linux-kernel - [194/200] KVM: Fix wallclock version writing race

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Message-Id: <20100701174302.895625064@clark.site>
Date:	Thu, 01 Jul 2010 10:44:44 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Glauber Costa <glommer@...hat.com>, Avi Kivity <avi@...hat.com>
Subject: [194/200] KVM: Fix wallclock version writing race

2.6.34-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Avi Kivity <avi@...hat.com>

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@...hat.com>
Signed-off-by: Avi Kivity <avi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

(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)
 		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));
 


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