Merge branch kvm-arm64/nv-eret-pauth into kvmarm-master/next

* kvm-arm64/nv-eret-pauth:
  : .
  : Add NV support for the ERETAA/ERETAB instructions. From the cover letter:
  :
  : "Although the current upstream NV support has *some* support for
  : correctly emulating ERET, that support is only partial as it doesn't
  : support the ERETAA and ERETAB variants.
  :
  : Supporting these instructions was cast aside for a long time as it
  : involves implementing some form of PAuth emulation, something I wasn't
  : overly keen on. But I have reached a point where enough of the
  : infrastructure is there that it actually makes sense. So here it is!"
  : .
  KVM: arm64: nv: Work around lack of pauth support in old toolchains
  KVM: arm64: Drop trapping of PAuth instructions/keys
  KVM: arm64: nv: Advertise support for PAuth
  KVM: arm64: nv: Handle ERETA[AB] instructions
  KVM: arm64: nv: Add emulation for ERETAx instructions
  KVM: arm64: nv: Add kvm_has_pauth() helper
  KVM: arm64: nv: Reinject PAC exceptions caused by HCR_EL2.API==0
  KVM: arm64: nv: Handle HCR_EL2.{API,APK} independently
  KVM: arm64: nv: Honor HFGITR_EL2.ERET being set
  KVM: arm64: nv: Fast-track 'InHost' exception returns
  KVM: arm64: nv: Add trap forwarding for ERET and SMC
  KVM: arm64: nv: Configure HCR_EL2 for FEAT_NV2
  KVM: arm64: nv: Drop VCPU_HYP_CONTEXT flag
  KVM: arm64: Constraint PAuth support to consistent implementations
  KVM: arm64: Add helpers for ESR_ELx_ERET_ISS_ERET*
  KVM: arm64: Harden __ctxt_sys_reg() against out-of-range values

Signed-off-by: Marc Zyngier <maz@kernel.org>

9 files changed, 454 insertions, 108 deletions

diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
index c0c050e53157..04882b577575 100644
--- a/arch/arm64/kvm/Makefile
+++ b/arch/arm64/kvm/Makefile
@@ -23,6 +23,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
 	 vgic/vgic-its.o vgic/vgic-debug.o
 
 kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o pmu.o
+kvm-$(CONFIG_ARM64_PTR_AUTH)  += pauth.o
 
 always-y := hyp_constants.h hyp-constants.s
 
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index e57aa3502e13..c93ac8a77c27 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -35,10 +35,11 @@
 #include <asm/virt.h>
 #include <asm/kvm_arm.h>
 #include <asm/kvm_asm.h>
+#include <asm/kvm_emulate.h>
 #include <asm/kvm_mmu.h>
 #include <asm/kvm_nested.h>
 #include <asm/kvm_pkvm.h>
-#include <asm/kvm_emulate.h>
+#include <asm/kvm_ptrauth.h>
 #include <asm/sections.h>
 
 #include <kvm/arm_hypercalls.h>
@@ -218,6 +219,40 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	kvm_arm_teardown_hypercalls(kvm);
 }
 
