With protected VMs, forwarding guest HVC/SMCs happens at two interfaces:

     pKVM [EL2] <--> KVM [EL1] <--> VMM [EL0]

so it might be possible for EL0 to successfully register with EL1 to
handle guest SMCCC calls but never see the KVM_EXIT_HYPERCALL, even if
the calls are properly issued by the guest, due to EL2 handling them so
that (host) EL1 never gets a chance to exit to EL0.

Instead, avoid that confusing situation and make userspace fail early by
disallowing KVM_ARM_VM_SMCCC_FILTER-ing calls from protected guests in
the KVM FID range (which pKVM re-uses).

DEN0028 defines 65536 "Vendor Specific Hypervisor Service Calls":

- the first ARM_SMCCC_KVM_NUM_FUNCS (128) can be custom-defined
- the following 3 are currently standardized
- the rest is "reserved for future expansion"

so reserve them all, like commit 821d935c87bc ("KVM: arm64: Introduce
support for userspace SMCCC filtering") with the Arm Architecture Calls.

Alternatively, we could have only reserved the ARM_SMCCC_KVM_NUM_FUNCS
(or even a subset of it) and the "Call UID Query" but that would have
risked future conflicts between that uAPI and an extension of the SMCCC
or of the pKVM ABI.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
---
 Documentation/virt/kvm/devices/vm.rst | 11 ++++++++++
 arch/arm64/kvm/hypercalls.c           | 39 +++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+)

diff --git a/Documentation/virt/kvm/devices/vm.rst b/Documentation/virt/kvm/devices/vm.rst
index a4d39fa1b0834b090318250db3b670b0b3174259..9ed4b7afe022023346d4835d33a901e557fc4765 100644
--- a/Documentation/virt/kvm/devices/vm.rst
+++ b/Documentation/virt/kvm/devices/vm.rst
@@ -400,3 +400,14 @@ will reject attempts to define a filter for any portion of these ranges:
         0x8000_0000 0x8000_FFFF
         0xC000_0000 0xC000_FFFF
         =========== ===============
+
+Protected KVM (pKVM) reserves the 'Vendor Specific Hypervisor Service Calls'
+range of function IDs and will reject attempts to define a filter for any
+portion of these ranges for a protected VM (``KVM_VM_TYPE_ARM_PROTECTED``):
+
+        =========== ===============
+        Start       End (inclusive)
+        =========== ===============
+        0x8600_0000 0x8600_FFFF
+        0xC600_0000 0xC600_FFFF
+        =========== ===============
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 58c5fe7d757274d9079606fcc378485980c6c0f8..5ddcdd70a6b280914048e7683dafb778d0f24658 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -135,6 +135,21 @@ static bool kvm_smccc_test_fw_bmap(struct kvm_vcpu *vcpu, u32 func_id)
 						   ARM_SMCCC_SMC_64,		\
 						   0, ARM_SMCCC_FUNC_MASK)
 
+#define SMC32_VHYP_RANGE_BEGIN	ARM_SMCCC_VENDOR_HYP_KVM_FEATURES_FUNC_ID
+#define SMC32_VHYP_RANGE_END	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,		\
+						   ARM_SMCCC_SMC_32,		\
+						   ARM_SMCCC_OWNER_VENDOR_HYP,	\
+						   ARM_SMCCC_FUNC_MASK)
+
+#define SMC64_VHYP_RANGE_BEGIN	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,		\
+						   ARM_SMCCC_SMC_64,		\
+						   ARM_SMCCC_OWNER_VENDOR_HYP,	\
+						   0)
+#define SMC64_VHYP_RANGE_END	ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL,		\
+						   ARM_SMCCC_SMC_64,		\
+						   ARM_SMCCC_OWNER_VENDOR_HYP,	\
+						   ARM_SMCCC_FUNC_MASK)
+
 static int kvm_smccc_filter_insert_reserved(struct kvm *kvm)
 {
 	int r;
@@ -158,6 +173,30 @@ static int kvm_smccc_filter_insert_reserved(struct kvm *kvm)
 	if (r)
 		goto out_destroy;
 
+	/*
+	 * Prevent userspace from registering to handle any SMCCC call in the
+	 * vendor hypervisor range for pVMs, avoiding the confusion of guest
+	 * calls seemingly not resulting in KVM_RUN exits because pKVM handles
+	 * them without ever returning to the host. This is NOT for security.
+	 */
+	if (kvm_vm_is_protected(kvm)) {
+		r = mtree_insert_range(&kvm->arch.smccc_filter,
+				       SMC32_VHYP_RANGE_BEGIN,
+				       SMC32_VHYP_RANGE_END,
+				       xa_mk_value(KVM_SMCCC_FILTER_HANDLE),
+				       GFP_KERNEL_ACCOUNT);
+		if (r)
+			goto out_destroy;
+
+		r = mtree_insert_range(&kvm->arch.smccc_filter,
+				       SMC64_VHYP_RANGE_BEGIN,
+				       SMC64_VHYP_RANGE_END,
+				       xa_mk_value(KVM_SMCCC_FILTER_HANDLE),
+				       GFP_KERNEL_ACCOUNT);
+		if (r)
+			goto out_destroy;
+	}
+
 	return 0;
 out_destroy:
 	mtree_destroy(&kvm->arch.smccc_filter);

---
base-commit: 7d0a66e4bb9081d75c82ec4957c50034cb0ea449
change-id: 20251201-smccc-filter-588ddf12b355

Best regards,
-- 
Pierre-Clément Tosi <ptosi@google.com>