[PATCH v7 23/45] KVM: arm64: Validate register access for a Realm VM

Steven Price
2025-02-13T16:16:26Z

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@arm.com>
---
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 | 40 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 40 insertions(+)

diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 2196979a24a3..ff0306650b39 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;
@@ -783,12 +801,34 @@ 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