+static bool kvm_has_full_ptr_auth(void)
+{
+	bool apa, gpa, api, gpi, apa3, gpa3;
+	u64 isar1, isar2, val;
+
+	/*
+	 * Check that:
+	 *
+	 * - both Address and Generic auth are implemented for a given
+         *   algorithm (Q5, IMPDEF or Q3)
+	 * - only a single algorithm is implemented.
+	 */
+	if (!system_has_full_ptr_auth())
+		return false;
+
+	isar1 = read_sanitised_ftr_reg(SYS_ID_AA64ISAR1_EL1);
+	isar2 = read_sanitised_ftr_reg(SYS_ID_AA64ISAR2_EL1);
+
+	apa = !!FIELD_GET(ID_AA64ISAR1_EL1_APA_MASK, isar1);
+	val = FIELD_GET(ID_AA64ISAR1_EL1_GPA_MASK, isar1);
+	gpa = (val == ID_AA64ISAR1_EL1_GPA_IMP);
+
+	api = !!FIELD_GET(ID_AA64ISAR1_EL1_API_MASK, isar1);
+	val = FIELD_GET(ID_AA64ISAR1_EL1_GPI_MASK, isar1);
+	gpi = (val == ID_AA64ISAR1_EL1_GPI_IMP);
+
+	apa3 = !!FIELD_GET(ID_AA64ISAR2_EL1_APA3_MASK, isar2);
+	val  = FIELD_GET(ID_AA64ISAR2_EL1_GPA3_MASK, isar2);
+	gpa3 = (val == ID_AA64ISAR2_EL1_GPA3_IMP);
+
+	return (apa == gpa && api == gpi && apa3 == gpa3 &&
+		(apa + api + apa3) == 1);
+}
+
 int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 {
 	int r;
@@ -311,7 +346,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		break;
 	case KVM_CAP_ARM_PTRAUTH_ADDRESS:
 	case KVM_CAP_ARM_PTRAUTH_GENERIC:
-		r = system_has_full_ptr_auth();
+		r = kvm_has_full_ptr_auth();
 		break;
 	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
 		if (kvm)
@@ -422,6 +457,44 @@ void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
 
 }
 
+static void vcpu_set_pauth_traps(struct kvm_vcpu *vcpu)
+{
+	if (vcpu_has_ptrauth(vcpu)) {
+		/*
+		 * Either we're running running an L2 guest, and the API/APK
+		 * bits come from L1's HCR_EL2, or API/APK are both set.
+		 */
+		if (unlikely(vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu))) {
+			u64 val;
+
+			val = __vcpu_sys_reg(vcpu, HCR_EL2);
+			val &= (HCR_API | HCR_APK);
+			vcpu->arch.hcr_el2 &= ~(HCR_API | HCR_APK);
+			vcpu->arch.hcr_el2 |= val;
+		} else {
+			vcpu->arch.hcr_el2 |= (HCR_API | HCR_APK);
+		}
+
+		/*
+		 * Save the host keys if there is any chance for the guest
+		 * to use pauth, as the entry code will reload the guest
+		 * keys in that case.
+		 * Protected mode is the exception to that rule, as the
+		 * entry into the EL2 code eagerly switch back and forth
+		 * between host and hyp keys (and kvm_hyp_ctxt is out of
+		 * reach anyway).
+		 */
+		if (is_protected_kvm_enabled())
+			return;
+
+		if (vcpu->arch.hcr_el2 & (HCR_API | HCR_APK)) {
+			struct kvm_cpu_context *ctxt;
+			ctxt = this_cpu_ptr_hyp_sym(kvm_hyp_ctxt);
+			ptrauth_save_keys(ctxt);
+		}
+	}
+}
+
 void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	struct kvm_s2_mmu *mmu;
@@ -460,8 +533,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	else
 		vcpu_set_wfx_traps(vcpu);
 
-	if (vcpu_has_ptrauth(vcpu))
-		vcpu_ptrauth_disable(vcpu);
+	vcpu_set_pauth_traps(vcpu);
+
 	kvm_arch_vcpu_load_debug_state_flags(vcpu);
 
 	if (!cpumask_test_cpu(cpu, vcpu->kvm->arch.supported_cpus))
@@ -1264,7 +1337,7 @@ static unsigned long system_supported_vcpu_features(void)
 	if (!system_supports_sve())
 		clear_bit(KVM_ARM_VCPU_SVE, &features);
 
-	if (!system_has_full_ptr_auth()) {
+	if (!kvm_has_full_ptr_auth()) {
 		clear_bit(KVM_ARM_VCPU_PTRAUTH_ADDRESS, &features);
 		clear_bit(KVM_ARM_VCPU_PTRAUTH_GENERIC, &features);
 	}
diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index 4697ba41b3a9..72d733c74a38 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2117,6 +2117,26 @@ inject:
 	return true;
 }
 
