KVM: arm64: Use different pointer authentication keys for pKVM

When the use of pointer authentication is enabled in the kernel it
applies to both the kernel itself as well as KVM's nVHE hypervisor. The
same keys are used for both the kernel and the nVHE hypervisor, which is
less than desirable for pKVM as the host is not trusted at runtime.

Naturally, the fix is to use a different set of keys for the hypervisor
when running in protected mode. Have the host generate a new set of keys
for the hypervisor before deprivileging the kernel. While there might be
other sources of random directly available at EL2, this keeps the
implementation simple, and the host is trusted anyways until it is
deprivileged.

Since the host and hypervisor no longer share a set of pointer
authentication keys, start context switching them on the host entry/exit
path exactly as we do for guest entry/exit. There is no need to handle
CPU migration as the nVHE code is not migratable in the first place.
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20230614122600.2098901-1-smostafa@google.comSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/arm.c

@@ -51,6 +51,8 @@ DECLARE_KVM_HYP_PER_CPU(unsigned long, kvm_hyp_vector);
 DEFINE_PER_CPU(unsigned long, kvm_arm_hyp_stack_page);
 DECLARE_KVM_NVHE_PER_CPU(struct kvm_nvhe_init_params, kvm_init_params);
 
+DECLARE_KVM_NVHE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
+
 static bool vgic_present;
 
 static DEFINE_PER_CPU(unsigned char, kvm_arm_hardware_enabled);
@@ -2067,6 +2069,26 @@ static int __init kvm_hyp_init_protection(u32 hyp_va_bits)
 	return 0;
 }
 
+static void pkvm_hyp_init_ptrauth(void)
+{
+	struct kvm_cpu_context *hyp_ctxt;
+	int cpu;
+
+	for_each_possible_cpu(cpu) {
+		hyp_ctxt = per_cpu_ptr_nvhe_sym(kvm_hyp_ctxt, cpu);
+		hyp_ctxt->sys_regs[APIAKEYLO_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APIAKEYHI_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APIBKEYLO_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APIBKEYHI_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APDAKEYLO_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APDAKEYHI_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APDBKEYLO_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APDBKEYHI_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APGAKEYLO_EL1] = get_random_long();
+		hyp_ctxt->sys_regs[APGAKEYHI_EL1] = get_random_long();
+	}
+}
+
 /* Inits Hyp-mode on all online CPUs */
 static int __init init_hyp_mode(void)
 {
@@ -2228,6 +2250,10 @@ static int __init init_hyp_mode(void)
 	kvm_hyp_init_symbols();
 
 	if (is_protected_kvm_enabled()) {
+		if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL) &&
+		    cpus_have_const_cap(ARM64_HAS_ADDRESS_AUTH))
+			pkvm_hyp_init_ptrauth();
+
 		init_cpu_logical_map();
 
 		if (!init_psci_relay()) {

arch/arm64/kvm/hyp/nvhe/host.S

@@ -10,6 +10,7 @@
 #include <asm/kvm_arm.h>
 #include <asm/kvm_asm.h>
 #include <asm/kvm_mmu.h>
+#include <asm/kvm_ptrauth.h>
 
 	.text
 
@@ -37,10 +38,43 @@ SYM_FUNC_START(__host_exit)
 
 	/* Save the host context pointer in x29 across the function call */
 	mov	x29, x0
+
+#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
+alternative_if_not ARM64_HAS_ADDRESS_AUTH
+b __skip_pauth_save
+alternative_else_nop_endif
+
+alternative_if ARM64_KVM_PROTECTED_MODE
+	/* Save kernel ptrauth keys. */
+	add x18, x29, #CPU_APIAKEYLO_EL1
+	ptrauth_save_state x18, x19, x20
+
+	/* Use hyp keys. */
+	adr_this_cpu x18, kvm_hyp_ctxt, x19
+	add x18, x18, #CPU_APIAKEYLO_EL1
+	ptrauth_restore_state x18, x19, x20
+	isb
+alternative_else_nop_endif
+__skip_pauth_save:
+#endif /* CONFIG_ARM64_PTR_AUTH_KERNEL */
+
 	bl	handle_trap
 
-	/* Restore host regs x0-x17 */
 __host_enter_restore_full:
+	/* Restore kernel keys. */
+#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
+alternative_if_not ARM64_HAS_ADDRESS_AUTH
+b __skip_pauth_restore
+alternative_else_nop_endif
+
+alternative_if ARM64_KVM_PROTECTED_MODE
+	add x18, x29, #CPU_APIAKEYLO_EL1
+	ptrauth_restore_state x18, x19, x20
+alternative_else_nop_endif
+__skip_pauth_restore:
+#endif /* CONFIG_ARM64_PTR_AUTH_KERNEL */
+
+	/* Restore host regs x0-x17 */
 	ldp	x0, x1,   [x29, #CPU_XREG_OFFSET(0)]
 	ldp	x2, x3,   [x29, #CPU_XREG_OFFSET(2)]
 	ldp	x4, x5,   [x29, #CPU_XREG_OFFSET(4)]