KVM: VMX: Disable preemption when probing user return MSRs

Disable preemption when probing a user return MSR via RDSMR/WRMSR.  If
the MSR holds a different value per logical CPU, the WRMSR could corrupt
the host's value if KVM is preempted between the RDMSR and WRMSR, and
then rescheduled on a different CPU.

Opportunistically land the helper in common x86, SVM will use the helper
in a future commit.

Fixes: 4be5341 ("KVM: VMX: Initialize vmx->guest_msrs[] right after allocation")
Cc: stable@vger.kernel.org
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210504171734.1434054-6-seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1777,6 +1777,7 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
 		    unsigned long icr, int op_64_bit);
 
 void kvm_define_user_return_msr(unsigned index, u32 msr);
+int kvm_probe_user_return_msr(u32 msr);
 int kvm_set_user_return_msr(unsigned index, u64 val, u64 mask);
 
 u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc);

arch/x86/kvm/vmx/vmx.c

@@ -6914,12 +6914,9 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
 
 	for (i = 0; i < ARRAY_SIZE(vmx_uret_msrs_list); ++i) {
 		u32 index = vmx_uret_msrs_list[i];
-		u32 data_low, data_high;
 		int j = vmx->nr_uret_msrs;
 
-		if (rdmsr_safe(index, &data_low, &data_high) < 0)
-			continue;
-		if (wrmsr_safe(index, data_low, data_high) < 0)
+		if (kvm_probe_user_return_msr(index))
 			continue;
 
 		vmx->guest_uret_msrs[j].slot = i;

arch/x86/kvm/x86.c

@@ -339,6 +339,22 @@ static void kvm_on_user_return(struct user_return_notifier *urn)
 	}
 }
 
+int kvm_probe_user_return_msr(u32 msr)
+{
+	u64 val;
+	int ret;
+
+	preempt_disable();
+	ret = rdmsrl_safe(msr, &val);
+	if (ret)
+		goto out;
+	ret = wrmsrl_safe(msr, val);
+out:
+	preempt_enable();
+	return ret;
+}
+EXPORT_SYMBOL_GPL(kvm_probe_user_return_msr);
+
 void kvm_define_user_return_msr(unsigned slot, u32 msr)
 {
 	BUG_ON(slot >= KVM_MAX_NR_USER_RETURN_MSRS);