Re: [PATCH v6 4/6] arm64/kvm: add a userspace option to enable pointer authentication

[Amit Daniel Kachhap]

Hi,

On 2/21/19 6:04 PM, Mark Rutland wrote:
> On Tue, Feb 19, 2019 at 02:54:29PM +0530, Amit Daniel Kachhap wrote:
> > This feature will allow the KVM guest to allow the handling of
> > pointer authentication instructions or to treat them as undefined
> > if not set. It uses the existing vcpu API KVM_ARM_VCPU_INIT to
> > supply this parameter instead of creating a new API.
> >
> > A new register is not created to pass this parameter via
> > SET/GET_ONE_REG interface as just a flag (KVM_ARM_VCPU_PTRAUTH)
> > supplied is enough to enable this feature.
> >
> > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> > Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> > ---
> >   Documentation/arm64/pointer-authentication.txt |  9 +++++----
> >   Documentation/virtual/kvm/api.txt              |  4 ++++
> >   arch/arm64/include/asm/kvm_host.h              |  3 ++-
> >   arch/arm64/include/uapi/asm/kvm.h              |  1 +
> >   arch/arm64/kvm/handle_exit.c                   |  2 +-
> >   arch/arm64/kvm/hyp/ptrauth-sr.c                | 16 +++++++++++++++-
> >   arch/arm64/kvm/reset.c                         |  3 +++
> >   arch/arm64/kvm/sys_regs.c                      | 26 +++++++++++++-------------
> >   include/uapi/linux/kvm.h                       |  1 +
> >   9 files changed, 45 insertions(+), 20 deletions(-)
> >
> > diff --git a/Documentation/arm64/pointer-authentication.txt b/Documentation/arm64/pointer-authentication.txt
> > index a25cd21..0529a7d 100644
> > --- a/Documentation/arm64/pointer-authentication.txt
> > +++ b/Documentation/arm64/pointer-authentication.txt
> > @@ -82,7 +82,8 @@ pointers).
> >   Virtualization
> >   --------------
> >   
> > -Pointer authentication is not currently supported in KVM guests. KVM
> > -will mask the feature bits from ID_AA64ISAR1_EL1, and attempted use of
> > -the feature will result in an UNDEFINED exception being injected into
> > -the guest.
> > +Pointer authentication is enabled in KVM guest when virtual machine is
> > +created by passing a flag (KVM_ARM_VCPU_PTRAUTH) requesting this feature
> > +to be enabled. Without this flag, pointer authentication is not enabled
> > +in KVM guests and attempted use of the feature will result in an UNDEFINED
> > +exception being injected into the guest.
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index 356156f..1e646fb 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -2642,6 +2642,10 @@ Possible features:
> >   	  Depends on KVM_CAP_ARM_PSCI_0_2.
> >   	- KVM_ARM_VCPU_PMU_V3: Emulate PMUv3 for the CPU.
> >   	  Depends on KVM_CAP_ARM_PMU_V3.
> > +	- KVM_ARM_VCPU_PTRAUTH: Emulate Pointer authentication for the CPU.
> > +	  Depends on KVM_CAP_ARM_PTRAUTH and only on arm64 architecture. If
> > +	  set, then the KVM guest allows the execution of pointer authentication
> > +	  instructions. Otherwise, KVM treats these instructions as undefined.
>
> I think that we should have separate flags for address auth and generic
> auth, to match the ID register split.
>
> For now, we can have KVM only support the case where both are set, but
> it gives us freedom to support either in isolation if we have to in
> future, without an ABI break.
yes agree with you about having two address and generic ptrauth flags. 
Will add in next iteration.

Thanks,
Amit D
> Thanks,
> Mark.