KVM: nVMX: Document that ignoring memory failures for VMCLEAR is deliberate

Explicitly drop the result of kvm_vcpu_write_guest() when writing the
"launch state" as part of VMCLEAR emulation, and add a comment to call
out that KVM's behavior is architecturally valid.  Intel's pseudocode
effectively says that VMCLEAR is a nop if the target VMCS address isn't
in memory, e.g. if the address points at MMIO.

Add a FIXME to call out that suppressing failures on __copy_to_user() is
wrong, as memory (a memslot) does exist in that case.  Punt the issue to
the future as open coding kvm_vcpu_write_guest() just to make sure the
guest dies with -EFAULT isn't worth the extra complexity.  The flaw will
need to be addressed if KVM ever does something intelligent on uaccess
failures, e.g. to support post-copy demand paging, but in that case KVM
will need a more thorough overhaul, i.e. VMCLEAR shouldn't need to open
code a core KVM helper.

No functional change intended.
Reported-by: coverity-bot <keescook+coverity-bot@chromium.org>
Addresses-Coverity-ID: 1527765 ("Error handling issues")
Fixes: 587d7e72 ("kvm: nVMX: VMCLEAR should not cause the vCPU to shut down")
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20221220154224.526568-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -5296,7 +5296,16 @@ static int handle_vmclear(struct kvm_vcpu *vcpu)
 		if (vmptr == vmx->nested.current_vmptr)
 			nested_release_vmcs12(vcpu);
 
-		kvm_vcpu_write_guest(vcpu,
+		/*
+		 * Silently ignore memory errors on VMCLEAR, Intel's pseudocode
+		 * for VMCLEAR includes a "ensure that data for VMCS referenced
+		 * by the operand is in memory" clause that guards writes to
+		 * memory, i.e. doing nothing for I/O is architecturally valid.
+		 *
+		 * FIXME: Suppress failures if and only if no memslot is found,
+		 * i.e. exit to userspace if __copy_to_user() fails.
+		 */
+		(void)kvm_vcpu_write_guest(vcpu,
 					   vmptr + offsetof(struct vmcs12,
 							    launch_state),
 					   &zero, sizeof(zero));