| 1 | package remote |
| 2 | |
| 3 | import ( |
| 4 | "bufio" |
| 5 | "bytes" |
| 6 | "context" |
| 7 | "errors" |
| 8 | "fmt" |
| 9 | "net" |
| 10 | "os" |
| 11 | "path/filepath" |
| 12 | "strings" |
| 13 | "sync" |
| 14 | |
| 15 | "golang.org/x/crypto/ssh" |
| 16 | "golang.org/x/crypto/ssh/knownhosts" |
| 17 | ) |
| 18 | |
| 19 | // HostKeyQuestion describes a first-seen (TOFU) host key awaiting the user's |
| 20 | // decision. |
| 21 | type HostKeyQuestion struct { |
| 22 | Host string // display label (user@host:port or alias) |
| 23 | Address string // the network address that presented the key |
| 24 | KeyType string // e.g. "ssh-ed25519" |
| 25 | Fingerprint string // ssh.FingerprintSHA256(key) |
| 26 | } |
| 27 | |
| 28 | // KnownHostLocation identifies the OpenSSH record that conflicts with a |
| 29 | // presented host key. It is intentionally structured so desktop clients can |
| 30 | // keep machine-local paths out of the primary error message while still |
| 31 | // exposing the exact record in an explicit security-details view. |
| 32 | type KnownHostLocation struct { |
| 33 | Filename string |
| 34 | Line int |
| 35 | } |
| 36 | |
| 37 | // HostKeyMismatchError describes a presented key that contradicts an existing |
| 38 | // known_hosts record. It unwraps to ErrHostKeyMismatch so callers can retain |
| 39 | // the existing fail-closed classification without parsing error strings. |
| 40 | type HostKeyMismatchError struct { |
| 41 | Host string |
| 42 | PresentedFingerprint string |
| 43 | Locations []KnownHostLocation |
| 44 | } |
| 45 | |
| 46 | func (e *HostKeyMismatchError) Error() string { |
| 47 | var b strings.Builder |
| 48 | fmt.Fprintf(&b, "%s for %s: presented %s; known_hosts records a different key", |
| 49 | ErrHostKeyMismatch, e.Host, e.PresentedFingerprint) |
| 50 | for _, location := range e.Locations { |
| 51 | if location.Filename != "" { |
| 52 | fmt.Fprintf(&b, " (%s:%d)", location.Filename, location.Line) |
| 53 | } |
| 54 | } |
| 55 | return b.String() |
| 56 | } |
| 57 | |
| 58 | func (e *HostKeyMismatchError) Unwrap() error { return ErrHostKeyMismatch } |
| 59 | |
| 60 | // HostKeyPrompt is called for an unknown host key. Returning (true, nil) |
| 61 | // accepts and persists it (trust on first use); (false, nil) rejects; a |
| 62 | // non-nil error aborts the dial. A nil prompt means strict mode: unknown hosts |
| 63 | // are rejected. |
| 64 | type HostKeyPrompt func(ctx context.Context, q HostKeyQuestion) (accept bool, err error) |
| 65 | |
| 66 | // HostKeyPolicy verifies presented host keys against the user's OpenSSH |
| 67 | // known_hosts files (read-only) and a Reasonix-managed file (read-write, TOFU). |
| 68 | type HostKeyPolicy struct { |
| 69 | // SystemKnownHosts are OpenSSH known_hosts files consulted read-only. |
| 70 | // Empty => [~/.ssh/known_hosts, ~/.ssh/known_hosts2] when they exist. |
| 71 | SystemKnownHosts []string |
| 72 | // ManagedPath is the Reasonix-managed known_hosts file that accepted TOFU |
| 73 | // keys are appended to. Empty => config.RemoteKnownHostsPath(). |
| 74 | ManagedPath string |
| 75 | // Prompt decides unknown (first-seen) keys. Nil => strict reject. |
| 76 | Prompt HostKeyPrompt |
| 77 | // Verified observes a key only after the known_hosts check (and, for TOFU, |
| 78 | // the user's acceptance and durable append) succeeded. It lets an assembly |
| 79 | // layer bind higher-level capabilities to the peer actually authenticated by |
| 80 | // this transport without weakening HostKeyCallback authority. |
| 81 | Verified func(HostKeyQuestion) |
| 82 | |
| 83 | mu sync.Mutex // serializes appends to ManagedPath |
| 84 | } |
| 85 | |
| 86 | // Callback builds an ssh.HostKeyCallback enforcing this policy for host (the |
