kami
7ab9b48259
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.
112 lines
3.8 KiB
Go
112 lines
3.8 KiB
Go
package main
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// Writing the wrapped-key blob (Vikunja #14).
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//
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// The blob is the only thing that opens the database on a cold-started box, so
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// the two rules here are about not losing it.
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//
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// # It is rewritten on every assertion, so the write must be atomic
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//
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// mavweb calls StoreEncryptionKey after every successful assertion, not only
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// after enrolment. os.WriteFile truncates in place: a power cut or an OOM kill
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// between the truncate and the write left a zero-length blob and no previous
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// contents, on the path of every routine step-up. Write to a temp file in the
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// same directory, fsync it, rename over the target, then fsync the directory.
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//
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// # Only one authenticator can hold the cold-start key
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//
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// A blob is wrapped under one credential's PRF output and nothing else opens
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// it. mavweb sends an empty allowCredentials list and the credential store
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// keeps more than one passkey, so an unconditional rewrite meant the last
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// authenticator to assert silently locked out every other one — including the
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// backup hardware key enrolled for exactly the cold-start case. So: a blob
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// that already opens under this secret and already wraps this key is left
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// alone, a v1 blob is upgraded in place, and a v2 blob belonging to a
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// different credential is refused rather than overwritten.
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import (
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"bytes"
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"errors"
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"fmt"
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"os"
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"path/filepath"
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"github.com/kami/maven/internal/webauthn"
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)
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// errForeignBlob — the wrapped key on disk belongs to another credential.
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// Refusing is the point: overwriting would lock that authenticator out.
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var errForeignBlob = errors.New("wrapped key belongs to a different credential")
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// wrapKeyToFile wraps key under secret and persists it at path, unless the
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// blob already there says not to. Reports whether it wrote anything.
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func wrapKeyToFile(path string, key, secret []byte) (wrote bool, err error) {
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existing, err := os.ReadFile(path)
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switch {
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case err == nil:
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plain, version, uerr := webauthn.UnwrapKey(existing, secret)
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switch {
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case uerr == nil && version == webauthn.BlobV2 && bytes.Equal(plain, key):
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// Already wrapped under this secret, around this key. The
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// common case on every assertion after the first.
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return false, nil
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case uerr != nil && version == webauthn.BlobV2:
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return false, fmt.Errorf("%w: %s does not open under this assertion's PRF output, so another passkey holds the cold-start key; delete it deliberately to re-wrap", errForeignBlob, path)
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}
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// A v1 blob (upgrade it), or a v2 blob wrapping a stale key under
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// this same secret (the key was rotated). Both are rewrites.
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case errors.Is(err, os.ErrNotExist):
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// First wrap.
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default:
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return false, fmt.Errorf("read wrapped key: %w", err)
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}
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blob, err := webauthn.WrapKey(key, secret)
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if err != nil {
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return false, fmt.Errorf("wrap encryption key: %w", err)
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}
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if err := writeFileAtomic(path, blob, 0o600); err != nil {
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return false, fmt.Errorf("write wrapped key: %w", err)
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}
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return true, nil
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}
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// writeFileAtomic writes data to path so that a reader sees either the whole
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// new file or the whole old one, never a truncated blob.
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func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
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dir := filepath.Dir(path)
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f, err := os.CreateTemp(dir, filepath.Base(path)+".tmp*")
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if err != nil {
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return err
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}
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tmp := f.Name()
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defer os.Remove(tmp) // no-op once the rename succeeded
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if err := f.Chmod(perm); err != nil {
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f.Close()
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return err
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}
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if _, err := f.Write(data); err != nil {
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f.Close()
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return err
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}
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if err := f.Sync(); err != nil {
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f.Close()
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return err
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}
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if err := f.Close(); err != nil {
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return err
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}
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if err := os.Rename(tmp, path); err != nil {
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return err
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}
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// The rename itself needs to reach the disk, or a crash can resurrect the
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// old directory entry pointing at a file that is gone.
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d, err := os.Open(dir)
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if err != nil {
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return err
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}
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defer d.Close()
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return d.Sync()
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}
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