[PATCH v3] KVM: arm64: Prevent the host from using an smc with imm16 != 0

[Sebastian Ene <sebastianene@google.com>]

The ARM Service Calling Convention (SMCCC) specifies that the function
identifier and parameters should be passed in registers, leaving the
16-bit immediate field un-handled in pKVM when an SMC instruction is
trapped.
Since the HVC is a private interface between EL2 and the host,
enforce the host kernel running under pKVM to use an immediate value
of 0 only when using SMCs to make it clear for non-compliant software
talking to Trustzone that we only use SMCCC.

Signed-off-by: Sebastian Ene <sebastianene@google.com>
---
v2 -> current:
 - move the ESR decoding of the imm16 in the handle_host_smc where it
   was supposed to be
 - updated the commit message to include the reason behind while we are
   not doing for HVCs as well
 - updated the commit message to clarify that pKVM is not handling the
   imm16

v1 -> v2:
 - Dropped injecting an UNDEF and return an error instead
   (SMCCC_RET_NOT_SUPPORTED)
 - Used the mask ESR_ELx_xVC_IMM_MASK instead of masking with U16_MAX
 - Updated the title of the commit message from:
   "[PATCH] KVM: arm64: Inject UNDEF when host is executing an
    smc with imm16 != 0"

Link to v2:
https://lore.kernel.org/all/20260325113138.4171430-1-sebastianene@google.com/
Link to v1:
https://lore.kernel.org/all/20260324135728.3532400-1-sebastianene@google.com/

---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index e7790097db93..461cf5cb5ac7 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -676,8 +676,14 @@ static void default_host_smc_handler(struct kvm_cpu_context *host_ctxt)
 static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(u64, func_id, host_ctxt, 0);
+	u64 esr = read_sysreg_el2(SYS_ESR);
 	bool handled;
 
+	if (esr & ESR_ELx_xVC_IMM_MASK) {
+		cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
+		goto exit_skip_instr;
+	}
+
 	func_id &= ~ARM_SMCCC_CALL_HINTS;
 
 	handled = kvm_host_psci_handler(host_ctxt, func_id);
@@ -686,6 +692,7 @@ static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
 	if (!handled)
 		default_host_smc_handler(host_ctxt);
 
+exit_skip_instr:
 	/* SMC was trapped, move ELR past the current PC. */
 	kvm_skip_host_instr();
 }
-- 
2.53.0.1018.g2bb0e51243-goog