Subject: [PATCH v3 6/6] KVM: arm64: Disable privileged hypercalls after pKVM finalisation

From: Will Deacon <[email-masked]>
Date: Fri, 8 Oct 2021 14:58:39 +0100

After pKVM has been 'finalised' using the __pkvm_prot_finalize hypercall,
the calling CPU will have a Stage-2 translation enabled to prevent access
to memory pages owned by EL2.

Although this forms a significant part of the process to deprivilege the
host kernel, we also need to ensure that the hypercall interface is
reduced so that the EL2 code cannot, for example, be re-initialised using
a new set of vectors.

Re-order the hypercalls so that only a suffix remains available after
finalisation of pKVM.

Cc: Marc Zyngier <[email-masked]>
Cc: Quentin Perret <[email-masked]>
Signed-off-by: Will Deacon <[email-masked]>
---
 arch/arm64/include/asm/kvm_asm.h   | 25 +++++++++++---------
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 37 +++++++++++++++++++++---------
 2 files changed, 40 insertions(+), 22 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 43b5e213ae43..4654d27fd221 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -50,27 +50,30 @@
 #include <linux/mm.h>
 
 enum __kvm_host_smccc_func {
+	/* Hypercalls available only prior to pKVM finalisation */
 	/* __KVM_HOST_SMCCC_FUNC___kvm_hyp_init */
-	__KVM_HOST_SMCCC_FUNC___kvm_vcpu_run = __KVM_HOST_SMCCC_FUNC___kvm_hyp_init + 1,
+	__KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2 = __KVM_HOST_SMCCC_FUNC___kvm_hyp_init + 1,
+	__KVM_HOST_SMCCC_FUNC___pkvm_init,
+	__KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping,
+	__KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector,
+	__KVM_HOST_SMCCC_FUNC___kvm_enable_ssbs,
+	__KVM_HOST_SMCCC_FUNC___vgic_v3_init_lrs,
+	__KVM_HOST_SMCCC_FUNC___vgic_v3_get_gic_config,
+	__KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize,
+
+	/* Hypercalls available after pKVM finalisation */
+	__KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp,
+	__KVM_HOST_SMCCC_FUNC___kvm_adjust_pc,
+	__KVM_HOST_SMCCC_FUNC___kvm_vcpu_run,
 	__KVM_HOST_SMCCC_FUNC___kvm_flush_vm_context,
 	__KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid_ipa,
 	__KVM_HOST_SMCCC_FUNC___kvm_tlb_flush_vmid,
 	__KVM_HOST_SMCCC_FUNC___kvm_flush_cpu_context,
 	__KVM_HOST_SMCCC_FUNC___kvm_timer_set_cntvoff,
-	__KVM_HOST_SMCCC_FUNC___kvm_enable_ssbs,
-	__KVM_HOST_SMCCC_FUNC___vgic_v3_get_gic_config,
 	__KVM_HOST_SMCCC_FUNC___vgic_v3_read_vmcr,
 	__KVM_HOST_SMCCC_FUNC___vgic_v3_write_vmcr,
-	__KVM_HOST_SMCCC_FUNC___vgic_v3_init_lrs,
-	__KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2,
 	__KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs,
 	__KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs,
-	__KVM_HOST_SMCCC_FUNC___pkvm_init,
-	__KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp,
-	__KVM_HOST_SMCCC_FUNC___pkvm_create_private_mapping,
-	__KVM_HOST_SMCCC_FUNC___pkvm_cpu_set_vector,
-	__KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize,
-	__KVM_HOST_SMCCC_FUNC___kvm_adjust_pc,
 };
 
 #define DECLARE_KVM_VHE_SYM(sym)	extern char sym[]
diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 2da6aa8da868..8566805ef62c 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -165,36 +165,51 @@ typedef void (*hcall_t)(struct kvm_cpu_context *);
 #define HANDLE_FUNC(x)	[__KVM_HOST_SMCCC_FUNC_##x] = (hcall_t)handle_##x
 
 static const hcall_t host_hcall[] = {
-	HANDLE_FUNC(__kvm_vcpu_run),
+	/* ___kvm_hyp_init */
+	HANDLE_FUNC(__kvm_get_mdcr_el2),
+	HANDLE_FUNC(__pkvm_init),
+	HANDLE_FUNC(__pkvm_create_private_mapping),
+	HANDLE_FUNC(__pkvm_cpu_set_vector),
+	HANDLE_FUNC(__kvm_enable_ssbs),
+	HANDLE_FUNC(__vgic_v3_init_lrs),
+	HANDLE_FUNC(__vgic_v3_get_gic_config),
+	HANDLE_FUNC(__pkvm_prot_finalize),
+
+	HANDLE_FUNC(__pkvm_host_share_hyp),
 	HANDLE_FUNC(__kvm_adjust_pc),
+	HANDLE_FUNC(__kvm_vcpu_run),
 	HANDLE_FUNC(__kvm_flush_vm_context),
 	HANDLE_FUNC(__kvm_tlb_flush_vmid_ipa),
 	HANDLE_FUNC(__kvm_tlb_flush_vmid),
 	HANDLE_FUNC(__kvm_flush_cpu_context),
 	HANDLE_FUNC(__kvm_timer_set_cntvoff),
-	HANDLE_FUNC(__kvm_enable_ssbs),
-	HANDLE_FUNC(__vgic_v3_get_gic_config),
 	HANDLE_FUNC(__vgic_v3_read_vmcr),
 	HANDLE_FUNC(__vgic_v3_write_vmcr),
-	HANDLE_FUNC(__vgic_v3_init_lrs),
-	HANDLE_FUNC(__kvm_get_mdcr_el2),
 	HANDLE_FUNC(__vgic_v3_save_aprs),
 	HANDLE_FUNC(__vgic_v3_restore_aprs),
-	HANDLE_FUNC(__pkvm_init),
-	HANDLE_FUNC(__pkvm_cpu_set_vector),
-	HANDLE_FUNC(__pkvm_host_share_hyp),
-	HANDLE_FUNC(__pkvm_create_private_mapping),
-	HANDLE_FUNC(__pkvm_prot_finalize),
 };
 
 static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(unsigned long, id, host_ctxt, 0);
+	unsigned long hcall_min = 0;
 	hcall_t hfn;
 
+	/*
+	 * If pKVM has been initialised then reject any calls to the
+	 * early "privileged" hypercalls. Note that we cannot reject
+	 * calls to __pkvm_prot_finalize for two reasons: (1) The static
+	 * key used to determine initialisation must be toggled prior to
+	 * finalisation and (2) finalisation is performed on a per-CPU
+	 * basis. This is all fine, however, since __pkvm_prot_finalize
+	 * returns -EPERM after the first call for a given CPU.
+	 */
+	if (static_branch_unlikely(&kvm_protected_mode_initialized))
+		hcall_min = __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize;
+
 	id -= KVM_HOST_SMCCC_ID(0);
 
-	if (unlikely(id >= ARRAY_SIZE(host_hcall)))
+	if (unlikely(id < hcall_min || id >= ARRAY_SIZE(host_hcall)))
 		goto inval;
 
 	hfn = host_hcall[id];
-- 
2.33.0.882.g93a45727a2-goog

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