[PATCH v6 22/43] KVM: arm64: Validate register access for a Realm VM

[Steven Price <steven.price@xxxxxxx>]

The RMM only allows setting the GPRS (x0-x30) and PC for a realm
guest. Check this in kvm_arm_set_reg() so that the VMM can receive a
suitable error return if other registers are accessed.

Signed-off-by: Steven Price <steven.price@xxxxxxx>
---
Changes since v5:
 * Upper GPRS can be set as part of a HOST_CALL return, so fix up the
   test to allow them.
---
 arch/arm64/kvm/guest.c | 43 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 12dad841f2a5..1ee2fe072f1a 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -73,6 +73,24 @@ static u64 core_reg_offset_from_id(u64 id)
 	return id & ~(KVM_REG_ARCH_MASK | KVM_REG_SIZE_MASK | KVM_REG_ARM_CORE);
 }
 
+static bool kvm_realm_validate_core_reg(u64 off)
+{
+	/*
+	 * Note that GPRs can only sometimes be controlled by the VMM.
+	 * For PSCI only X0-X6 are used, higher registers are ignored (restored
+	 * from the REC).
+	 * For HOST_CALL all of X0-X30 are copied to the RsiHostCall structure.
+	 * For emulated MMIO X0 is always used.
+	 */
+	switch (off) {
+	case KVM_REG_ARM_CORE_REG(regs.regs[0]) ...
+	     KVM_REG_ARM_CORE_REG(regs.regs[30]):
+	case KVM_REG_ARM_CORE_REG(regs.pc):
+		return true;
+	}
+	return false;
+}
+
 static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
 {
 	int size;
@@ -115,6 +133,9 @@ static int core_reg_size_from_offset(const struct kvm_vcpu *vcpu, u64 off)
 	if (vcpu_has_sve(vcpu) && core_reg_offset_is_vreg(off))
 		return -EINVAL;
 
+	if (kvm_is_realm(vcpu->kvm) && !kvm_realm_validate_core_reg(off))
+		return -EPERM;
+
 	return size;
 }
 
@@ -783,12 +804,34 @@ int kvm_arm_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 	return kvm_arm_sys_reg_get_reg(vcpu, reg);
 }
 
+/*
+ * The RMI ABI only enables setting some GPRs and PC. The selection of GPRs
+ * that are available depends on the Realm state and the reason for the last
+ * exit.  All other registers are reset to architectural or otherwise defined
+ * reset values by the RMM, except for a few configuration fields that
+ * correspond to Realm parameters.
+ */
+static bool validate_realm_set_reg(struct kvm_vcpu *vcpu,
+				   const struct kvm_one_reg *reg)
+{
+	if ((reg->id & KVM_REG_ARM_COPROC_MASK) == KVM_REG_ARM_CORE) {
+		u64 off = core_reg_offset_from_id(reg->id);
+
+		return kvm_realm_validate_core_reg(off);
+	}
+
+	return false;
+}
+
 int kvm_arm_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
 {
 	/* We currently use nothing arch-specific in upper 32 bits */
 	if ((reg->id & ~KVM_REG_SIZE_MASK) >> 32 != KVM_REG_ARM64 >> 32)
 		return -EINVAL;
 
+	if (kvm_is_realm(vcpu->kvm) && !validate_realm_set_reg(vcpu, reg))
+		return -EINVAL;
+
 	switch (reg->id & KVM_REG_ARM_COPROC_MASK) {
 	case KVM_REG_ARM_CORE:	return set_core_reg(vcpu, reg);
 	case KVM_REG_ARM_FW:
-- 
2.43.0