Subject: [PATCH] KVM: arm64: Use different pointer authentication keys for pKVM

From: Mostafa Saleh <[email-masked]>
Date: Tue, 16 May 2023 14:15:31 +0000

When the kernel is compiled with CONFIG_ARM64_PTR_AUTH_KERNEL, it
uses Armv8.3-Pauth for return address protection for the kernel code
including nvhe code in EL2.

Same keys are used in both kernel(EL1) and nvhe code(EL2), this is
fine for nvhe but not when running in protected mode(pKVM) as the host
can't be trusted.

The keys for the hypervisor are generated from the kernel before it
de-privileges, each cpu has different keys, this relies on nvhe code
not being migratable while running.

This patch adds host/hyp save/restore for the keys.
For guest/hyp, they are already handled in common kvm code in
__guest_enter, where they are saved/restored if they are not
trapped.

Signed-off-by: Mostafa Saleh <[email-masked]>
---
 arch/arm64/kvm/arm.c           | 26 +++++++++++++++++++++++++
 arch/arm64/kvm/hyp/nvhe/host.S | 35 +++++++++++++++++++++++++++++++++-
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 14391826241c..dd03b52f035d 100644
--- a/arch/arm64/kvm/arm.c
+++ b/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()) {
diff --git a/arch/arm64/kvm/hyp/nvhe/host.S b/arch/arm64/kvm/hyp/nvhe/host.S
index b6c0188c4b35..255ba4af911b 100644
--- a/arch/arm64/kvm/hyp/nvhe/host.S
+++ b/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 +38,42 @@ 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
+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)]
-- 
2.40.1.606.ga4b1b128d6-goog

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