+static bool forward_traps(struct kvm_vcpu *vcpu, u64 control_bit)
+{
+	bool control_bit_set;
+
+	if (!vcpu_has_nv(vcpu))
+		return false;
+
+	control_bit_set = __vcpu_sys_reg(vcpu, HCR_EL2) & control_bit;
+	if (!is_hyp_ctxt(vcpu) && control_bit_set) {
+		kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+		return true;
+	}
+	return false;
+}
+
+bool forward_smc_trap(struct kvm_vcpu *vcpu)
+{
+	return forward_traps(vcpu, HCR_TSC);
+}
+
 static u64 kvm_check_illegal_exception_return(struct kvm_vcpu *vcpu, u64 spsr)
 {
 	u64 mode = spsr & PSR_MODE_MASK;
@@ -2152,37 +2172,39 @@ static u64 kvm_check_illegal_exception_return(struct kvm_vcpu *vcpu, u64 spsr)
 
 void kvm_emulate_nested_eret(struct kvm_vcpu *vcpu)
 {
-	u64 spsr, elr, mode;
-	bool direct_eret;
+	u64 spsr, elr, esr;
 
 	/*
-	 * Going through the whole put/load motions is a waste of time
-	 * if this is a VHE guest hypervisor returning to its own
-	 * userspace, or the hypervisor performing a local exception
-	 * return. No need to save/restore registers, no need to
-	 * switch S2 MMU. Just do the canonical ERET.
+	 * Forward this trap to the virtual EL2 if the virtual
+	 * HCR_EL2.NV bit is set and this is coming from !EL2.
 	 */
-	spsr = vcpu_read_sys_reg(vcpu, SPSR_EL2);
-	spsr = kvm_check_illegal_exception_return(vcpu, spsr);
-
-	mode = spsr & (PSR_MODE_MASK | PSR_MODE32_BIT);
-
-	direct_eret  = (mode == PSR_MODE_EL0t &&
-			vcpu_el2_e2h_is_set(vcpu) &&
-			vcpu_el2_tge_is_set(vcpu));
-	direct_eret |= (mode == PSR_MODE_EL2h || mode == PSR_MODE_EL2t);
-
-	if (direct_eret) {
-		*vcpu_pc(vcpu) = vcpu_read_sys_reg(vcpu, ELR_EL2);
-		*vcpu_cpsr(vcpu) = spsr;
-		trace_kvm_nested_eret(vcpu, *vcpu_pc(vcpu), spsr);
+	if (forward_traps(vcpu, HCR_NV))
 		return;
+
+	/* Check for an ERETAx */
+	esr = kvm_vcpu_get_esr(vcpu);
+	if (esr_iss_is_eretax(esr) && !kvm_auth_eretax(vcpu, &elr)) {
+		/*
+		 * Oh no, ERETAx failed to authenticate.  If we have
+		 * FPACCOMBINE, deliver an exception right away.  If we
+		 * don't, then let the mangled ELR value trickle down the
+		 * ERET handling, and the guest will have a little surprise.
+		 */
+		if (kvm_has_pauth(vcpu->kvm, FPACCOMBINE)) {
+			esr &= ESR_ELx_ERET_ISS_ERETA;
+			esr |= FIELD_PREP(ESR_ELx_EC_MASK, ESR_ELx_EC_FPAC);
+			kvm_inject_nested_sync(vcpu, esr);
+			return;
+		}
 	}
 
 	preempt_disable();
 	kvm_arch_vcpu_put(vcpu);
 
-	elr = __vcpu_sys_reg(vcpu, ELR_EL2);
+	spsr = __vcpu_sys_reg(vcpu, SPSR_EL2);
+	spsr = kvm_check_illegal_exception_return(vcpu, spsr);
+	if (!esr_iss_is_eretax(esr))
+		elr = __vcpu_sys_reg(vcpu, ELR_EL2);
 
 	trace_kvm_nested_eret(vcpu, elr, spsr);
 
