[PATCH] KVM: arm64: Set a linux errno on SMCCC error in kvm_call_hyp_nvhe()

[Vincent Donnefort]

If the HVC called in kvm_call_hyp_nvhe() fails with an SMCCC error code,
we WARN. However, the returned value isn't initialized and the caller
might get garbage or 0 which is likely to be interpreted as success.

Set a default -EPERM error value, ensuring all callers get the message
when SMCCC calls fail.

Signed-off-by: Vincent Donnefort <vdonnefort at google.com>

---

I have encountered this issue while working on a follow-up contribution to the
hypervisor tracing. In that case it completely crashed the kernel because
IS_ERR() failed on that res.a1 value.

Now, if it makes that function more robust, I do not believe it is fixing any
existing bug which is why I haven't added a "Fixes:" tag. 

In case we want to stick one, here it is:

Fixes: 054698316d87 ("KVM: arm64: nVHE: Migrate hyp interface to SMCCC")

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index a49042bfa801..6b8fd494792c 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1273,13 +1273,14 @@ void kvm_arm_resume_guest(struct kvm *kvm);
 #define vcpu_has_run_once(vcpu)	(!!READ_ONCE((vcpu)->pid))
 
 #ifndef __KVM_NVHE_HYPERVISOR__
-#define kvm_call_hyp_nvhe(f, ...)						\
+#define kvm_call_hyp_nvhe(f, ...)					\
 	({								\
 		struct arm_smccc_res res;				\
 									\
 		arm_smccc_1_1_hvc(KVM_HOST_SMCCC_FUNC(f),		\
 				  ##__VA_ARGS__, &res);			\
-		WARN_ON(res.a0 != SMCCC_RET_SUCCESS);			\
+		if (WARN_ON(res.a0 != SMCCC_RET_SUCCESS))		\
+			res.a1 = -EPERM;				\
 									\
 		res.a1;							\
 	})

base-commit: e43ffb69e0438cddd72aaa30898b4dc446f664f8
-- 
2.54.0.1032.g2f8565e1d1-goog