diff --git a/fs/ntfs3/index.c b/fs/ntfs3/index.c
index 5868964e129c..ade276225999 100644
--- a/fs/ntfs3/index.c
+++ b/fs/ntfs3/index.c
@@ -1862,6 +1862,20 @@ indx_insert_into_buffer(struct ntfs_index *indx, struct ntfs_inode *ni,
 	memcpy(up_e, sp, sp_size);
 
 	used1 = le32_to_cpu(hdr1->used);
+
+	/*
+	 * hdr_find_split does not validate per-entry sizes, so a crafted
+	 * NTFS_DE whose le16 size field is out of range can place sp such
+	 * that (PtrOffset(hdr1, sp) + sp_size) exceeds used1. Without this
+	 * guard the u32 'used = used1 - to_copy - sp_size' underflows and
+	 * the subsequent memmove count becomes a near-4-GiB value,
+	 * triggering an out-of-bounds kernel write.
+	 */
+	if (PtrOffset(hdr1, sp) + sp_size > used1) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	hdr1_saved = kmemdup(hdr1, used1, GFP_NOFS);
 	if (!hdr1_saved) {
 		err = -ENOMEM;