gix-pack: uncapped Tree::with_capacity from pack-header object count causes OOM/process abort on clone/fetch

Summary

During any clone/fetch, gix-pack sizes its delta-resolution Tree directly from the attacker-controlled 32-bit object count in the 12-byte pack header, before any object body is read. The backing allocation uses Vec::reserve_exact (not try_reserve), so a header claiming a huge object count forces a multi-hundred-gigabyte reservation that aborts the process via handle_alloc_error. This is a pre-authentication, ~12-byte remote denial of service against any application built on gix/gix-pack that clones or fetches from an untrusted remote.

Root Cause

write_data_iter_to_stream (gix-pack/src/index/write/mod.rs:123) does:

let (anticipated_num_objects, upper_bound) = entries.size_hint();
let worst_case_num_objects_after_thin_pack_resolution = upper_bound.unwrap_or(anticipated_num_objects);
let mut tree = Tree::with_capacity(worst_case_num_objects_after_thin_pack_resolution)?;

Tree::with_capacity (gix-pack/src/cache/delta/tree.rs:51) calls exact_vec(num_objects/2) twice, and exact_vec (gix-pack/src/lib.rs:100-104) is:

fn exact_vec<T>(capacity: usize) -> Vec<T> {
    let mut v = Vec::new();
    v.reserve_exact(capacity);   // aborts on failure; does NOT return Err
    v
}

reserve_exact aborts via handle_alloc_error on failure — it does not return Err — so the Result return type of with_capacity does NOT guard the allocation. The upper_bound derives from size_hint(): BytesToEntriesIter::size_hint returns Some(objects_left) where objects_left = the raw num_objects: u32 read from the pack header (gix-pack/src/data/header.rs:18), and LookupRefDeltaObjectsIter::size_hint doubles it. So worst_case = 2 * num_objects.

Impact

num_objects = 0xFFFFFFFF yields two reserve_exact(~4.29e9) allocations of Vec<Item<TreeEntry>> (~76 bytes/element ≈ 326 GB each), attempted before any entry body is decompressed. On default-overcommit Linux (vm.overcommit_memory=0, heuristic mode rejects the oversized single request) and on Windows (commit charge), the allocation fails and the process aborts (SIGABRT). No num_objects sanity check exists before the allocation.

Proof of Concept

Serve an upload-pack whose pack stream begins with a valid header 50 41 43 4B 00 00 00 02 FF FF FF FF ("PACK", version 2, num_objects=0xFFFFFFFF) to a victim running gix clone/fetch. The Tree::with_capacity call attempts ~326 GB reservations and aborts the process before reading any object body.

Attack Chain

1. Entry: attacker-served (or MITM-redirected) upload-pack → victim gix clone/fetch. Guard: none (serving a pack needs no auth). Bypass proof: N/A.
2. Check: data::header::decode (header.rs:12-21) accepts any u32 object count. Guard: header validation. Bypass proof: only "PACK" magic + version (2 or 3) are checked; num_objects has no upper bound.
3. Processing: size_hint() → objects_left as usize → LookupRefDeltaObjectsIter::size_hint doubles it → worst_case = 2 * num_objects (index/write/mod.rs:122).
4. Sink: Tree::with_capacity(2*num_objects) → exact_vec(n/2) → Vec::reserve_exact (lib.rs:102). Guard: alloc_limit_bytes (added by the GHSA-x494-mj8g-cj27 follow-up fix ace687ce). Bypass proof: git show ace687ce -- gix-pack/src/index/write/mod.rs shows the param was added to the function signature and forwarded ONLY to resolve() (line 207); the Tree::with_capacity call at line 123 takes no limit and runs earlier.
5. Impact: reserve_exact of hundreds of GB fails and calls handle_alloc_error → process abort. Pre-auth, ~12 attacker bytes.

Bypass Evidence

• git show gix-pack-v0.74.2:gix-pack/src/lib.rs confirms exact_vec uses v.reserve_exact(capacity) with no try_reserve and no cap.
• git show gix-pack-v0.74.2:gix-pack/src/index/write/mod.rs confirms line 123 Tree::with_capacity(worst_case...)? is unguarded by alloc_limit_bytes.
• The published advisory GHSA-x494-mj8g-cj27 covers the DIFFERENT allocation sink bytes_to_entries.rs:109 (Vec::with_capacity(decompressed_size)); this Tree::with_capacity sink is untouched by its fix (commit ace687ce added alloc_limit_bytes only to resolve()).

Affected Versions

gix-pack <= 0.74.2 (latest release tag gix-pack-v0.74.2; code present on HEAD e3a6fa15). No post-tag fix.

Suggested Fix

Cap worst_case_num_objects_after_thin_pack_resolution against a configurable maximum (like git's transfer.maxPackSize), and change exact_vec to use try_reserve_exact and propagate the error through Tree::with_capacity's existing Result.

Caveat

On kernels with vm.overcommit_memory=1 (always-overcommit) the virtual reservation succeeds and the cheap crash degrades (the attacker must then actually stream billions of entries). Hence severity MEDIUM rather than HIGH. On CI/CD auto-clone (UI:N) severity rises to ~7.5.

--- Reported by zx (Jace) — GitHub: @manus-use