[PATCH v17 06/14] KVM: arm64: Validate GCS exception lock when emulating ERET

[Mark Brown <broonie@kernel.org>]

As per DDI0487 R_TYTWB GCS adds an additional case where an illegal
exception return can be generated. If all of:

 - PSTATE.EXLOCK is 0.
 - The EL is not being changed by the ERET.
 - GCSCR_ELx.EXLOCKEN is 1.

are true then the return is illegal. Emulate this behaviour when
emulating ERET for nested guests, while we're at it using the symbolic
definition for EXLOCK in SPSR.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/kvm_nested.h | 39 +++++++++++++++++++++++++++++++++++++
 arch/arm64/kvm/emulate-nested.c     |  5 ++++-
 arch/arm64/kvm/hyp/vhe/switch.c     |  4 ++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
index 012d711034d1..b2343f67d15b 100644
--- a/arch/arm64/include/asm/kvm_nested.h
+++ b/arch/arm64/include/asm/kvm_nested.h
@@ -240,6 +240,45 @@ static inline bool kvm_auth_eretax(struct kvm_vcpu *vcpu, u64 *elr)
 }
 #endif
 
+#ifdef CONFIG_ARM64_GCS
+/*
+ * A subset of the pseudocode ELFromSPSR(), validity checks are
+ * assumed to have been done in code that is not GCS specific.
+ */
+static inline int exlock_el_from_spsr(u64 spsr)
+{
+	return FIELD_GET(GENMASK(3, 2), spsr);
+}
+
+/* See IllegalExceptionReturn() pseudocode */
+static inline bool kvm_check_illegal_exlock_return(struct kvm_vcpu *vcpu,
+						   u64 spsr)
+{
+	u64 cur_el, target_el;
+
+	if (!kvm_has_gcs(vcpu->kvm))
+		return false;
+
+	if (vcpu->arch.ctxt.regs.pstate & PSR_EXLOCK_BIT)
+		return false;
+
+	cur_el = exlock_el_from_spsr(vcpu->arch.ctxt.regs.pstate);
+	target_el = exlock_el_from_spsr(spsr);
+
+	if (cur_el != target_el)
+		return false;
+
+	return read_sysreg_el1(SYS_GCSCR) & GCSCR_ELx_EXLOCKEN;
+}
+
+#else
+static inline bool kvm_check_illegal_exlock_return(struct kvm_vcpu *vcpu,
+						   u64 spsr)
+{
+	return false;
+}
+#endif
+
 #define KVM_NV_GUEST_MAP_SZ	(KVM_PGTABLE_PROT_SW1 | KVM_PGTABLE_PROT_SW0)
 
 static inline u64 kvm_encode_nested_level(struct kvm_s2_trans *trans)
diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
index b32742d9dd73..0f0723f22000 100644
--- a/arch/arm64/kvm/emulate-nested.c
+++ b/arch/arm64/kvm/emulate-nested.c
@@ -2740,10 +2740,13 @@ static u64 kvm_check_illegal_exception_return(struct kvm_vcpu *vcpu, u64 spsr)
 	 * - trying to return to an illegal M value
 	 * - trying to return to a 32bit EL
 	 * - trying to return to EL1 with HCR_EL2.TGE set
+	 * - GCSCR_ELx.EXLOCKEN is 1 and PSTATE.EXLOCK is 0 when attempting
+	 *   to return from ELx the same EL.
 	 */
 	if (mode == PSR_MODE_EL3t   || mode == PSR_MODE_EL3h ||
 	    mode == 0b00001         || (mode & BIT(1))       ||
 	    (spsr & PSR_MODE32_BIT) ||
+	    kvm_check_illegal_exlock_return(vcpu, spsr) ||
 	    (vcpu_el2_tge_is_set(vcpu) && (mode == PSR_MODE_EL1t ||
 					   mode == PSR_MODE_EL1h))) {
 		u64 mask;
@@ -2770,7 +2773,7 @@ static u64 kvm_check_illegal_exception_return(struct kvm_vcpu *vcpu, u64 spsr)
 
 		mask = PSR_MODE_MASK | PSR_MODE32_BIT;
 		if (kvm_has_feat(vcpu->kvm, ID_AA64PFR1_EL1, GCS, IMP))
-			mask |= BIT_ULL(34);	/* PSTATE.EXLOCK */
+			mask |= PSR_EXLOCK_BIT;
 
 		spsr |= *vcpu_cpsr(vcpu) & mask;
 		spsr |= PSR_IL_BIT;
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index bbe9cebd3d9d..e09d9a425689 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -371,6 +371,10 @@ static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
 		return false;
 	}
 
+	/* Push GCS exception lock failures into the slow path */
+	if (kvm_check_illegal_exlock_return(vcpu, spsr))
+		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)))

-- 
2.47.3