KVM: x86: Prevent host from panicking on shared MSR writes.

The previous patch blocked invalid writes directly when the MSR
is written.  As a precaution, prevent future similar mistakes by
gracefulling handle GPs caused by writes to shared MSRs.

Cc: stable@vger.kernel.org
Signed-off-by: Andrew Honig <ahonig@google.com>
[Remove parts obsoleted by Nadav's patch. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 13 insertions, 5 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 148020a7dd98..7e2c098b59c9 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2659,12 +2659,15 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
         default:
                 msr = find_msr_entry(vmx, msr_index);
                 if (msr) {
+                        u64 old_msr_data = msr->data;
                         msr->data = data;
                         if (msr - vmx->guest_msrs < vmx->save_nmsrs) {
                                 preempt_disable();
-                                kvm_set_shared_msr(msr->index, msr->data,
-                                                   msr->mask);
+                                ret = kvm_set_shared_msr(msr->index, msr->data,
+                                                         msr->mask);
                                 preempt_enable();
+                                if (ret)
+                                        msr->data = old_msr_data;
                         }
                         break;
                 }
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5a7195573a32..0033df32a745 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -229,20 +229,25 @@ static void kvm_shared_msr_cpu_online(void)
                 shared_msr_update(i, shared_msrs_global.msrs[i]);
 }
 
-void kvm_set_shared_msr(unsigned slot, u64 value, u64 mask)
+int kvm_set_shared_msr(unsigned slot, u64 value, u64 mask)
 {
         unsigned int cpu = smp_processor_id();
         struct kvm_shared_msrs *smsr = per_cpu_ptr(shared_msrs, cpu);
+        int err;
 
         if (((value ^ smsr->values[slot].curr) & mask) == 0)
-                return;
+                return 0;
         smsr->values[slot].curr = value;
-        wrmsrl(shared_msrs_global.msrs[slot], value);
+        err = wrmsrl_safe(shared_msrs_global.msrs[slot], value);
+        if (err)
+                return 1;
+
         if (!smsr->registered) {
                 smsr->urn.on_user_return = kvm_on_user_return;
                 user_return_notifier_register(&smsr->urn);
                 smsr->registered = true;
         }
+        return 0;
 }
 EXPORT_SYMBOL_GPL(kvm_set_shared_msr);