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 617ae6dea5d5..b037f0a0e27e 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -56,6 +56,13 @@ static int handle_hvc(struct kvm_vcpu *vcpu)
 static int handle_smc(struct kvm_vcpu *vcpu)
 {
 	/*
+	 * Forward this trapped smc instruction to the virtual EL2 if
+	 * the guest has asked for it.
+	 */
+	if (forward_smc_trap(vcpu))
+		return 1;
+
+	/*
 	 * "If an SMC instruction executed at Non-secure EL1 is
 	 * trapped to EL2 because HCR_EL2.TSC is 1, the exception is a
 	 * Trap exception, not a Secure Monitor Call exception [...]"
@@ -207,19 +214,40 @@ static int handle_sve(struct kvm_vcpu *vcpu)
 }
 
 /*
- * Guest usage of a ptrauth instruction (which the guest EL1 did not turn into
- * a NOP). If we get here, it is that we didn't fixup ptrauth on exit, and all
- * that we can do is give the guest an UNDEF.
+ * Two possibilities to handle a trapping ptrauth instruction:
+ *
+ * - Guest usage of a ptrauth instruction (which the guest EL1 did not
+ *   turn into a NOP). If we get here, it is because we didn't enable
+ *   ptrauth for the guest. This results in an UNDEF, as it isn't
+ *   supposed to use ptrauth without being told it could.
+ *
+ * - Running an L2 NV guest while L1 has left HCR_EL2.API==0, and for
+ *   which we reinject the exception into L1.
+ *
+ * Anything else is an emulation bug (hence the WARN_ON + UNDEF).
  */
 static int kvm_handle_ptrauth(struct kvm_vcpu *vcpu)
 {
+	if (!vcpu_has_ptrauth(vcpu)) {
+		kvm_inject_undefined(vcpu);
+		return 1;
+	}
+
+	if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu)) {
+		kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+		return 1;
+	}
+
+	/* Really shouldn't be here! */
+	WARN_ON_ONCE(1);
 	kvm_inject_undefined(vcpu);
 	return 1;
 }
 
 static int kvm_handle_eret(struct kvm_vcpu *vcpu)
 {
-	if (kvm_vcpu_get_esr(vcpu) & ESR_ELx_ERET_ISS_ERET)
+	if (esr_iss_is_eretax(kvm_vcpu_get_esr(vcpu)) &&
+	    !vcpu_has_ptrauth(vcpu))
 		return kvm_handle_ptrauth(vcpu);
 
 	/*
diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index 2629420d0659..61772bf1d3e6 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -27,6 +27,7 @@
 #include <asm/kvm_hyp.h>
 #include <asm/kvm_mmu.h>
 #include <asm/kvm_nested.h>
+#include <asm/kvm_ptrauth.h>
 #include <asm/fpsimd.h>
 #include <asm/debug-monitors.h>
 #include <asm/processor.h>
@@ -271,10 +272,8 @@ static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
 	__deactivate_traps_hfgxtr(vcpu);
 }
 
-static inline void ___activate_traps(struct kvm_vcpu *vcpu)
+static inline void ___activate_traps(struct kvm_vcpu *vcpu, u64 hcr)
 {
-	u64 hcr = vcpu->arch.hcr_el2;
-
 	if (cpus_have_final_cap(ARM64_WORKAROUND_CAVIUM_TX2_219_TVM))
 		hcr |= HCR_TVM;
 
@@ -449,60 +448,6 @@ static inline bool handle_tx2_tvm(struct kvm_vcpu *vcpu)
 	return true;
 }
 
