Re: [PATCH v7 7/10] KVM: arm/arm64: context-switch ptrauth registers

[Kristina Martsenko]

On 19/03/2019 08:30, Amit Daniel Kachhap wrote:
> From: Mark Rutland <mark.rutland@xxxxxxx>
> 
> 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 preperation
> 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@xxxxxxx>
> [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@xxxxxxx>
> Reviewed-by: Julien Thierry <julien.thierry@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Christoffer Dall <christoffer.dall@xxxxxxx>
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx

[...]

> +/* SPDX-License-Identifier: GPL-2.0
> + * arch/arm64/include/asm/kvm_ptrauth_asm.h: Guest/host ptrauth save/restore
> + * Copyright 2019 Arm Limited
> + * Author: Mark Rutland <mark.rutland@xxxxxxx>
> + *         Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> + */

I think the license needs to be in its own comment, like

/* SPDX-License-Identifier: GPL-2.0 */
/* arch/arm64/include/asm/kvm_ptrauth_asm.h: ...
 * ...
 */

> +
> +#ifndef __ASM_KVM_ASM_PTRAUTH_H
> +#define __ASM_KVM_ASM_PTRAUTH_H

__ASM_KVM_PTRAUTH_ASM_H ? (to match the file name)

> +	if (test_bit(KVM_ARM_VCPU_PTRAUTH_ADDRESS, vcpu->arch.features) ||
> +		test_bit(KVM_ARM_VCPU_PTRAUTH_GENERIC, vcpu->arch.features)) {
> +		/* Verify that KVM startup matches the conditions for ptrauth */
> +		if (WARN_ON(!vcpu_has_ptrauth(vcpu)))
> +			return -EINVAL;
> +	}

I think this now needs to have "goto out;" instead of "return -EINVAL;",
since 5.1-rcX contains commit e761a927bc9a ("KVM: arm/arm64: Reset the
VCPU without preemption and vcpu state loaded") which changed some of
this code.

> @@ -385,6 +385,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		vcpu_clear_wfe_traps(vcpu);
>  	else
>  		vcpu_set_wfe_traps(vcpu);
> +
> +	kvm_arm_vcpu_ptrauth_setup_lazy(vcpu);

This version of the series seems to have lost the arch/arm/ definition
of kvm_arm_vcpu_ptrauth_setup_lazy (previously
kvm_arm_vcpu_ptrauth_reset), so KVM no longer compiles for arch/arm/ :(

Thanks,
Kristina