diff --git a/FABRIC-3.7.md b/FABRIC-3.7.md new file mode 100644 index 00000000..06fc8a1f --- /dev/null +++ b/FABRIC-3.7.md @@ -0,0 +1,124 @@ +# FABRIC-3.7.md — Phase 8 PKI: the elevation entrypoint, without the pointer-confusion hole + +**Status: OPEN — design only, no code written or authorized.** Successor to +`FABRIC-3.5.md`/`FABRIC-3.6.md` (both CLOSED/ARCHIVAL at `v2.1.0`) for exactly one topic: the +Ed25519-challenge-response elevation entrypoint that `.claude/CLAUDE.md`'s ACL section names as +Phase 8, the last open item in the word-level ACL system. This is a **new document**, not a +reopening of `FABRIC-3.6.md` — that document's own close header says future fleet work gets its +own document, and this is that. + +**Provenance.** Written 2026-09-22, immediately after `FABRIC-3.6.md`'s close, from a design +conversation with Captain Bob about a security concern he raised directly: how to rebuild the +elevation entrypoint that Phase 4's Category B strip left dangling, without reopening a hole. +The design below was proposed, and Captain Bob asked for it in writing here rather than left +only in session memory. + +--- + +## 1. What's dangling, and why + +`FABRIC-3.6.md` Phase 4 (Category B strip, 2026-09-22) deleted `capsules/common/messaging.4th` +after every FORTH-owned message type had been cut over to kernel-Hermes. One casualty was +collateral, not intended: `SEND-ELEVATE-REQUEST` (`messaging.4th` block 5040) was the only +caller of both `KH-ELEVATE-SEND` (`src/starkernel/repl.c`) and, transitively, `ELEVATE-GRANT` +(`capsules/zuse-eligibility.4th`, blocks 4021–4022). All three still exist in the tree. +`ELEVATE-GRANT` is still loaded at boot (`capsules/init.4th:18`). Nothing can call it any more. + +Captain Bob's decision at the time (`FABRIC-3.6.md`'s own Phase 4 entry): leave it unreachable, +don't patch a caller back in as part of that strip. Phase 8 builds its own entrypoint instead of +resuming this one. **This document is that entrypoint's design.** + +## 2. The security hole in the old mechanism — found before any code was written + +`ELEVATE-GRANT`'s signature, unchanged since it was written: + +``` +ELEVATE-GRANT ( waddr wu pk0 pk1 pk2 pk3 -- ) +``` + +`waddr`/`wu` are an address/length pair meant to point at the string naming the word to elevate. +`pk0`–`pk3` are the caller's Ed25519 pubkey, packed 8 bytes per cell (`ELEVATE-PUBKEY-UNPACK`, +`mama_forth_words.c`). + +**The old `SEND-ELEVATE-REQUEST` computed `waddr` in the *sending* VM's own address space, but +`ELEVATE-GRANT` always executes on Hera** (`ELEVATE-GRANT always runs on Hera` — `repl.c`'s own +comment on `KH-ELEVATE-SEND`, still there). A word's name string lives in the sending VM's +memory. `ELEVATE-GRANT` dereferences `waddr` in Hera's memory. Those are not the same address +space by construction — `vaddr_t` is per-VM. + +**Consequence:** whoever controls `waddr` controls what bytes `NAME>XT` reads and resolves as a +word name, in Hera's dictionary, not the caller's. This is not "the string might be malformed" — +it is a primitive for making Hera's own `ELEVATE-GRANT` grant `ACL-ALLOW!`/`ACL-TTL!` on +*whatever dictionary entry the attacker's chosen `waddr` happens to land on*, regardless of what +word name the caller claims to be requesting elevation for. A caller who can influence `waddr` +at all — not forge a signature, not defeat `zuse_eligibility_is_member()`, just choose a number +— has a privilege-escalation primitive against the fleet governor. + +This was never exploited (the entrypoint has had zero live callers since the file that called it +was deleted), and is reported here as a design defect found by inspection, not a live incident. + +## 3. The fix: never cross an address, only ever cross bytes + +This project already solved the general version of this problem once, this same session +(`FABRIC-3.6.md` tasks 3.8/3.9, the payload-aliasing fix): a kernel-Hermes message's payload +must be **copied into the message's own storage**, never a pointer into the sender's memory that +might be reused or freed before the receiver drains it. `SkHermesMessage.payload_buf` +(`include/starkernel/vm/kernel_hermes.h:160`, `SK_HERMES_CHUNK_MAX_PAYLOAD` = 1024 bytes) is +exactly that fix, already built, already proven on all three architectures. + +**The elevation entrypoint's hole is the same defect one level up: an address crossing a +boundary it isn't valid on the other side of.