KVM: arm/arm64: Context-switch ptrauth registers

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: Christoffer Dall <christoffer.dall@arm.com>
Cc: kvmarm@lists.cs.columbia.edu
[maz: various fixups]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

1 files changed, 42 insertions, 8 deletions

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 7046c7686321..12bd72e42b91 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1007,6 +1007,37 @@ static bool access_pmuserenr(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
 	{ SYS_DESC(SYS_PMEVTYPERn_EL0(n)),					\
 	  access_pmu_evtyper, reset_unknown, (PMEVTYPER0_EL0 + n), }
 
+static bool trap_ptrauth(struct kvm_vcpu *vcpu,
+			 struct sys_reg_params *p,
+			 const struct sys_reg_desc *rd)
+{
+	kvm_arm_vcpu_ptrauth_trap(vcpu);
+
+	/*
+	 * Return false for both cases as we never skip the trapped
+	 * instruction:
+	 *
+	 * - Either we re-execute the same key register access instruction
+	 *   after enabling ptrauth.
+	 * - Or an UNDEF is injected as ptrauth is not supported/enabled.
+	 */
+	return false;
+}
+
+static unsigned int ptrauth_visibility(const struct kvm_vcpu *vcpu,
+			const struct sys_reg_desc *rd)
+{
+	return vcpu_has_ptrauth(vcpu) ? 0 : REG_HIDDEN_USER | REG_HIDDEN_GUEST;
+}
+
+#define __PTRAUTH_KEY(k)						\
+	{ SYS_DESC(SYS_## k), trap_ptrauth, reset_unknown, k,		\
+	.visibility = ptrauth_visibility}
+
+#define PTRAUTH_KEY(k)							\
+	__PTRAUTH_KEY(k ## KEYLO_EL1),					\
+	__PTRAUTH_KEY(k ## KEYHI_EL1)
+
 static bool access_arch_timer(struct kvm_vcpu *vcpu,
 			      struct sys_reg_params *p,
 			      const struct sys_reg_desc *r)
@@ -1053,14 +1084,11 @@ 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 (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;
@@ -1460,6 +1488,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 	{ SYS_DESC(SYS_TTBR1_EL1), access_vm_reg, reset_unknown, TTBR1_EL1 },
 	{ SYS_DESC(SYS_TCR_EL1), access_vm_reg, reset_val, TCR_EL1, 0 },
 
+	PTRAUTH_KEY(APIA),
+	PTRAUTH_KEY(APIB),
+	PTRAUTH_KEY(APDA),
+	PTRAUTH_KEY(APDB),
+	PTRAUTH_KEY(APGA),
+
 	{ SYS_DESC(SYS_AFSR0_EL1), access_vm_reg, reset_unknown, AFSR0_EL1 },
 	{ SYS_DESC(SYS_AFSR1_EL1), access_vm_reg, reset_unknown, AFSR1_EL1 },
 	{ SYS_DESC(SYS_ESR_EL1), access_vm_reg, reset_unknown, ESR_EL1 },