KVM: arm64: Save the host's PtrAuth keys in non-preemptible context

BugLink: https://bugs.launchpad.net/bugs/1883938

commit ef3e40a upstream.

When using the PtrAuth feature in a guest, we need to save the host's
keys before allowing the guest to program them. For that, we dump
them in a per-CPU data structure (the so called host context).

But both call sites that do this are in preemptible context,
which may end up in disaster should the vcpu thread get preempted
before reentering the guest.

Instead, save the keys eagerly on each vcpu_load(). This has an
increased overhead, but is at least safe.

Cc: stable@vger.kernel.org
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

arch/arm/include/asm/kvm_emulate.h

@@ -367,6 +367,7 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
 	}
 }
 
-static inline void vcpu_ptrauth_setup_lazy(struct kvm_vcpu *vcpu) {}
+static inline bool vcpu_has_ptrauth(struct kvm_vcpu *vcpu) { return false; }
+static inline void vcpu_ptrauth_disable(struct kvm_vcpu *vcpu) { }
 
 #endif /* __ARM_KVM_EMULATE_H__ */

arch/arm64/include/asm/kvm_emulate.h

@@ -111,12 +111,6 @@ static inline void vcpu_ptrauth_disable(struct kvm_vcpu *vcpu)
 	vcpu->arch.hcr_el2 &= ~(HCR_API | HCR_APK);
 }
 
-static inline void vcpu_ptrauth_setup_lazy(struct kvm_vcpu *vcpu)
-{
-	if (vcpu_has_ptrauth(vcpu))
-		vcpu_ptrauth_disable(vcpu);
-}
-
 static inline unsigned long vcpu_get_vsesr(struct kvm_vcpu *vcpu)
 {
 	return vcpu->arch.vsesr_el2;

arch/arm64/kvm/handle_exit.c

@@ -162,31 +162,16 @@ static int handle_sve(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	return 1;
 }
 
-#define __ptrauth_save_key(regs, key)						\
-({										\
-	regs[key ## KEYLO_EL1] = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);	\
-	regs[key ## KEYHI_EL1] = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);	\
-})
-
 /*
  * Handle the guest trying to use a ptrauth instruction, or trying to access a
  * ptrauth register.
  */
 void kvm_arm_vcpu_ptrauth_trap(struct kvm_vcpu *vcpu)
 {
-	struct kvm_cpu_context *ctxt;
-
-	if (vcpu_has_ptrauth(vcpu)) {
+	if (vcpu_has_ptrauth(vcpu))
 		vcpu_ptrauth_enable(vcpu);
-		ctxt = vcpu->arch.host_cpu_context;
-		__ptrauth_save_key(ctxt->sys_regs, APIA);
-		__ptrauth_save_key(ctxt->sys_regs, APIB);
-		__ptrauth_save_key(ctxt->sys_regs, APDA);
-		__ptrauth_save_key(ctxt->sys_regs, APDB);
-		__ptrauth_save_key(ctxt->sys_regs, APGA);
-	} else {
+	else
 		kvm_inject_undefined(vcpu);
-	}
 }
 
 /*

virt/kvm/arm/arm.c

@@ -332,6 +332,16 @@ void kvm_arch_vcpu_unblocking(struct kvm_vcpu *vcpu)
 	preempt_enable();
 }
 
+#ifdef CONFIG_ARM64
+#define __ptrauth_save_key(regs, key)						\
+({										\
+	regs[key ## KEYLO_EL1] = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);	\
+	regs[key ## KEYHI_EL1] = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);	\
+})
+#else
+#define  __ptrauth_save_key(regs, key)	do { } while (0)
+#endif
+
 void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
 	int *last_ran;
@@ -365,7 +375,17 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	else
 		vcpu_set_wfx_traps(vcpu);
 
-	vcpu_ptrauth_setup_lazy(vcpu);
+	if (vcpu_has_ptrauth(vcpu)) {
+		struct kvm_cpu_context __maybe_unused *ctxt = vcpu->arch.host_cpu_context;
+
+		__ptrauth_save_key(ctxt->sys_regs, APIA);
+		__ptrauth_save_key(ctxt->sys_regs, APIB);
+		__ptrauth_save_key(ctxt->sys_regs, APDA);
+		__ptrauth_save_key(ctxt->sys_regs, APDB);
+		__ptrauth_save_key(ctxt->sys_regs, APGA);
+
+		vcpu_ptrauth_disable(vcpu);
+	}
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)