linux-kernel - Re: [PATCH v10 2/5] KVM: arm/arm64: context-switch ptrauth registers

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Message-ID: <20190424143041.GF17864@e103592.cambridge.arm.com>
Date:   Wed, 24 Apr 2019 14:30:43 +0000
From:   Dave P Martin <Dave.Martin@....com>
To:     Marc Zyngier <Marc.Zyngier@....com>
CC:     Amit Kachhap <Amit.Kachhap@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        Kristina Martsenko <Kristina.Martsenko@....com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v10 2/5] KVM: arm/arm64: context-switch ptrauth registers

On Wed, Apr 24, 2019 at 03:29:26PM +0100, Marc Zyngier wrote:
> On 24/04/2019 14:39, Dave Martin wrote:
> > On Tue, Apr 23, 2019 at 10:12:35AM +0530, Amit Daniel Kachhap wrote:
> >> From: Mark Rutland <mark.rutland@....com>
> >>
> >> When pointer authentication is supported, a guest may wish to use it.
> >> This patch adds the necessary KVM infrastructure for this to work, with
> >> a semi-lazy context switch of the pointer auth state.
> >>
> >> Pointer authentication feature is only enabled when VHE is built
> >> in the kernel and present in the CPU implementation so only VHE code
> >> paths are modified.
> >>
> >> When we schedule a vcpu, we disable guest usage of pointer
> >> authentication instructions and accesses to the keys. While these are
> >> disabled, we avoid context-switching the keys. When we trap the guest
> >> trying to use pointer authentication functionality, we change to eagerly
> >> context-switching the keys, and enable the feature. The next time the
> >> vcpu is scheduled out/in, we start again. However the host key save is
> >> optimized and implemented inside ptrauth instruction/register access
> >> trap.
> >>
> >> Pointer authentication consists of address authentication and generic
> >> authentication, and CPUs in a system might have varied support for
> >> either. Where support for either feature is not uniform, it is hidden
> >> from guests via ID register emulation, as a result of the cpufeature
> >> framework in the host.
> >>
> >> Unfortunately, address authentication and generic authentication cannot
> >> be trapped separately, as the architecture provides a single EL2 trap
> >> covering both. If we wish to expose one without the other, we cannot
> >> prevent a (badly-written) guest from intermittently using a feature
> >> which is not uniformly supported (when scheduled on a physical CPU which
> >> supports the relevant feature). Hence, this patch expects both type of
> >> authentication to be present in a cpu.
> >>
> >> This switch of key is done from guest enter/exit assembly as preparation
> >> for the upcoming in-kernel pointer authentication support. Hence, these
> >> key switching routines are not implemented in C code as they may cause
> >> pointer authentication key signing error in some situations.
> >>
> >> Signed-off-by: Mark Rutland <mark.rutland@....com>
> >> [Only VHE, key switch in full assembly, vcpu_has_ptrauth checks
> >> , save host key in ptrauth exception trap]
> >> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
> >> Reviewed-by: Julien Thierry <julien.thierry@....com>
> >> Cc: Marc Zyngier <marc.zyngier@....com>
> >> Cc: Christoffer Dall <christoffer.dall@....com>
> >> Cc: kvmarm@...ts.cs.columbia.edu
> >> ---
> >> Changes since v9:
> >>
> >> * Removed hardcoding of enum values[Mark Zyngier].
> >> * Changed kvm_ptrauth_asm.h to kvm_ptrauth.h[Mark Zyngier].
> >> * Removed macro __ptrauth_save_state and applied inline [Marc Zyngier].
> >> * Moved kvm_arm_vcpu_ptrauth_setup_lazy, kvm_arm_vcpu_ptrauth_enable and
> >>   kvm_arm_vcpu_ptrauth_disable from *.c to kvm_emulate.h file [Marc Zyngier].
> >> * Added/Modified comments at few places [Marc Zyngier].

[...]

> >  if (id == SYS_ID_AA64PFR0_EL1 && !vcpu_has_sve(vcpu)) {
> >  val &= ~(0xfUL << ID_AA64PFR0_SVE_SHIFT);
> > -} else if (id == SYS_ID_AA64ISAR1_EL1) {
> > -const u64 ptrauth_mask = (0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> > - (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> > - (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> > - (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
> > -if (!vcpu_has_ptrauth(vcpu)) {
> > -if (val & ptrauth_mask)
> > -kvm_debug("ptrauth unsupported for guests, suppressing\n");
> > -val &= ~ptrauth_mask;
> > -}
> > +} else if (id == SYS_ID_AA64ISAR1_EL1 && !vcpu_has_ptrauth(vcpu)) {
> > +val &= ~((0xfUL << ID_AA64ISAR1_APA_SHIFT) |
> > + (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> > + (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> > + (0xfUL << ID_AA64ISAR1_GPI_SHIFT));
> >  }
> >
> >  return val;
> >
>
> Since the series isn't in -next yet, I've squashed the above with the
> other nits I had in -queue.

Okey doke.

Thanks
---Dave
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