Validate IndexHNSW2Level storage type during deserialization and search (#5113)

Summary:
Pull Request resolved: #5113

Add validation that IndexHNSW2Level (fourcc "IHN2") has storage of an appropriate type, both at deserialization time and at search time.

IndexHNSW2Level::search() uses dynamic_cast to dispatch between Index2Layer and IndexIVFPQ storage types. When storage is null or a different type (e.g. IndexFlat from corrupt serialized data, or a programmatically misconfigured index), the dynamic_cast returns nullptr which is then unconditionally dereferenced, causing a segfault.

Deserialization-time fix: After reading the HNSW storage sub-index for IHN2, validate that storage is non-null and is either Index2Layer or IndexIVFPQ.

Search-time defense-in-depth: Add a FAISS_THROW_IF_NOT check on the dynamic_cast result in IndexHNSW2Level::search() before dereferencing. This protects against programmatically constructed indexes that bypass deserialization validation.

Reviewed By: mnorris11

Differential Revision: D101243603

Author: scsiguy

faiss/IndexHNSW.cpp

@@ -865,6 +865,9 @@ void IndexHNSW2Level::search(
 
         const IndexIVFPQ* index_ivfpq =
                 dynamic_cast<const IndexIVFPQ*>(storage);
+        FAISS_THROW_IF_NOT_MSG(
+                index_ivfpq,
+                "IndexHNSW2Level mixed search requires IndexIVFPQ storage");
 
         int nprobe = index_ivfpq->nprobe;
 

faiss/impl/index_read.cpp

@@ -1982,6 +1982,15 @@ std::unique_ptr<Index> read_index_up(IOReader* f, int io_flags) {
                     idxhnsw->storage->d,
                     idxhnsw->d);
         }
+        if (h == fourcc("IHN2")) {
+            FAISS_THROW_IF_NOT_MSG(
+                    idxhnsw->storage,
+                    "IndexHNSW2Level requires non-null storage");
+            FAISS_THROW_IF_NOT_MSG(
+                    dynamic_cast<Index2Layer*>(idxhnsw->storage) ||
+                            dynamic_cast<IndexIVFPQ*>(idxhnsw->storage),
+                    "IndexHNSW2Level storage must be Index2Layer or IndexIVFPQ");
+        }
         if (h == fourcc("IHNp") && !(io_flags & IO_FLAG_PQ_SKIP_SDC_TABLE)) {
             auto* storage_pq = dynamic_cast<IndexPQ*>(idxhnsw->storage);
             FAISS_THROW_IF_NOT_MSG(

tests/test_read_index_deserialize.cpp

@@ -1467,6 +1467,38 @@ TEST(ReadIndexDeserialize, HNSWValidNeighborsSearchWorks) {
     EXPECT_LT(labels[0], ntotal);
 }
 
+// -----------------------------------------------------------------------
+// Test: IndexHNSW2Level with wrong storage type is rejected.
+// Protects against corrupt serialized data where storage is not
+// Index2Layer or IndexIVFPQ, causing null-deref from failed dynamic_cast.
+// -----------------------------------------------------------------------
+TEST(ReadIndexDeserialize, HNSW2LevelWrongStorageType) {
+    // Build an IHN2 with IndexFlat storage (wrong type — must be
+    // Index2Layer or IndexIVFPQ).
+    std::vector<uint8_t> buf;
+    push_fourcc(buf, "IHN2");
+    push_index_header(buf, /*d=*/4, /*ntotal=*/0);
+    push_minimal_hnsw(buf, /*ntotal=*/0);
+    // IndexFlat storage — wrong type for HNSW2Level
+    push_minimal_flat(buf, /*d=*/4, /*ntotal=*/0);
+
+    expect_read_throws_with(buf, "Index2Layer or IndexIVFPQ");
+}
+
+// -----------------------------------------------------------------------
+// Test: IndexHNSW2Level with null storage is rejected.
+// -----------------------------------------------------------------------
+TEST(ReadIndexDeserialize, HNSW2LevelNullStorage) {
+    std::vector<uint8_t> buf;
+    push_fourcc(buf, "IHN2");
+    push_index_header(buf, /*d=*/4, /*ntotal=*/0);
+    push_minimal_hnsw(buf, /*ntotal=*/0);
+    // Null storage
+    push_fourcc(buf, "null");
+
+    expect_read_throws_with(buf, "non-null storage");
+}
+
 // -----------------------------------------------------------------------
 // Test: NSG ntotal != index ntotal.
 // -----------------------------------------------------------------------