fix(gc): reject fabricated Map/Set headers in plausible_gc_header - #8251
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classify_arena validates that an address and addr-8 are in heap space,
then pattern-matches a GcHeader at addr-8 — but never checks addr-8 is
an object START. An 8-aligned interior arena pointer (a word in a live
object's payload) supplies a fabricated GcHeader whose:
- obj_type = low byte = 0x08 = GC_TYPE_MAP (the only valid type
that is a multiple of 8)
- size = top 32 bits ≈ 1024 (always passes the old [8, 2^34] range
check)
- gc_flags = second byte, needs GC_FLAG_ARENA (0x02) — ~coin flip
move_young then copy_nonoverlapping's ~1024 bytes from that interior
address, and the remembered-set rebuild reads byte 8 of the copied data
as MapHeader.entries — a NaN-boxed JSValue carrying 0x7FFD in the top
bits, which crashes on dereference.
Three-layer fix:
1. Primary: tighten plausible_gc_header so fixed-layout types (Map, Set)
must have size == their known constant total (GC_HEADER_SIZE + 16 =
24). A fabricated header has size ≈ 1024, which is rejected. The
invariant holds because:
- The nursery bump-allocator always sets size = total
- The old-gen allocator uses exact-match free-list reuse and sets
size = total
- The nursery free-list reuse path is inert (hot_arena_free_list is
never populated); it is also fixed to set size = total and use
exact-match for safety
2. Allocator hardening: the nursery free-list reuse path now sets
size = total (was: retained the stale slot size) and uses exact-match
only (was: best-fit), mirroring arena_alloc_gc_old's
old_free_take_exact. This prevents a future activation of the free
list from breaking the fixed-layout invariant.
3. Defensive tripwire: the Map and Set descriptor arms now reject an
entries/elements pointer whose top bits (>>47) are non-zero — an
impossible x86-64 user-space address, and the exact signature of a
NaN-boxed JSValue misread as a pointer. This is a backstop; the
primary fix stops fabrication at classify_arena.
Regression test: four deterministic tests in
gc/tests/copying/fabricated_map_rejection.rs that drive a fabricated
Map header (size=1024) through plausible_gc_header and classify_arena
and assert rejection, plus positive tests for genuine headers and
variable-size types.
Verification on the sfw-registry --help workload (firewall repo,
iovalkey forced, loop polls compiled and run):
- Plain arm: 120/120 PASS (0% failure; 95% upper bound ~2.5%)
- Seeded arm (rate=0.05, 120 seeds): 110/120 PASS, 10 FAIL — all
TypeError ("Cannot convert undefined or null to object"), 0 SIGSEGV.
The TypeError failures are a SEPARATE rooting bug, not the
fabricated-Map bug. The fabricated-Map SIGSEGV is eliminated.
- Full cargo test -p perry-runtime --lib: 2535/2535 PASS
The PerryTS#7161 stopgap (moving loop polls default-OFF) is already reverted
(default ON since PerryTS#7682). This fix eliminates the SIGSEGV class of
failures that justified the stopgap. However, a second bug (rooting
TypeError) remains under seeded schedules, so the default-ON state is
not yet fully safe under adversarial GC timing.
📝 WalkthroughWalkthroughThe runtime now reuses only exact-size arena slots, updates reused header sizes, validates fixed-layout Map and Set headers, rejects implausible pointers, and adds copying-GC regression tests. ChangesGC validation hardening
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: 🟠 High · up to Although the PR tightens Map/Set size validation and hardens allocator reuse, a correctly sized fabricated interior Map header can still be treated as a real object, while the new pointer checks can accept invalid low addresses. That leaves a concrete invalid-memory-access risk in garbage collection, so the PR is not merge-ready until object-start validation and shared heap-address validation are added. Possibly related PRs
Suggested reviewers: 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@crates/perry-runtime/src/gc/copying_pointer_set.rs`:
- Around line 268-271: Update the allocation classification logic in
copying_pointer_set.rs around plausible_gc_header so it verifies header is an
arena allocation start, not merely a valid-sized header; retain the fixed-size
check and reject interior or fabricated addresses. In
crates/perry-runtime/src/gc/tests/copying/fabricated_map_rejection.rs lines
193-223, add or update coverage requiring None for a fabricated Map header whose
size equals MAP_FIXED_TOTAL.
In `@crates/perry-runtime/src/gc/layout_slot_visit.rs`:
- Around line 204-220: Replace the local high-bit validation in the Map layout
visitor with crate::value::addr_class::is_plausible_heap_addr for
MapHeader.entries, preserving the diagnostic and early return behavior. Apply
the same predicate to SetHeader.elements in
crates/perry-runtime/src/set.rs:558-569; both sites must use the shared runtime
heap-address classifier rather than independent bit checks.
