Milestone 6: mkcapsule signing + capsule_birth.c wiring, WARN-only

First attempt shelled out to `openssl pkeyutl -sign` (fork/execlp, not system() -- avoided shell string interpolation of the key path). Corrected on request: no new external host binary dependency when the repo's own code can do the job -- same standing preference as the earlier anti-file correction. Rewritten to link ed25519_sign() (already verified against OpenSSL in Phase B) directly into mkcapsule. New tools/pkcs8_ed25519.c: a narrow DER walker (same shape as x509_ed25519.c, deliberately not shared -- small enough that duplicating a few TLV-walking lines beat threading a header between the kernel crypto tree and host tooling) extracting the raw seed from the intermediate's PKCS#8 private key, plus a minimal self-written base64 decoder (PEM is openssl genpkey's default output; no decoder existed anywhere in the repo). Verified end-to-end before wiring anything in: the extracted seed's derived pubkey matches the cert's exactly, and a full self-contained sign+verify round-trip (zero openssl) passes. CapsuleDesc had no spare bytes, so signatures live in a new parallel CapsuleSigEntry array, emitted by a new `mkcapsule --sign-key <path>` flag (omitted/missing key -> has_sig=0 everywhere, graceful, not a build failure -- CI has no access to the offline key). New capsule_sig.c/.h: capsule_verify_signature(), a separate function, not folded into the already-tested capsule_validate(). Finds and caches the embedded intermediate cert's pubkey once per boot, then verifies against it. Wired into all three capsule_validate() call sites in capsule_birth.c via log_message(LOG_WARN, ...) -- never refuses yet, per the earlier staged-rollout decision. Verified independently, both directions, live in the real kernel: a full clean build (38 signed capsules) boots clean on all three architectures with zero warnings. Separately, hand-corrupted one byte of Mama's own init.4th capsule's stored signature (not its payload/hash, which capsule_validate() already catches and would have masked the test) and rebuilt just the changed object: produced exactly "capsule sig: init.4th: INVALID -- signature does not verify" on boot, and the kernel still reached ok> -- proving warn-only doesn't refuse anything yet. Reverted before the final, untampered 3-arch acceptance pass. Still open: flipping WARN to hard-refuse (separate, deliberate step) and the BLOCK_MAP.md signature-status column. Documented in FABRIC-3.md. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01U14ET9CWAtbQMbYqomKgXd
2026-08-26 21:32:29 -04:00
parent 431bcb1f34
commit 2fc55f47e1
16 changed files with 45762 additions and 11 deletions
@@ -0,0 +1,68 @@
+/* capsule_sig.c -- see capsule_sig.h. */
+#include "starkernel/capsule_sig.h"
+#include "starkernel/x509_ed25519.h"
+#include "starkernel/ed25519.h"
+#include <string.h>
+
+#define SNAKEOIL_CERT_NAME "pki:snakeoil-intermediate.der"
+
+/* Cached once per boot: the cert doesn't change mid-boot, and every
+ * capsule birth (Mama, then every child VM) would otherwise re-find and
+ * re-parse the same cert. -1 = not yet attempted, 0 = failed, 1 = ready. */
+static int pubkey_state = -1;
+static uint8_t cached_pubkey[32];
+
+static int find_and_cache_pubkey(const CapsuleDesc *descs,
+                                  const CapsuleNameEntry *names,
+                                  const uint8_t *arena_base,
+                                  uint32_t desc_count) {
+    if (pubkey_state != -1) return pubkey_state;
+
+    pubkey_state = 0; /* assume failure unless every step below succeeds */
+    for (uint32_t i = 0; i < desc_count; i++) {
+        if (strcmp(names[i].name, SNAKEOIL_CERT_NAME) == 0) {
+            const CapsuleDesc *cert_desc = &descs[i];
+            if (x509_extract_ed25519_pubkey(arena_base + cert_desc->offset,
+                                             (size_t)cert_desc->length,
+                                             cached_pubkey) == 0) {
+                pubkey_state = 1;
+            }
+            break;
+        }
+    }
+    return pubkey_state;
+}
+
+CapsuleSigResult capsule_verify_signature(
+    const CapsuleDesc *descs, const CapsuleNameEntry *names,
+    const CapsuleSigEntry *sigs, const uint8_t *arena_base,
+    uint32_t desc_count, int index) {
+
+    if (!descs || !names || !sigs || !arena_base || index < 0 ||
+        (uint32_t)index >= desc_count) {
+        return CAPSULE_SIG_NO_ROOT_KEY; /* malformed call -- treat like "can't check" */
+    }
+
+    if (!find_and_cache_pubkey(descs, names, arena_base, desc_count)) {
+        return CAPSULE_SIG_NO_ROOT_KEY;
+    }
+
+    if (!sigs[index].has_sig) {
+        return CAPSULE_SIG_MISSING;
+    }
+
+    const CapsuleDesc *d = &descs[index];
+    int ok = ed25519_verify(cached_pubkey, arena_base + d->offset,
+                             (size_t)d->length, sigs[index].sig);
+    return ok ? CAPSULE_SIG_OK : CAPSULE_SIG_INVALID;
+}
+
+const char *capsule_sig_result_str(CapsuleSigResult result) {
+    switch (result) {
+        case CAPSULE_SIG_OK:          return "ok";
+        case CAPSULE_SIG_MISSING:     return "missing (unsigned)";
+        case CAPSULE_SIG_INVALID:     return "INVALID -- signature does not verify";
+        case CAPSULE_SIG_NO_ROOT_KEY: return "no root key (cert not found/parseable)";
+        default:                      return "unknown";
+    }
+}