[PATCH v2] KVM: arm64: Reject non compliant SMCCC function calls in pKVM

[Sebastian Ene <sebastianene@google.com>]

Prevent the propagation of a function-id that has the top bits set since
this is not compliant with the SMCCC spec and can overlap with the
already known function-id decoders. (eg. if we invoke an smc with
0xffffffffc4000012 it will be decoded as a PSCI reset call). Instead,
make it clear that we don't support it and return an error.

Signed-off-by: Sebastian Ene <sebastianene@google.com>
---
NOTE: This is based on linux-next, next-20260407 to avoid a minor
conflict with a previously submitted patch (commit cf6348af645b).  

Changelog:

v1 -> v2:
* dropped the changes to the function signature that were accepting
  64-bit function-ids.
* applied Mark's suggestion to make it clear that we don't accept non
  standard SMCCC calls.
* revised commit message & updated the title.

Link to v1: 
https://lore.kernel.org/all/20260401123201.389906-1-sebastianene@google.com/

---
 arch/arm64/kvm/hyp/nvhe/hyp-main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
index 73f2e0221e70..cca4b07c8d61 100644
--- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
+++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
@@ -805,6 +805,10 @@ static void handle_host_smc(struct kvm_cpu_context *host_ctxt)
 	}
 
 	func_id &= ~ARM_SMCCC_CALL_HINTS;
+	if (upper_32_bits(func_id)) {
+		cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
+		goto exit_skip_instr;
+	}
 
 	handled = kvm_host_psci_handler(host_ctxt, func_id);
 	if (!handled)
-- 
2.53.0.1213.gd9a14994de-goog