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

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>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 4a02dee1f2b56286ecd113b0b9c89e034902ee39..d7aebafffdfe0cbf5bcb27801db4421e4132cd14 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2094,6 +2094,7 @@ 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);
@@ -2124,6 +2125,7 @@ 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);
@@ -2496,9 +2498,6 @@ 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);