Re: [PATCH 5/5] KVM: arm64: Disable privileged hypercalls after pKVM finalisation

On Thu, 23 Sep 2021 14:02:11 +0100,
Will Deacon [off-list ref] wrote:

On Thu, Sep 23, 2021 at 01:56:21PM +0100, Marc Zyngier wrote:

quoted

On Thu, 23 Sep 2021 12:22:56 +0100,
Will Deacon [off-list ref] wrote:

[...]

quoted

quoted

 static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
 {
 	DECLARE_REG(unsigned long, id, host_ctxt, 0);
+	unsigned long hcall_min = 0;
 	hcall_t hfn;
 
+	if (static_branch_unlikely(&kvm_protected_mode_initialized))
+		hcall_min = __KVM_HOST_SMCCC_FUNC___pkvm_prot_finalize;
+
 	id -= KVM_HOST_SMCCC_ID(0);
 
-	if (unlikely(id >= ARRAY_SIZE(host_hcall)))
+	if (unlikely(id < hcall_min || id >= ARRAY_SIZE(host_hcall)))

So I can still issue a pkvm_prot_finalize after finalisation? Seems
odd. As hcall_min has to be inclusive, you probably want it to be set
to __KVM_HOST_SMCCC_FUNC___pkvm_host_share_hyp once protected.

Yeah, I ended up addresing that one in the previous patch. The problem is
that we need to allow pkvm_prot_finalize to be called on each CPU, so I
think we'd end up having an extra "really finalize damnit!" call to be
issued _once_ after each CPU is done with the finalisation if we want
to lock it down.

The approach I took instead is to make pkvm_prot_finalize return -EBUSY
if it's called on a CPU where it's already been called.

Ah, I see. Serves me right for reading patches out of order. Finalise
is of course per-CPU, and the static key global. Epic fail.

Probably deserves a comment, because I'm surely going to jump at that
again in three months.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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