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Subject[075/136] KVM: MMU: fix missing locking in alloc_mmu_pages
2.6.31-stable review patch.  If anyone has any objections, please let us know.

From: Marcelo Tosatti <>

(cherry picked from commit 6a1ac77110ee3e8d8dfdef8442f3b30b3d83e6a2)

n_requested_mmu_pages/n_free_mmu_pages are used by
kvm_mmu_change_mmu_pages to calculate the number of pages to zap.

alloc_mmu_pages, called from the vcpu initialization path, modifies this
variables without proper locking, which can result in a negative value
in kvm_mmu_change_mmu_pages (say, with cpu hotplug).

Signed-off-by: Marcelo Tosatti <>
Signed-off-by: Greg Kroah-Hartman <>

arch/x86/kvm/mmu.c | 2 ++
1 file changed, 2 insertions(+)

--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2713,12 +2713,14 @@ static int alloc_mmu_pages(struct kvm_vc


+ spin_lock(&vcpu->kvm->mmu_lock);
if (vcpu->kvm->arch.n_requested_mmu_pages)
vcpu->kvm->arch.n_free_mmu_pages =
vcpu->kvm->arch.n_free_mmu_pages =
+ spin_unlock(&vcpu->kvm->mmu_lock);
* When emulating 32-bit mode, cr3 is only 32 bits even on x86_64.
* Therefore we need to allocate shadow page tables in the first

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