| 87 | // display label used in prompts). ctx bounds any interactive prompt. |
| 88 | func (p *HostKeyPolicy) Callback(ctx context.Context, host string) (ssh.HostKeyCallback, error) { |
| 89 | base, managed, err := p.loadCallback() |
| 90 | if err != nil { |
| 91 | return nil, err |
| 92 | } |
| 93 | |
| 94 | return func(hostname string, remote net.Addr, key ssh.PublicKey) error { |
| 95 | if base != nil { |
| 96 | err := base(hostname, remote, key) |
| 97 | if err == nil { |
| 98 | p.notifyVerified(host, hostname, remote, key) |
| 99 | return nil |
| 100 | } |
| 101 | var keyErr *knownhosts.KeyError |
| 102 | if !errors.As(err, &keyErr) { |
| 103 | return err |
| 104 | } |
| 105 | if len(keyErr.Want) > 0 { |
| 106 | // A different key is on record for this host: hard fail, never |
| 107 | // promptable. Name the file:line so the user can inspect it. |
| 108 | return newHostKeyMismatchError(host, ssh.FingerprintSHA256(key), keyErr) |
| 109 | } |
| 110 | // len(Want)==0 => host unknown. Fall through to TOFU. |
| 111 | } |
| 112 | if err := p.tofu(ctx, host, hostname, remote, key, managed); err != nil { |
| 113 | return err |
| 114 | } |
| 115 | p.notifyVerified(host, hostname, remote, key) |
| 116 | return nil |
| 117 | }, nil |
| 118 | } |
| 119 | |
| 120 | func (p *HostKeyPolicy) notifyVerified(host, hostname string, remoteAddr net.Addr, key ssh.PublicKey) { |
| 121 | if p == nil || p.Verified == nil || key == nil { |
| 122 | return |
| 123 | } |
| 124 | address := hostname |
| 125 | if remoteAddr != nil && strings.TrimSpace(remoteAddr.String()) != "" { |
| 126 | address = remoteAddr.String() |
| 127 | } |
| 128 | p.Verified(HostKeyQuestion{ |
| 129 | Host: host, Address: address, KeyType: key.Type(), Fingerprint: ssh.FingerprintSHA256(key), |
| 130 | }) |
| 131 | } |
| 132 | |
| 133 | // HostKeyAlgorithms returns host-key algorithms in negotiation order, |
| 134 | // preferring algorithms compatible with ordinary host identities already |
| 135 | // recorded for hostname. Certificate-authority records are deliberately not |
| 136 | // treated as host keys: the CA algorithm does not describe the certified host |
| 137 | // key. The strict callback remains the authority for every negotiated key. |
| 138 | func (p *HostKeyPolicy) HostKeyAlgorithms(hostname string, remote net.Addr) ([]string, error) { |
| 139 | base, _, err := p.loadCallback() |
| 140 | if err != nil || base == nil { |
| 141 | return nil, err |
| 142 | } |
| 143 | err = base(hostname, remote, hostKeyLookupProbe{}) |
| 144 | if err == nil { |
| 145 | return nil, nil |
| 146 | } |
| 147 | var keyErr *knownhosts.KeyError |
| 148 | if !errors.As(err, &keyErr) { |
| 149 | return nil, err |
| 150 | } |
| 151 | if len(keyErr.Want) == 0 { |
| 152 | return nil, nil |
| 153 | } |
| 154 | |
| 155 | preferred := make(map[string]bool, len(keyErr.Want)) |
| 156 | for _, known := range keyErr.Want { |
| 157 | if known.Key == nil { |
| 158 | continue |
| 159 | } |
| 160 | marker, err := knownHostMarker(known) |
| 161 | if err != nil { |
| 162 | return nil, err |
| 163 | } |
| 164 | if marker != "" { |
| 165 | continue |
| 166 | } |
| 167 | keyType := known.Key.Type() |
| 168 | preferred[keyType] = true |
| 169 | switch keyType { |
| 170 | case ssh.KeyAlgoRSA: |
| 171 | // An ssh-rsa public key can use the SHA-2 signature algorithms; |
| 172 | preferred[ssh.KeyAlgoRSASHA512] = true |
| 173 | preferred[ssh.KeyAlgoRSASHA256] = true |
| 174 | case ssh.CertAlgoRSAv01: |
| 175 | // RSA host certificates likewise support SHA-2 signature |
| 176 | // algorithms even though their public key format is ssh-rsa. |
| 177 | preferred[ssh.CertAlgoRSASHA512v01] = true |
| 178 | preferred[ssh.CertAlgoRSASHA256v01] = true |
| 179 | } |
| 180 | } |
| 181 | |
