Re: [PATCH 2/2] arm64: Add support for SMCCC TRNG firmware interface

[Ard Biesheuvel]

On Wed, 7 Oct 2020 at 16:44, André Przywara <andre.przywara@xxxxxxx> wrote:
>
> On 07/10/2020 15:16, James Morse wrote:
>
> Hi,
>
> > On 06/10/2020 21:18, Andre Przywara wrote:
> >> The ARM architected TRNG firmware interface, described in ARM spec
> >> DEN0098[1], defines an ARM SMCCC based interface to a true random number
> >> generator, provided by firmware.
> >> This can be discovered via the SMCCC >=v1.1 interface, and provides
> >> up to 192 bits of entropy per call.
> >>
> >> Hook this SMC call into arm64's arch_get_random_*() implementation,
> >> coming to the rescue when the CPU does not implement the ARM v8.5 RNG
> >> system registers.
> >>
> >> For the detection, we piggy back on the PSCI/SMCCC discovery (which gives
> >> us the conduit to use: hvc or smc), then try to call the
> >> ARM_SMCCC_TRNG_VERSION function, which returns -1 if this interface is
> >> not implemented.
> >
> >>  arch/arm64/include/asm/archrandom.h | 83 +++++++++++++++++++++++++----
> >>  1 file changed, 73 insertions(+), 10 deletions(-)
> >
> >> diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
> >> index ffb1a40d5475..b6c291c42a48 100644
> >> --- a/arch/arm64/include/asm/archrandom.h
> >> +++ b/arch/arm64/include/asm/archrandom.h
> >> @@ -7,6 +7,13 @@
> >>  #include <linux/bug.h>
> >>  #include <linux/kernel.h>
> >>  #include <asm/cpufeature.h>
> >> +#include <linux/arm-smccc.h>
> >> +
> >> +static enum smc_trng_status {
> >> +    SMC_TRNG_UNKNOWN,
> >> +    SMC_TRNG_NOT_SUPPORTED,
> >> +    SMC_TRNG_SUPPORTED
> >> +} smc_trng_status = SMC_TRNG_UNKNOWN;
> >
> > Doesn't this static variable in a header file mean each file that includes this has its
> > own copy? Is that intentional?
>
> Right, and it's not intentional. It doesn't really break, but since
> random.h includes archrandom.h, we get an instance everywhere :-(
>
> I wasn't too happy with this detection method to begin with (and also
> not with stuffing everything into a header file), but wanted to
> accommodate the early case, where PSCI hasn't been initialised yet, and
> so we don't know the SMCCC conduit. A static key sounds better, but gets
> a bit hairy with this scenario, I think.
>
> Any ideas here?

I think the early case isn't worth obsessing about. PSCI is
initialized in setup_arch(), which gets called way before
rand_initialize(), which is where this functionality will get used the
first time typically. And kaslr_early_init() is called extremely
early, i.e., straight from head.S, and we should avoid adding any more
code there that sets global state (if kaslr_early_init() exits
successfully, the kernel will be unmapped and remapped again in a
different place, and BSS cleared again etc etc)

> I could copy Ard's solution and introduce random.c, if that makes more
> sense.
>
> Cheers,
> Andre