-static inline bool esr_is_ptrauth_trap(u64 esr)
-{
-	switch (esr_sys64_to_sysreg(esr)) {
-	case SYS_APIAKEYLO_EL1:
-	case SYS_APIAKEYHI_EL1:
-	case SYS_APIBKEYLO_EL1:
-	case SYS_APIBKEYHI_EL1:
-	case SYS_APDAKEYLO_EL1:
-	case SYS_APDAKEYHI_EL1:
-	case SYS_APDBKEYLO_EL1:
-	case SYS_APDBKEYHI_EL1:
-	case SYS_APGAKEYLO_EL1:
-	case SYS_APGAKEYHI_EL1:
-		return true;
-	}
-
-	return false;
-}
-
-#define __ptrauth_save_key(ctxt, key)					\
-	do {								\
-	u64 __val;                                                      \
-	__val = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);                \
-	ctxt_sys_reg(ctxt, key ## KEYLO_EL1) = __val;                   \
-	__val = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);                \
-	ctxt_sys_reg(ctxt, key ## KEYHI_EL1) = __val;                   \
-} while(0)
-
-DECLARE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
-
-static bool kvm_hyp_handle_ptrauth(struct kvm_vcpu *vcpu, u64 *exit_code)
-{
-	struct kvm_cpu_context *ctxt;
-	u64 val;
-
-	if (!vcpu_has_ptrauth(vcpu))
-		return false;
-
-	ctxt = this_cpu_ptr(&kvm_hyp_ctxt);
-	__ptrauth_save_key(ctxt, APIA);
-	__ptrauth_save_key(ctxt, APIB);
-	__ptrauth_save_key(ctxt, APDA);
-	__ptrauth_save_key(ctxt, APDB);
-	__ptrauth_save_key(ctxt, APGA);
-
-	vcpu_ptrauth_enable(vcpu);
-
-	val = read_sysreg(hcr_el2);
-	val |= (HCR_API | HCR_APK);
-	write_sysreg(val, hcr_el2);
-
-	return true;
-}
-
 static bool kvm_hyp_handle_cntpct(struct kvm_vcpu *vcpu)
 {
 	struct arch_timer_context *ctxt;
@@ -590,9 +535,6 @@ static bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
 	    __vgic_v3_perform_cpuif_access(vcpu) == 1)
 		return true;
 
-	if (esr_is_ptrauth_trap(kvm_vcpu_get_esr(vcpu)))
-		return kvm_hyp_handle_ptrauth(vcpu, exit_code);
-
 	if (kvm_hyp_handle_cntpct(vcpu))
 		return true;
 
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 1f82d531a494..563604b55ea2 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -40,7 +40,7 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
 {
 	u64 val;
 
-	___activate_traps(vcpu);
+	___activate_traps(vcpu, vcpu->arch.hcr_el2);
 	__activate_traps_common(vcpu);
 
 	val = vcpu->arch.cptr_el2;
@@ -191,7 +191,6 @@ static const exit_handler_fn hyp_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };
 
@@ -203,7 +202,6 @@ static const exit_handler_fn pvm_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };
 
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 0027cc9b985e..23794589c52e 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -33,11 +33,43 @@ DEFINE_PER_CPU(struct kvm_host_data, kvm_host_data);
 DEFINE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
 DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);
 
+/*
+ * HCR_EL2 bits that the NV guest can freely change (no RES0/RES1
+ * semantics, irrespective of the configuration), but that cannot be
+ * applied to the actual HW as things would otherwise break badly.
+ *
+ * - TGE: we want the guest to use EL1, which is incompatible with
+ *   this bit being set
+ *
+ * - API/APK: they are already accounted for by vcpu_load(), and can
+ *   only take effect across a load/put cycle (such as ERET)
+ */
+#define NV_HCR_GUEST_EXCLUDE	(HCR_TGE | HCR_API | HCR_APK)
+
+static u64 __compute_hcr(struct kvm_vcpu *vcpu)
+{
+	u64 hcr = vcpu->arch.hcr_el2;
+
+	if (!vcpu_has_nv(vcpu))
+		return hcr;
+
+	if (is_hyp_ctxt(vcpu)) {
+		hcr |= HCR_NV | HCR_NV2 | HCR_AT | HCR_TTLB;
+
+		if (!vcpu_el2_e2h_is_set(vcpu))
+			hcr |= HCR_NV1;
+
+		write_sysreg_s(vcpu->arch.ctxt.vncr_array, SYS_VNCR_EL2);
+	}
+
+	return hcr | (__vcpu_sys_reg(vcpu, HCR_EL2) & ~NV_HCR_GUEST_EXCLUDE);
+}
+
 static void __activate_traps(struct kvm_vcpu *vcpu)
 {
 	u64 val;
 
