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coldstart: recover v1 boxes, and make key wrapping an explicit act

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Three ways the cold-start path could lose the database.

A box enrolled before the PRF change could never cold-start again. UnwrapKey
still read v1 blobs, but the only caller stopped supplying the v1 secret: the
assertion handler sends the PRF output and nothing looks up the credential
public key any more. On such a box the daemon read the blob, took the v1
branch, failed to decrypt, and stayed locked while a valid passkey was
asserted at it. The escape hatch was gone too, because WrapKeyFn was wired
only in env-key mode and a locked boot is by definition the mode with no env
key. The recovery was to put MAVEN_DB_KEY back in the environment, which is
the thing cold-start unlock exists to avoid. AssertFinish now retries a failed
PRF unwrap with the credential public key, and WrapKeyFn is wired in locked
mode too, so the box that came up on a v1 blob can be moved to v2.

Wrapping ran on every successful assertion. That made a routine step-up
rewrite the one file that opens the database, under whatever 32 bytes the page
posted. A compromised /auth/webauthn converted one legitimate touch into
permanent offline recovery of the at-rest key, and a second enrolled
authenticator silently locked out the first. Wrapping is now an act of its
own: a plain assertion may write the blob only when none exists, and replacing
one takes the rewrite button, which is the only caller that sets the new
explicit flag. The daemon still refuses to overwrite a v2 blob that does not
open under the presented secret.

The write was os.WriteFile, which truncates in place. A power cut between the
truncate and the write left a zero-length blob and no previous contents, on
the path of every step-up. It is now a temp file in the same directory, fsync,
rename, fsync of the directory.

Two smaller things on the same path. The v2 unwrap checked the secret length
but not the all-zero case the wrap side rejects, so the two ends disagreed
about what a valid secret is. And the handler logged "daemon unlocked via
credential" when an env-key daemon had answered unknown method, and again when
an already-unlocked daemon had done nothing.

Left alone deliberately: the PRF value is client-supplied and not covered by
the assertion signature. That is inherent to PRF key wrapping, since the salt
has to be fixed for the blob to open on the next boot. It is recorded as a
known property where the secret enters the handler.

