linux-kernel - Re: [PATCH v2 3/5] ARM: implement support for SMCCC TRNG entropy source

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Date:   Fri, 6 Nov 2020 16:30:50 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Marc Zyngier <maz@...terjones.org>
Cc:     Andre Przywara <andre.przywara@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 3/5] ARM: implement support for SMCCC TRNG entropy source

On Fri, 6 Nov 2020 at 16:30, Marc Zyngier <maz@...terjones.org> wrote:
>
> On 2020-11-05 12:56, Andre Przywara wrote:
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Implement arch_get_random_seed_*() for ARM based on the firmware
> > or hypervisor provided entropy source described in ARM DEN0098.
> >
> > This will make the kernel's random number generator consume entropy
> > provided by this interface, at early boot, and periodically at
> > runtime when reseeding.
> >
> > Cc: Linus Walleij <linus.walleij@...aro.org>
> > Cc: Russell King <linux@...linux.org.uk>
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > [Andre: rework to be initialised by the SMCCC firmware driver]
> > Signed-off-by: Andre Przywara <andre.przywara@....com>
> > ---
> >  arch/arm/Kconfig                  |  4 ++
> >  arch/arm/include/asm/archrandom.h | 64 +++++++++++++++++++++++++++++++
> >  2 files changed, 68 insertions(+)
> >
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index fe2f17eb2b50..06fda4f954fd 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -1667,6 +1667,10 @@ config STACKPROTECTOR_PER_TASK
> >         Enable this option to switch to a different method that uses a
> >         different canary value for each task.
> >
> > +config ARCH_RANDOM
> > +     def_bool y
> > +     depends on HAVE_ARM_SMCCC
> > +
> >  endmenu
> >
> >  menu "Boot options"
> > diff --git a/arch/arm/include/asm/archrandom.h
> > b/arch/arm/include/asm/archrandom.h
> > index a8e84ca5c2ee..f3e96a5b65f8 100644
> > --- a/arch/arm/include/asm/archrandom.h
> > +++ b/arch/arm/include/asm/archrandom.h
> > @@ -2,9 +2,73 @@
> >  #ifndef _ASM_ARCHRANDOM_H
> >  #define _ASM_ARCHRANDOM_H
> >
> > +#ifdef CONFIG_ARCH_RANDOM
> > +
> > +#include <linux/arm-smccc.h>
> > +#include <linux/kernel.h>
> > +
> > +#define ARM_SMCCC_TRNG_MIN_VERSION     0x10000UL
> > +
> > +extern bool smccc_trng_available;
> > +
> > +static inline bool __init smccc_probe_trng(void)
> > +{
> > +     struct arm_smccc_res res;
> > +
> > +     arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_VERSION, &res);
> > +     if ((s32)res.a0 < 0)
> > +             return false;
> > +     if (res.a0 >= ARM_SMCCC_TRNG_MIN_VERSION) {
> > +             /* double check that the 32-bit flavor is available */
> > +             arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_FEATURES,
> > +                                  ARM_SMCCC_TRNG_RND32,
> > +                                  &res);
> > +             if ((s32)res.a0 >= 0)
> > +                     return true;
> > +     }
> > +
> > +     return false;
> > +}
> > +
> > +static inline bool __must_check arch_get_random_long(unsigned long *v)
> > +{
> > +     return false;
> > +}
> > +
> > +static inline bool __must_check arch_get_random_int(unsigned int *v)
> > +{
> > +     return false;
> > +}
> > +
> > +static inline bool __must_check arch_get_random_seed_long(unsigned
> > long *v)
> > +{
> > +     struct arm_smccc_res res;
> > +
> > +     if (smccc_trng_available) {
> > +             arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND32, 8 * sizeof(*v), &res);
> > +
> > +             if (res.a0 != 0)
> > +                     return false;
> > +
> > +             *v = res.a3;
> > +             return true;
> > +     }
> > +
> > +     return false;
> > +}
> > +
> > +static inline bool __must_check arch_get_random_seed_int(unsigned int
> > *v)
> > +{
> > +     return arch_get_random_seed_long((unsigned long *)v);
>
> I don't think this cast is safe. At least not on 64bit.

True, but this is arch/arm