From: Greg KH on
2.6.34-stable review patch. If anyone has any objections, please let me know.


From: Avi Kivity <avi(a)>

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(a)>
Signed-off-by: Avi Kivity <avi(a)>
Signed-off-by: Greg Kroah-Hartman <gregkh(a)>

(Cherry-picked from commit 9ed3c444ab8987c7b219173a2f7807e3f71e234e)
arch/x86/kvm/x86.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -695,14 +695,22 @@ static int do_set_msr(struct kvm_vcpu *v

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)

- 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));

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