Found in review of #77.
This commit is contained in:
kami
2026-08-01 14:05:13 +04:00
parent 7f42cc73be
commit 7ab9b48259
13 changed files with 654 additions and 65 deletions
Download Patch File Download Diff File Expand all files Collapse all files
cmd/mavend/main.go
+65 -22
View File
@@ -25,7 +25,7 @@
// When a passkey credential is enrolled AND no env key is set, the daemon
// starts in LOCKED mode: the IPC server runs but rejects all store methods
// except MethodAssertStepUp and MethodUnlock. A passkey assertion followed
// by MethodUnlock (with the same credential's public key) unwraps the at-rest
// by MethodUnlock (with that credential's WebAuthn PRF output) unwraps the at-rest
// AES-256 key from a wrapped blob on disk (HKDF-SHA256 + AES-GCM) and opens
// the encrypted store. After unlock, the daemon wires voice, loop, and
// delivery and runs normally.
@@ -34,10 +34,13 @@
// starts unlocked from the env key (pre-unlock behavior). Enrolling a passkey
// while unlocked calls MethodStoreEncryptionKey to wrap the env key and
// persist the wrapped blob — enabling cold-start unlock on the next boot
// after the env key is removed.
// after the env key is removed. That write happens once, when no blob
// exists; replacing an existing one takes an explicit request, see
// cmd/mavend/keyfile.go.
package main
import (
"bytes"
"context"
"encoding/json"
"errors"
@@ -48,6 +51,7 @@ import (
"os"
"os/signal"
"sync"
"sync/atomic"
"syscall"
"time"
@@ -164,11 +168,28 @@ func run(args []string) error {
var st *store.Store
var envKeyBytes []byte // kept for WrapKeyFn (enrollment wraps this key)
// dbKey — the plaintext at-rest key, once the daemon has one. Set at boot
// in env-key mode and inside UnlockFn after a cold start. WrapKeyFn reads
// it from an IPC goroutine, hence the atomic: srv's function fields are
// installed before Serve and must not be reassigned afterwards.
var dbKey atomic.Pointer[[]byte]
// wrappedPath resolves the blob location the same way for both the read
// at boot and every write, so a default-path deployment cannot wrap to
// one file and unwrap from another.
wrappedPath := func() string {
if *wrappedKeyPath != "" {
return *wrappedKeyPath
}
return cfg.DefaultWrappedKeyPath()
}
if !locked {
// Normal boot: env key or plaintext (dev/CI)
if envKey != nil {
envKeyBytes = make([]byte, len(envKey))
copy(envKeyBytes, envKey)
dbKey.Store(&envKeyBytes)
st, err = store.OpenEncrypted(ctx, cfg.DBPath, cfg.DBTmpfs, envKey)
} else {
st, err = store.Open(ctx, cfg.DBPath)
@@ -387,27 +408,44 @@ func run(args []string) error {
wireSpeaker(srv, st, cfg)
}
// WrapKeyFn — wraps the env key under the passkey PRF secret and persists
// the wrapped blob. Only wired when the daemon has the key in memory (env
// key mode). Called by mavweb after passkey enrollment.
// WrapKeyFn — wraps the at-rest key under the passkey PRF secret and
// persists the wrapped blob. Called by mavweb after every assertion.
//
// It is wired in locked mode too, not only in env-key mode, and that is
// what makes a v1 blob recoverable. A box enrolled before Vikunja #14
// cold-starts through the legacy public-key retry in mavweb, and the
// StoreEncryptionKey that follows rewrites the blob as v2. Without this
// the only escape from a v1 blob was putting MAVEN_DB_KEY back in the
// environment, which is the thing cold-start unlock exists to avoid.
//
// webauthn.WrapKey refuses anything that is not a 32-byte PRF output, so
// an authenticator without PRF support produces no wrapped file at all
// rather than a file that looks protected and is not.
if envKeyBytes != nil {
srv.WrapKeyFn = func(ctx context.Context, secret []byte) error {
blob, err := webauthn.WrapKey(envKeyBytes, secret)
if envKeyBytes != nil || locked {
srv.WrapKeyFn = func(ctx context.Context, secret []byte, explicit bool) error {
kp := dbKey.Load()
if kp == nil {
return errors.New("wrap encryption key: the daemon is locked and has no key yet (unlock first)")
}
wp := wrappedPath()
// Asserting a passkey is not a request to rewrite the cold-start
// key. Without this an assertion carrying a substituted PRF value
// re-wrapped the real database key under it, and a second
// authenticator silently replaced the first one's blob.
if !explicit {
if _, err := os.Stat(wp); err == nil {
return nil
} else if !errors.Is(err, os.ErrNotExist) {
return fmt.Errorf("check wrapped key: %w", err)
}
}
wrote, err := wrapKeyToFile(wp, *kp, secret)
if err != nil {
return fmt.Errorf("wrap encryption key: %w", err)
return err
}
wp := *wrappedKeyPath
if wp == "" {
wp = cfg.DefaultWrappedKeyPath()
if wrote {
log.Printf("mavend: wrapped encryption key under this passkey's PRF output → %s", wp)
}
if err := os.WriteFile(wp, blob, 0o600); err != nil {
return fmt.Errorf("write wrapped key: %w", err)
}
log.Printf("mavend: wrapped encryption key with passkey credential (%d bytes)", len(blob))
return nil
}
}
@@ -428,15 +466,17 @@ func run(args []string) error {
return nil // already unlocked; the caller does not need to know
}
// The wire cannot authenticate its caller — the socket is
// same-uid — so the unlock path requires a passkey assertion
// that mavweb verified cryptographically first. Without this,
// MethodUnlock is reachable by anything on the box.
// Depth, not a boundary. MethodAssertStepUp is AuthRead, so
// anything that can open the same-uid socket can flip the
// session and reach MethodUnlock. What actually stops a local
// attacker is the 32-byte PRF output they do not have, and that
// was true before this check. What this check stops is an
// accidental unlock attempt from an unrelated local caller.
if !passkeySess.IsStepUp() {
return errors.New("unlock: no verified passkey assertion (assert first)")
}
wp := *wrappedKeyPath
wp := wrappedPath()
blob, err := os.ReadFile(wp)
if err != nil {
return fmt.Errorf("read wrapped key: %w", err)
@@ -446,8 +486,11 @@ func run(args []string) error {
return fmt.Errorf("unwrap key: %w", err)
}
if version == webauthn.BlobV1 {
log.Printf("SECURITY: %s was unwrapped from a %s blob. The wrapping key is derived from the credential PUBLIC key, which mavweb also writes to its passkeys.json — anyone holding both files can recover the database key with no authenticator. Re-enroll the passkey on an authenticator that supports the PRF extension to rewrite it as v2.", wp, version)
log.Printf("SECURITY: %s was unwrapped from a %s blob. The wrapping key is derived from the credential PUBLIC key, which mavweb also writes to its passkeys.json — anyone holding both files can recover the database key with no authenticator. Use the \"rewrite cold-start key\" button on /auth/webauthn with a PRF-capable authenticator to replace it with a v2 blob.", wp, version)
}
// WrapKeyFn needs the key to be able to rewrite the blob later.
keyCopy := bytes.Clone(key)
dbKey.Store(&keyCopy)
// Open the store with the unwrapped key.
st, err = store.OpenEncrypted(ctx, cfg.DBPath, cfg.DBTmpfs, key)
if err != nil {