocated restrict memory
> > > > > are instantiated when requested by user-space. This instantiation can
> > > > > fail if OP-TEE doesn't support the requested use-case of restricted
> > > > > memory.
> > > > >
> > > > > Restricted memory pools based on a static carveout or dynamic allocation
> > > > > can coexist for different use-cases. We use only dynamic allocation with
> > > > > FF-A.
> > > > >
> > > > > Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
> > > > > ---
> > > > > drivers/tee/optee/Makefile | 1 +
> > > > > drivers/tee/optee/ffa_abi.c | 143 ++++++++++++-
> > > > > drivers/tee/optee/optee_private.h | 13 +-
> > > > > drivers/tee/optee/rstmem.c | 329 ++++++++++++++++++++++++++++++
> > > > > 4 files changed, 483 insertions(+), 3 deletions(-)
> > > > > create mode 100644 drivers/tee/optee/rstmem.c
> > > > >
> >
> > <snip>
> >
> > > > > diff --git a/drivers/tee/optee/rstmem.c b/drivers/tee/optee/rstmem.c
> > > > > new file mode 100644
> > > > > index 000000000000..ea27769934d4
> > > > > --- /dev/null
> > > > > +++ b/drivers/tee/optee/rstmem.c
> > > > > @@ -0,0 +1,329 @@
> > > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > > +/*
> > > > > + * Copyright (c) 2025, Linaro Limited
> > > > > + */
> > > > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > > > > +
> > > > > +#include <linux/errno.h>
> > > > > +#include <linux/genalloc.h>
> > > > > +#include <linux/slab.h>
> > > > > +#include <linux/string.h>
> > > > > +#include <linux/tee_core.h>
> > > > > +#include <linux/types.h>
> > > > > +#include "optee_private.h"
> > > > > +
> > > > > +struct optee_rstmem_cma_pool {
> > > > > + struct tee_rstmem_pool pool;
> > > > > + struct gen_pool *gen_pool;
> > > > > + struct optee *optee;
> > > > > + size_t page_count;
> > > > > + u16 *end_points;
> > > > > + u_int end_point_count;
> > > > > + u_int align;
> > > > > + refcount_t refcount;
> > > > > + u32 use_case;
> > > > > + struct tee_shm *rstmem;
> > > > > + /* Protects when initializing and tearing down this struct */
> > > > > + struct mutex mutex;
> > > > > +};
> > > > > +
> > > > > +static struct optee_rstmem_cma_pool *
> > > > > +to_rstmem_cma_pool(struct tee_rstmem_pool *pool)
> > > > > +{
> > > > > + return container_of(pool, struct optee_rstmem_cma_pool, pool);
> > > > > +}
> > > > > +
> > > > > +static int init_cma_rstmem(struct optee_rstmem_cma_pool *rp)
> > > > > +{
> > > > > + int rc;
> > > > > +
> > > > > + rp->rstmem = tee_shm_alloc_cma_phys_mem(rp->optee->ctx, rp->page_count,
> > > > > + rp->align);
> > > > > + if (IS_ERR(rp->rstmem)) {
> > > > > + rc = PTR_ERR(rp->rstmem);
> > > > > + goto err_null_rstmem;
> > > > > + }
> > > > > +
> > > > > + /*
> > > > > + * TODO unmap the memory range since the physical memory will
> > > > > + * become inaccesible after the lend_rstmem() call.
> > > > > + */
> > > >
> > > > What's your plan for this TODO? I think we need a CMA allocator here
> > > > which can allocate un-mapped memory such that any cache speculation
> > > > won't lead to CPU hangs once the memory restriction comes into picture.
> > >
> > > What happens is platform-specific. For some platforms, it might be
> > > enough to avoid explicit access. Yes, a CMA allocator with unmapped
> > > memory or where memory can be unmapped is one option.
> >
> > Did you get a chance to enable real memory protection on RockPi board?
>
> No, I don't think I have access to the needed documentation for the
> board to set it up for relevant peripherals.
>
> > This will atleast ensure that mapped restricted memory without explicit
> > access works fine. Since otherwise once people start to enable real
> > memory restriction in OP-TEE, there can be chances of random hang ups
> > due to cache speculation.
>
> A hypervisor in the normal world can also make the memory inaccessible
> to the kernel. That shouldn't cause any hangups due to cache
> speculation.

The hypervisor should unmap the memory from EL2 translation tables which
I think should disallow the cache speculation to take place. However,
without hypervisor here the memory remains mapped in normal world which
can lead to cache speculation for restricted buffers. That's why we
should atleast test on one platform with real memory protection enabled
to rule out any assumptions we make.

-Sumit

>
> Cheers,
> Jens
>
> >
> > MM folks,
> >
> > Basically what we are trying to achieve here is a "no-map" DT behaviour
> > [1] which is rather dynamic in nature. The use-case here is that a memory
> > block allocated from CMA can be marked restricted at runtime where we
> > would like the Linux not being able to directly or indirectly (cache
> > speculation) access it. Once memory restriction use-case has been
> > completed, the memory block can be marked as normal and freed for
> > further CMA allocation.
> >
> > It will be apprciated if you can guide us regarding the appropriate APIs
> > to use for un-mapping/mamping CMA allocations for this use-case.
> >
> > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/reserved-memory/reserved-memory.yaml#L79
> >
> > -Sumit