Reject nbits==0 ProductQuantizer on deserialization (T279464247) (#5483)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5483

Agentic fuzzing (faiss_rmdt_post_read_reconstruct_fuzzer) found a
null-pointer read in PQDecoderGeneric::decode(): a serialized index
whose ProductQuantizer declares nbits == 0 derives code_size == 0
(fbcode/faiss/impl/ProductQuantizer.cpp:69), which makes the codes-size
consistency check in the deserializer trivially satisfied by an empty
codes vector for any ntotal ("0 == ntotal * 0"). The first reconstruct()
call then hands codes.data() == nullptr down to PQDecoderGeneric::decode(),
which dereferences it unconditionally (reg = *code).

D112368333 patched this for the IxPq (IndexPQ) branch specifically by
adding a `code_size > 0 || ntotal == 0` guard local to that branch. That
missed every other deserialization path that shares the same
read_ProductQuantizer() entry point and the same nbits==0 degenerate
case -- in particular Index2Layer ("Ix2L"), whose pq.code_size feeds
code_size_2 via validate_code_size_match(), which only checks equality
and not positivity. The finding kept regressing (crashbot re-flagged it
on 2026-07-24, 07-27, 07-28) because the crash still reproduces, now via
Index2Layer::sa_decode -> ProductQuantizer::decode -> PQDecoderGeneric::decode.

Fix: move the validation to the shared deserializer entry point,
read_ProductQuantizer() (fbcode/faiss/impl/index_read.cpp), by tightening
the existing nbits upper-bound check into a range check requiring
nbits >= 1. This is "Option 1" from the original AI Security Assessment,
which the assessment itself noted is the correct place for the check:
nbits == 0 is a legitimate in-memory state for a default-constructed,
untrained PQ (ProductQuantizer() : ProductQuantizer(0, 1, 0)), so the
check can't live in set_derived_values(); it belongs at deserialization,
where nbits == 0 always means a corrupt/malicious index. This closes the
gap for every current and future caller of read_ProductQuantizer (IxPq,
IVFPQ, IVFPQR, Index2Layer, IMI, PQFastScan, ...), not just IndexPQ.

Reviewed By: mnorris11

Differential Revision: D114027516

fbshipit-source-id: 571ededc9bd055b2fd220f7d2431e01c9ba7fcbd

faiss/impl/index_read.cpp

@@ -737,7 +737,9 @@ void read_ProductQuantizer(ProductQuantizer* pq, IOReader* f) {
     FAISS_THROW_IF_NOT_FMT(
             pq->M > 0, "invalid ProductQuantizer M=%zd (must be > 0)", pq->M);
     FAISS_THROW_IF_NOT_FMT(
-            pq->nbits <= 24, "invalid ProductQuantizer nbits=%zd", pq->nbits);
+            pq->nbits >= 1 && pq->nbits <= 24,
+            "invalid ProductQuantizer nbits=%zd (must be in [1, 24])",
+            pq->nbits);
     {
         size_t ksub = size_t{1} << pq->nbits;
         size_t n = mul_no_overflow(pq->d, ksub, "PQ centroids");