From: Avi Kivity <avi@redhat.com>
Date: Tue, 4 May 2010 12:00:37 +0000 (+0300)
Subject: KVM: Fix wallclock version writing race
X-Git-Tag: v2.6.34.1~7
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=0119618803088bf8be6f287fc0e160e4a08fbfa6;p=karo-tx-linux.git

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

(Cherry-picked from commit 9ed3c444ab8987c7b219173a2f7807e3f71e234e)
---

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b5c4393385a7..ead3ea4070bf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -695,14 +695,22 @@ 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));