On 19/03/2019 08:30, Amit Daniel Kachhap wrote:

From: Mark Rutland <mark.rutland@arm.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 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@arm.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 <redacted>
Reviewed-by: Julien Thierry <redacted>
Cc: Marc Zyngier <redacted>
Cc: Christoffer Dall <redacted>
Cc: kvmarm@lists.cs.columbia.edu

[...]

+/* 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 [off-list ref]
+ *         Amit Daniel Kachhap [off-list ref]
+ */

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.

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

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