Patch "KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-x86-pvclock-handle-first-time-write-to-pvclock-page-contains-random-junk.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Liran Alon <[email protected]>
Date: Sun, 5 Nov 2017 16:11:30 +0200
Subject: KVM: x86: pvclock: Handle first-time write to pvclock-page contains random junk
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From: Liran Alon <[email protected]>

commit 51c4b8bba674cfd2260d173602c4dac08e4c3a99 upstream.

When guest passes KVM it's pvclock-page GPA via WRMSR to MSR_KVM_SYSTEM_TIME / MSR_KVM_SYSTEM_TIME_NEW, KVM don't initialize pvclock-page to some start-values. It just requests a clock-update which will happen before entering to guest.

The clock-update logic will call kvm_setup_pvclock_page() to update the pvclock-page with info. However, kvm_setup_pvclock_page() *wrongly* assumes that the version-field is initialized to an even number. This is wrong because at first-time write, field could be any-value.

Fix simply makes sure that if first-time version-field is odd, increment it once more to make it even and only then start standard logic. This follows same logic as done in other pvclock shared-pages (See kvm_write_wall_clock() and record_steal_time()).

---
 arch/x86/kvm/x86.c |    3 +++
 1 file changed, 3 insertions(+)

@@ -1812,6 +1812,9 @@ static int kvm_guest_time_update(struct
 	 */
 	BUILD_BUG_ON(offsetof(struct pvclock_vcpu_time_info, version) != 0);
 
+	if (guest_hv_clock.version & 1)
+		++guest_hv_clock.version;  /* first time write, random junk */
+
 	vcpu->hv_clock.version = guest_hv_clock.version + 1;
 	kvm_write_guest_cached(v->kvm, &vcpu->pv_time,
 				&vcpu->hv_clock,

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/kvm-x86-exit-to-user-mode-on-ud-intercept-when-emulator-requires.patch queue-4.4/kvm-x86-pvclock-handle-first-time-write-to-pvclock-page-contains-random-junk.patch