** The fix generalizes directly: + +1. **Never send `waddr`/`wu` across the kernel-Hermes boundary.** Send the pubkey (32 bytes, + already the right shape for `payload_buf`) and the target word's **name, as literal bytes**, + copied inline into the message payload — not an address, the actual characters. This is + already how `CONSOLE-CMD-EVENT`'s payload works (a command string's bytes, not a pointer to + one), so this isn't a new pattern, it's applying the existing one to the one caller that + still passed a raw address. + +2. **On receipt, kernel-Hermes's C drain-checkpoint copies those name bytes into a small, + fixed, kernel-owned buffer that already lives in Hera's own VM memory** — a receive-side + mirror of the existing send-side pattern (`g_kh_elevate_buf`, `repl.c:445`, is the + already-built precedent for "a static buffer this mechanism owns"; this needs its Hera-side + counterpart). The buffer's address is a compile-time constant, known to the kernel, never + computed from anything the caller supplied. + +3. **The FORTH command handed to `vm_interpret()` on Hera references only that fixed buffer's + address and length as plain integer literals.** Both are always kernel-controlled. Neither is + ever derived from caller input. `ELEVATE-GRANT` itself does not change — same signature, same + `zuse_eligibility_is_member()` check, same `ACL-ALLOW!`/`ACL-TTL!` grant. Policy logic stays + in FORTH, per `ACL.4th`'s own rule (no new C primitive for policy) — this fix is entirely + about how bytes get from one VM to another, not about who is allowed to grant what. + +**Why this closes the hole structurally, not by validation:** there is no string to sanitize and +no address to bounds-check, because the interpreted command never contains anything an attacker +touched except opaque data bytes that get copied, never dereferenced as a pointer, by the +receiving side. The class of bug (cross-address-space pointer confusion) becomes impossible by +construction, the same way `payload_buf` made use-after-free impossible by construction rather +than by careful lifetime tracking. + +## 4. What Phase 8 actually needs to build + +This document is the shape of the fix, not the implementation. Concretely, when Phase 8 starts: + +- A new `SkHermesMessage`-based sender (`KH-ELEVATE-SEND`'s replacement or a rewritten version of + it) that packs `[pubkey: 32 bytes][name_len: 1 byte][name bytes]` into `payload_buf`, instead + of taking a `waddr`/`wu` pair off the stack. +- A receive-side fixed buffer in Hera's memory (parallel to `g_kh_elevate_buf`'s existing + send-side declaration) that the drain-checkpoint path copies the name bytes into before + constructing the `vm_interpret()` command string. +- `ELEVATE-GRANT` unchanged. +- The Ed25519 **challenge-response** itself — proving the caller holds the *private* key, not + just presenting a pubkey that matches something in `zuse_eligibility_is_member()`'s list — is + a separate, larger piece of this same Phase 8 item and is not designed here. Today, WIREBIND/ + MINT trust whatever identity is stored on an attached thumbdrive with no challenge at all + (confirmed by grep: no `ed25519_sign`/`ed25519_verify` call anywhere in `capsule_wirebind.c` or + `capsule_mint.c`). This document only fixes the elevation entrypoint's own transport; the + broader "prove you hold the private key" mechanism is Phase 8's other open half. + +## 5. What this document is not + +Not a reopening of `FABRIC-3.6.md`, not a change to anything currently built, not an +authorization to write code. Per this series' own convention: design here, execution gets its +own document when the work actually starts, the same relationship `FABRIC-3.5.md` had to +`FABRIC-3.6.md`.