[PATCH] KVM: MMU: Fix free memory accounting race in mmu_alloc_roots()

[PATCH] KVM: MMU: Fix free memory accounting race in mmu_alloc_roots()

[Avi Kivity]

We drop the mmu lock between freeing memory and allocating the roots; this
allows some other vcpu to sneak in and allocate memory.

While the race is benign (resulting only in temporary overallocation, not oom)
it is simple and easy to fix by moving the freeing close to the allocation.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/mmu.c |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 95bee9d..6857a2f 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2067,6 +2067,7 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
 			root_gfn = 0;
 		}
 		spin_lock(&vcpu->kvm->mmu_lock);
+		kvm_mmu_free_some_pages(vcpu->kvm);
 		sp = kvm_mmu_get_page(vcpu, root_gfn, 0,
 				      PT64_ROOT_LEVEL, direct,
 				      ACC_ALL, NULL);
@@ -2097,6 +2098,7 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu)
 			root_gfn = i << 30;
 		}
 		spin_lock(&vcpu->kvm->mmu_lock);
+		kvm_mmu_free_some_pages(vcpu->kvm);
 		sp = kvm_mmu_get_page(vcpu, root_gfn, i << 30,
 				      PT32_ROOT_LEVEL, direct,
 				      ACC_ALL, NULL);
@@ -2470,9 +2472,6 @@ int kvm_mmu_load(struct kvm_vcpu *vcpu)
 	r = mmu_topup_memory_caches(vcpu);
 	if (r)
 		goto out;
-	spin_lock(&vcpu->kvm->mmu_lock);
-	kvm_mmu_free_some_pages(vcpu);
-	spin_unlock(&vcpu->kvm->mmu_lock);
 	r = mmu_alloc_roots(vcpu);
 	spin_lock(&vcpu->kvm->mmu_lock);
 	mmu_sync_roots(vcpu);
-- 
1.7.0.4

Re: [PATCH] KVM: MMU: Fix free memory accounting race in mmu_alloc_roots()

[Marcelo Tosatti]

On Mon, May 10, 2010 at 12:09:56PM +0300, Avi Kivity wrote:
> We drop the mmu lock between freeing memory and allocating the roots; this
> allows some other vcpu to sneak in and allocate memory.
> 
> While the race is benign (resulting only in temporary overallocation, not oom)
> it is simple and easy to fix by moving the freeing close to the allocation.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kvm/mmu.c |    5 ++---
>  1 files changed, 2 insertions(+), 3 deletions(-)

Applied, thanks.