KVM: Fix wallclock version writing race

Wallclock writing uses an unprotected global variable to hold the version;
this can cause one guest to interfere with another if both write their
wallclock at the same time.

Acked-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 10 insertions, 2 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index dff08e527ec7..54f73b6a006b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -754,14 +754,22 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 
 static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
 {
-        static int version;
+        int version;
+        int r;
         struct pvclock_wall_clock wc;
         struct timespec boot;
 
         if (!wall_clock)
                 return;
 
-        version++;
+        r = kvm_read_guest(kvm, wall_clock, &version, sizeof(version));
+        if (r)
+                return;
+
+        if (version & 1)
+                ++version;  /* first time write, random junk */
+
+        ++version;
 
         kvm_write_guest(kvm, wall_clock, &version, sizeof(version));