FAILED: patch "[PATCH] KVM: arm64: Reassign nested_mmus array behind mmu_lock" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 70543358fa08e0f7cebc3447c3b70fe97ad7aaa8
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026060912-erasure-visiting-baeb@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 70543358fa08e0f7cebc3447c3b70fe97ad7aaa8 Mon Sep 17 00:00:00 2001
From: Hyunwoo Kim <[email protected]>
Date: Fri, 5 Jun 2026 17:27:01 +0900
Subject: [PATCH] KVM: arm64: Reassign nested_mmus array behind mmu_lock

kvm->arch.nested_mmus[] is walked under kvm->mmu_lock, including from the MMU notifier path (kvm_unmap_gfn_range() -> kvm_nested_s2_unmap()), which can run at any time. kvm_vcpu_init_nested() reallocates the array and frees the old buffer while holding only kvm->arch.config_lock, so such a walker can reference the freed array.

Allocate the new array outside of mmu_lock, as the allocation can sleep. Under the lock, copy the existing entries, fix up the back pointers and reassign the array. Free the old buffer after dropping the lock, as kvfree() can sleep as well.

index 38f672e94087..6f7bc9a9992e 100644
@@ -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]);