-	___activate_traps(vcpu);
+	___activate_traps(vcpu, __compute_hcr(vcpu));
 
 	if (has_cntpoff()) {
 		struct timer_map map;
@@ -177,6 +209,59 @@ void kvm_vcpu_put_vhe(struct kvm_vcpu *vcpu)
 	host_data_ptr(host_ctxt)->__hyp_running_vcpu = NULL;
 }
 
+static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
+{
+	u64 esr = kvm_vcpu_get_esr(vcpu);
+	u64 spsr, elr, mode;
+
+	/*
+	 * Going through the whole put/load motions is a waste of time
+	 * if this is a VHE guest hypervisor returning to its own
+	 * userspace, or the hypervisor performing a local exception
+	 * return. No need to save/restore registers, no need to
+	 * switch S2 MMU. Just do the canonical ERET.
+	 *
+	 * Unless the trap has to be forwarded further down the line,
+	 * of course...
+	 */
+	if ((__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_NV) ||
+	    (__vcpu_sys_reg(vcpu, HFGITR_EL2) & HFGITR_EL2_ERET))
+		return false;
+
+	spsr = read_sysreg_el1(SYS_SPSR);
+	mode = spsr & (PSR_MODE_MASK | PSR_MODE32_BIT);
+
+	switch (mode) {
+	case PSR_MODE_EL0t:
+		if (!(vcpu_el2_e2h_is_set(vcpu) && vcpu_el2_tge_is_set(vcpu)))
+			return false;
+		break;
+	case PSR_MODE_EL2t:
+		mode = PSR_MODE_EL1t;
+		break;
+	case PSR_MODE_EL2h:
+		mode = PSR_MODE_EL1h;
+		break;
+	default:
+		return false;
+	}
+
+	/* If ERETAx fails, take the slow path */
+	if (esr_iss_is_eretax(esr)) {
+		if (!(vcpu_has_ptrauth(vcpu) && kvm_auth_eretax(vcpu, &elr)))
+			return false;
+	} else {
+		elr = read_sysreg_el1(SYS_ELR);
+	}
+
+	spsr = (spsr & ~(PSR_MODE_MASK | PSR_MODE32_BIT)) | mode;
+
+	write_sysreg_el2(spsr, SYS_SPSR);
+	write_sysreg_el2(elr, SYS_ELR);
+
+	return true;
+}
+
 static const exit_handler_fn hyp_exit_handlers[] = {
 	[0 ... ESR_ELx_EC_MAX]		= NULL,
 	[ESR_ELx_EC_CP15_32]		= kvm_hyp_handle_cp15_32,
@@ -186,7 +271,7 @@ static const exit_handler_fn hyp_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
+	[ESR_ELx_EC_ERET]		= kvm_hyp_handle_eret,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };
 
@@ -201,7 +286,7 @@ static void early_exit_filter(struct kvm_vcpu *vcpu, u64 *exit_code)
 	 * If we were in HYP context on entry, adjust the PSTATE view
 	 * so that the usual helpers work correctly.
 	 */
-	if (unlikely(vcpu_get_flag(vcpu, VCPU_HYP_CONTEXT))) {
+	if (vcpu_has_nv(vcpu) && (read_sysreg(hcr_el2) & HCR_NV)) {
 		u64 mode = *vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT);
 
 		switch (mode) {
@@ -243,11 +328,6 @@ static int __kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
 	sysreg_restore_guest_state_vhe(guest_ctxt);
 	__debug_switch_to_guest(vcpu);
 
-	if (is_hyp_ctxt(vcpu))
-		vcpu_set_flag(vcpu, VCPU_HYP_CONTEXT);
-	else
-		vcpu_clear_flag(vcpu, VCPU_HYP_CONTEXT);
-
 	do {
 		/* Jump in the fire! */
 		exit_code = __guest_enter(vcpu);
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index ced30c90521a..6813c7c7f00a 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -35,13 +35,9 @@ static u64 limit_nv_id_reg(u32 id, u64 val)
 		break;
 
