[PATCH] KVM: arm64: Reallocate the nested_mmus array under the mmu_lock

[Hyunwoo Kim <imv4bel@gmail.com>]

Code that walks kvm->arch.nested_mmus[] holds kvm->mmu_lock. By contrast,
kvm_vcpu_init_nested() reallocates the array and frees the old buffer while
holding only kvm->arch.config_lock, so a walker can reference the freed
array.

Allocate the new array outside the lock, as the allocation can sleep, and
do only the copy and the pointer swap under the mmu_lock. After the swap no
walker can reach the old buffer, so free it once the lock has been
released.

Fixes: 4f128f8e1aaac ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 arch/arm64/kvm/nested.c | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 38f672e940878..6f7bc9a9992e0 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -89,21 +89,28 @@ int kvm_vcpu_init_nested(struct kvm_vcpu *vcpu)
 	 * again, and there is no reason to affect the whole VM for this.
 	 */
 	num_mmus = atomic_read(&kvm->online_vcpus) * S2_MMU_PER_VCPU;
-	tmp = kvrealloc(kvm->arch.nested_mmus,
-			size_mul(sizeof(*kvm->arch.nested_mmus), num_mmus),
-			GFP_KERNEL_ACCOUNT | __GFP_ZERO);
-	if (!tmp)
-		return -ENOMEM;
 
-	swap(kvm->arch.nested_mmus, tmp);
+	if (num_mmus > kvm->arch.nested_mmus_size) {
+		tmp = kvcalloc(num_mmus, sizeof(*tmp), GFP_KERNEL_ACCOUNT);
+		if (!tmp)
+			return -ENOMEM;
 
-	/*
-	 * If we went through a realocation, adjust the MMU back-pointers in
-	 * the previously initialised kvm_pgtable structures.
-	 */
-	if (kvm->arch.nested_mmus != tmp)
-		for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
-			kvm->arch.nested_mmus[i].pgt->mmu = &kvm->arch.nested_mmus[i];
+		write_lock(&kvm->mmu_lock);
+
+		if (kvm->arch.nested_mmus_size) {
+			memcpy(tmp, kvm->arch.nested_mmus,
+			       size_mul(sizeof(*tmp), kvm->arch.nested_mmus_size));
+
+			for (int i = 0; i < kvm->arch.nested_mmus_size; i++)
+				tmp[i].pgt->mmu = &tmp[i];
+		}
+
+		swap(kvm->arch.nested_mmus, tmp);
+
+		write_unlock(&kvm->mmu_lock);
+
+		kvfree(tmp);
+	}
 
 	for (int i = kvm->arch.nested_mmus_size; !ret && i < num_mmus; i++)
 		ret = init_nested_s2_mmu(kvm, &kvm->arch.nested_mmus[i]);
-- 
2.43.0