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SubjectRe: [PATCH v10 2/5] KVM: arm/arm64: context-switch ptrauth registers
On Tue, Apr 23, 2019 at 10:12:35AM +0530, 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 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@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 <amit.kachhap@arm.com>
> Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Christoffer Dall <christoffer.dall@arm.com>
> Cc: kvmarm@lists.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].

[...]

> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c

[...]

> @@ -1058,9 +1087,11 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> (0xfUL << ID_AA64ISAR1_API_SHIFT) |
> (0xfUL << ID_AA64ISAR1_GPA_SHIFT) |
> (0xfUL << ID_AA64ISAR1_GPI_SHIFT);
> - if (val & ptrauth_mask)
> - kvm_debug("ptrauth unsupported for guests, suppressing\n");
> - val &= ~ptrauth_mask;
> + if (!vcpu_has_ptrauth(vcpu)) {
> + if (val & ptrauth_mask)
> + kvm_debug("ptrauth unsupported for guests, suppressing\n");
> + val &= ~ptrauth_mask;
> + }

Hmmm, didn't spot this before, but this error message no longer makes
sense now that KVM _does_ support pointer auth.

Without vcpu_has_ptrauth(vcpu), we should just silently mask out the
relevant ID fields now (same as for SVE).

The patch below should achieve that.

--8<--

From c6065122c5cccef57108dff990ce8fb43426f88e Mon Sep 17 00:00:00 2001
From: Dave Martin <Dave.Martin@arm.com>
Date: Wed, 24 Apr 2019 14:32:29 +0100
Subject: [PATCH] KVM: arm64: sys_regs: Remove warning about missing pointer
auth support

KVM does support pointer auth for guests now, so it is
inappropriate (and confusing) to print a warning to dmesg when
userspace explicitly does not ask for pointer auth to be turned on
for a vcpu.

So, just squash the virtual ptrauth ID_AA64ISAR1_EL1 fields when
appropriate and remove the warning.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
---
arch/arm64/kvm/sys_regs.c | 15 +++++----------
1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 7f06c2e..f599f5e 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1082,16 +1082,11 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,

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;
--
2.1.4
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