 	case SYS_ID_AA64ISAR1_EL1:
-		/* Support everything but PtrAuth and Spec Invalidation */
+		/* Support everything but Spec Invalidation */
 		val &= ~(GENMASK_ULL(63, 56)	|
-			 NV_FTR(ISAR1, SPECRES)	|
-			 NV_FTR(ISAR1, GPI)	|
-			 NV_FTR(ISAR1, GPA)	|
-			 NV_FTR(ISAR1, API)	|
-			 NV_FTR(ISAR1, APA));
+			 NV_FTR(ISAR1, SPECRES));
 		break;
 
 	case SYS_ID_AA64PFR0_EL1:
diff --git a/arch/arm64/kvm/pauth.c b/arch/arm64/kvm/pauth.c
new file mode 100644
index 000000000000..d5eb3ae876be
--- /dev/null
+++ b/arch/arm64/kvm/pauth.c
@@ -0,0 +1,206 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2024 - Google LLC
+ * Author: Marc Zyngier <maz@kernel.org>
+ *
+ * Primitive PAuth emulation for ERETAA/ERETAB.
+ *
+ * This code assumes that is is run from EL2, and that it is part of
+ * the emulation of ERETAx for a guest hypervisor. That's a lot of
+ * baked-in assumptions and shortcuts.
+ *
+ * Do no reuse for anything else!
+ */
+
+#include <linux/kvm_host.h>
+
+#include <asm/gpr-num.h>
+#include <asm/kvm_emulate.h>
+#include <asm/pointer_auth.h>
+
+/* PACGA Xd, Xn, Xm */
+#define PACGA(d,n,m)					\
+	asm volatile(__DEFINE_ASM_GPR_NUMS		\
+		     ".inst 0x9AC03000          |"	\
+		     "(.L__gpr_num_%[Rd] << 0)  |"	\
+		     "(.L__gpr_num_%[Rn] << 5)  |"	\
+		     "(.L__gpr_num_%[Rm] << 16)\n"	\
+		     : [Rd] "=r" ((d))			\
+		     : [Rn] "r" ((n)), [Rm] "r" ((m)))
+
+static u64 compute_pac(struct kvm_vcpu *vcpu, u64 ptr,
+		       struct ptrauth_key ikey)
+{
+	struct ptrauth_key gkey;
+	u64 mod, pac = 0;
+
+	preempt_disable();
+
+	if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
+		mod = __vcpu_sys_reg(vcpu, SP_EL2);
+	else
+		mod = read_sysreg(sp_el1);
+
+	gkey.lo = read_sysreg_s(SYS_APGAKEYLO_EL1);
+	gkey.hi = read_sysreg_s(SYS_APGAKEYHI_EL1);
+
+	__ptrauth_key_install_nosync(APGA, ikey);
+	isb();
+
+	PACGA(pac, ptr, mod);
+	isb();
+
+	__ptrauth_key_install_nosync(APGA, gkey);
+
+	preempt_enable();
+
+	/* PAC in the top 32bits */
+	return pac;
+}
+
+static bool effective_tbi(struct kvm_vcpu *vcpu, bool bit55)
+{
+	u64 tcr = vcpu_read_sys_reg(vcpu, TCR_EL2);
+	bool tbi, tbid;
+
+	/*
+	 * Since we are authenticating an instruction address, we have
+	 * to take TBID into account. If E2H==0, ignore VA[55], as
+	 * TCR_EL2 only has a single TBI/TBID. If VA[55] was set in
+	 * this case, this is likely a guest bug...
+	 */
+	if (!vcpu_el2_e2h_is_set(vcpu)) {
+		tbi = tcr & BIT(20);
+		tbid = tcr & BIT(29);
+	} else if (bit55) {
+		tbi = tcr & TCR_TBI1;
+		tbid = tcr & TCR_TBID1;
+	} else {
+		tbi = tcr & TCR_TBI0;
+		tbid = tcr & TCR_TBID0;
+	}
+
+	return tbi && !tbid;
+}
+
+static int compute_bottom_pac(struct kvm_vcpu *vcpu, bool bit55)
+{
+	static const int maxtxsz = 39; // Revisit these two values once