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crates/perry-runtime/src/arena/allocators.rscrates/perry-runtime/src/gc/copying_pointer_set.rscrates/perry-runtime/src/gc/layout_slot_visit.rscrates/perry-runtime/src/gc/tests/copying.rscrates/perry-runtime/src/gc/tests/copying/fabricated_map_rejection.rscrates/perry-runtime/src/set.rs
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The defensive entries/elements tripwires added in #8251 used a local `addr >> 47 != 0` cutoff to catch the NaN-box signature (top bits 0x7FFD) of a fabricated Map/Set header. CodeRabbit noted two gaps: * The cutoff only rejects the upper bits; a low garbage address (below the handle band) or a handle-band id reads as `0 >> 47 == 0` and is accepted, so the collector could still derive a slot range from an unmapped / unrelated low address. * The cutoff is platform-wrong on aarch64 Linux, where user space reaches bit 48 (HEAP_MAX = 0x1_0000_0000_0000) but `>> 47` rejects bit 47+, so a genuine entries pointer in that range would be false-rejected. Replace both local checks with the shared `crate::value::addr_class::is_plausible_heap_addr` predicate already used across the gc module (forwarding, fromspace_scan, dead_owner). It pairs `is_above_handle_band` with the platform-correct `is_valid_obj_ptr` range, so it rejects low / handle-band / NaN-box garbage while accepting every genuine entries/elements pointer. Safety: Map entries and Set elements are always system-allocator pointers (`std::alloc::alloc`), never arena or slab, and a capacity-0 alloc is coerced to 4 so the pointer is real and non-null. System malloc returns heap-range addresses above the handle band on every supported platform, so `is_plausible_heap_addr` accepts all genuine entries and the change is strictly more conservative — it can only reject more garbage, never a live object. Regression test: `test_gc_element_slot_range_rejects_implausible_elements` covers NaN-box, low-addr, and handle-band `elements` words (all now rejected) plus a genuine Set (accepted). `cargo test -p perry-runtime --lib`: 2556 passed, 0 failed, 4 ignored. Follow-up to #8251 (CodeRabbit Major finding on layout_slot_visit / set.rs).
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The bug
classify_arenavalidates that an address andaddr - 8are in heap space, then pattern-matches aGcHeaderataddr - 8. It never checks thataddr - 8is an object start. An 8-aligned interior arena pointer — a word in a live object's payload that happens to be an arena address — fabricates a fake object from the bytes preceding it.For an arena near
0x400_0000_0000, the fabricatedGcHeadersupplies:obj_type= low byte =0x08=GC_TYPE_MAP(the only valid type that is a multiple of 8)size= top 32 bits ≈ 1024 (always passes the old[8, 2^34]range check)gc_flags= second byte, needsGC_FLAG_ARENA(0x02) — ~coin flipmove_youngthencopy_nonoverlappings ~1024 bytes from that interior address, and the remembered-set rebuild reads byte 8 of the copied data asMapHeader.entries— a NaN-boxed JSValue carrying0x7FFDin the top bits, which crashes on dereference (SIGSEGV).GC_TYPE_SET(12) can never be fabricated this way (12 is not 8-aligned), so Map is the only reachable descriptor arm that derives a slot base from a payload word.The fix — three layers
1. Primary: tighten
plausible_gc_headerfor fixed-layout typesFor types whose payload layout is constant (Map, Set = 16-byte header → 24-byte total),
plausible_gc_headernow requiressize == 24. A fabricated header hassize ≈ 1024, which is rejected at the classification stage — beforemove_youngor the descriptor arm ever runs.The invariant holds because:
size = total(allocators.rs:504)size = total(allocators.rs:242)hot_arena_free_listis never populated (no.push()call exists in the codebase), so the branch that would retain a stale largersizeis dead code2. Allocator hardening (future-proofing)
The nursery free-list reuse path now:
(*header).size = total(was: retained the stale slot size from the original allocation)arena_alloc_gc_old'sold_free_take_exactThis prevents a future activation of the free list from breaking the fixed-layout invariant. Since the free list is currently dead code, this change is zero-risk.
3. Defensive tripwire in Map/Set descriptor arms
The Map descriptor (
layout_slot_visit.rs) and Set'sgc_element_slot_range(set.rs) now reject anentries/elementspointer whose top bits (>> 47) are non-zero — an impossible x86-64 user-space address, and the exact signature of a NaN-boxed JSValue misread as a pointer. This is a backstop; the primary fix stops fabrication atclassify_arena.Regression test
Four deterministic tests in
gc/tests/copying/fabricated_map_rejection.rs:test_plausible_gc_header_rejects_fabricated_map_size— fabricated Map (size=1024) rejected, genuine (size=24) accepted, wrong-size (32) rejectedtest_plausible_gc_header_rejects_fabricated_set_size— same for Settest_plausible_gc_header_still_accepts_variable_size_types— arrays/strings with arbitrary sizes still passtest_classify_arena_rejects_interior_pointer_as_map— end-to-end: a fabricated GcHeader written into an array's payload is rejected byclassify_arenaVerification
cargo test -p perry-runtime --libThe SIGSEGV class is eliminated. The baseline had "a mix of SIGSEGV and zod-core TypeErrors" (8/120 = 6.7%). After the fix, 0/120 plain runs fail and 0/120 seeded runs produce SIGSEGV.
A second bug remains. The 10 seeded failures are all
TypeError: Cannot convert undefined or null to object— a rooting bug (#7154 family) where a value live across a collection point is not rooted, and after the copying minor moves 6241 objects, the stale reference reads undefined. This is a different bug from the fabricated-Map issue and is not addressed by this PR.The #7161 stopgap
The #7161 stopgap (moving loop polls default-OFF) is already reverted —
PERRY_GC_MOVING_LOOP_POLLSdefaults to ON since #7682. This fix eliminates the SIGSEGV class of failures that originally justified the stopgap, making the default-ON state safer. However, because the rooting TypeError persists under seeded schedules, the default-ON state is not yet fully safe under adversarial GC timing. The revert is not earnable until the rooting bug is also fixed.Summary by CodeRabbit
Bug Fixes
Tests