+	static const int mintxsz = 16; // (if) we support TTST/LVA/LVA2
+	u64 tcr = vcpu_read_sys_reg(vcpu, TCR_EL2);
+	int txsz;
+
+	if (!vcpu_el2_e2h_is_set(vcpu) || !bit55)
+		txsz = FIELD_GET(TCR_T0SZ_MASK, tcr);
+	else
+		txsz = FIELD_GET(TCR_T1SZ_MASK, tcr);
+
+	return 64 - clamp(txsz, mintxsz, maxtxsz);
+}
+
+static u64 compute_pac_mask(struct kvm_vcpu *vcpu, bool bit55)
+{
+	int bottom_pac;
+	u64 mask;
+
+	bottom_pac = compute_bottom_pac(vcpu, bit55);
+
+	mask = GENMASK(54, bottom_pac);
+	if (!effective_tbi(vcpu, bit55))
+		mask |= GENMASK(63, 56);
+
+	return mask;
+}
+
+static u64 to_canonical_addr(struct kvm_vcpu *vcpu, u64 ptr, u64 mask)
+{
+	bool bit55 = !!(ptr & BIT(55));
+
+	if (bit55)
+		return ptr | mask;
+
+	return ptr & ~mask;
+}
+
+static u64 corrupt_addr(struct kvm_vcpu *vcpu, u64 ptr)
+{
+	bool bit55 = !!(ptr & BIT(55));
+	u64 mask, error_code;
+	int shift;
+
+	if (effective_tbi(vcpu, bit55)) {
+		mask = GENMASK(54, 53);
+		shift = 53;
+	} else {
+		mask = GENMASK(62, 61);
+		shift = 61;
+	}
+
+	if (esr_iss_is_eretab(kvm_vcpu_get_esr(vcpu)))
+		error_code = 2 << shift;
+	else
+		error_code = 1 << shift;
+
+	ptr &= ~mask;
+	ptr |= error_code;
+
+	return ptr;
+}
+
+/*
+ * Authenticate an ERETAA/ERETAB instruction, returning true if the
+ * authentication succeeded and false otherwise. In all cases, *elr
+ * contains the VA to ERET to. Potential exception injection is left
+ * to the caller.
+ */
+bool kvm_auth_eretax(struct kvm_vcpu *vcpu, u64 *elr)
+{
+	u64 sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL2);
+	u64 esr = kvm_vcpu_get_esr(vcpu);
+	u64 ptr, cptr, pac, mask;
+	struct ptrauth_key ikey;
+
+	*elr = ptr = vcpu_read_sys_reg(vcpu, ELR_EL2);
+
+	/* We assume we're already in the context of an ERETAx */
+	if (esr_iss_is_eretab(esr)) {
+		if (!(sctlr & SCTLR_EL1_EnIB))
+			return true;
+
+		ikey.lo = __vcpu_sys_reg(vcpu, APIBKEYLO_EL1);
+		ikey.hi = __vcpu_sys_reg(vcpu, APIBKEYHI_EL1);
+	} else {
+		if (!(sctlr & SCTLR_EL1_EnIA))
+			return true;
+
+		ikey.lo = __vcpu_sys_reg(vcpu, APIAKEYLO_EL1);
+		ikey.hi = __vcpu_sys_reg(vcpu, APIAKEYHI_EL1);
+	}
+
+	mask = compute_pac_mask(vcpu, !!(ptr & BIT(55)));
+	cptr = to_canonical_addr(vcpu, ptr, mask);
+
+	pac = compute_pac(vcpu, cptr, ikey);
+
+	/*
+	 * Slightly deviate from the pseudocode: if we have a PAC
+	 * match with the signed pointer, then it must be good.
+	 * Anything after this point is pure error handling.
+	 */
+	if ((pac & mask) == (ptr & mask)) {
+		*elr = cptr;
+		return true;
+	}
+
+	/*
+	 * Authentication failed, corrupt the canonical address if
+	 * PAuth2 isn't implemented, or some XORing if it is.
+	 */
+	if (!kvm_has_pauth(vcpu->kvm, PAuth2))
+		cptr = corrupt_addr(vcpu, cptr);
+	else
+		cptr = ptr ^ (pac & mask);
+
+	*elr = cptr;
+	return false;
+}