| 182 | candidates := hostKeyAlgorithmCandidates() |
| 183 | ordered := make([]string, 0, len(candidates)) |
| 184 | for _, algorithm := range candidates { |
| 185 | if preferred[algorithm] { |
| 186 | ordered = append(ordered, algorithm) |
| 187 | } |
| 188 | } |
| 189 | if len(ordered) == 0 { |
| 190 | return nil, nil |
| 191 | } |
| 192 | for _, algorithm := range candidates { |
| 193 | if !preferred[algorithm] { |
| 194 | ordered = append(ordered, algorithm) |
| 195 | } |
| 196 | } |
| 197 | return ordered, nil |
| 198 | } |
| 199 | |
| 200 | // hostKeyAlgorithmCandidates preserves the algorithms in the Go SSH default |
| 201 | // policy while keeping secure algorithms ahead of legacy fallbacks. Legacy |
| 202 | // algorithms are only promoted when their exact public key format is already |
| 203 | // recorded; the host-key callback must still verify the key material. |
| 204 | func hostKeyAlgorithmCandidates() []string { |
| 205 | secure := ssh.SupportedAlgorithms().HostKeys |
| 206 | legacy := ssh.InsecureAlgorithms().HostKeys |
| 207 | algorithms := make([]string, 0, len(secure)+len(legacy)) |
| 208 | seen := make(map[string]bool, cap(algorithms)) |
| 209 | for _, algorithm := range append(secure, legacy...) { |
| 210 | if !seen[algorithm] { |
| 211 | seen[algorithm] = true |
| 212 | algorithms = append(algorithms, algorithm) |
| 213 | } |
| 214 | } |
| 215 | return algorithms |
| 216 | } |
| 217 | |
| 218 | // knownHostMarker reads the original matching record so @cert-authority and |
| 219 | // @revoked entries cannot be mistaken for ordinary host identities. KnownKey |
| 220 | // exposes the exact file and line selected by knownhosts.New; ParseKnownHosts |
| 221 | // supplies OpenSSH marker semantics without duplicating its parser. |
| 222 | func knownHostMarker(known knownhosts.KnownKey) (string, error) { |
| 223 | if known.Filename == "" || known.Line <= 0 { |
| 224 | return "", fmt.Errorf("known_hosts record has no source location") |
| 225 | } |
| 226 | f, err := os.Open(known.Filename) |
| 227 | if err != nil { |
| 228 | return "", fmt.Errorf("open known_hosts record %s:%d: %w", known.Filename, known.Line, err) |
| 229 | } |
| 230 | defer f.Close() |
| 231 | |
| 232 | scanner := bufio.NewScanner(f) |
| 233 | for line := 1; scanner.Scan(); line++ { |
| 234 | if line != known.Line { |
| 235 | continue |
| 236 | } |
| 237 | marker, _, key, _, _, err := ssh.ParseKnownHosts(scanner.Bytes()) |
| 238 | if err != nil { |
| 239 | return "", fmt.Errorf("parse known_hosts record %s:%d: %w", known.Filename, known.Line, err) |
| 240 | } |
| 241 | if key == nil || known.Key == nil || !bytes.Equal(key.Marshal(), known.Key.Marshal()) { |
| 242 | return "", fmt.Errorf("known_hosts record changed while connecting: %s:%d", known.Filename, known.Line) |
| 243 | } |
| 244 | return marker, nil |
| 245 | } |
| 246 | if err := scanner.Err(); err != nil { |
| 247 | return "", fmt.Errorf("read known_hosts record %s:%d: %w", known.Filename, known.Line, err) |
| 248 | } |
| 249 | return "", fmt.Errorf("known_hosts record no longer exists: %s:%d", known.Filename, known.Line) |
| 250 | } |
| 251 | |
| 252 | // hostKeyLookupProbe deliberately cannot equal a parsed OpenSSH public key. |
| 253 | // Passing it through knownhosts.New lets us reuse the library's exact hostname, |
| 254 | // wildcard, hashed-host, port, and file matching and inspect KeyError.Want. |
| 255 | type hostKeyLookupProbe struct{} |
| 256 | |
| 257 | func (hostKeyLookupProbe) Type() string { return "reasonix-host-key-lookup-probe" } |
| 258 | func (hostKeyLookupProbe) Marshal() []byte { return []byte("reasonix-host-key-lookup-probe") } |
| 259 | func (hostKeyLookupProbe) Verify([]byte, *ssh.Signature) error { |
| 260 | return fmt.Errorf("host-key lookup probe cannot verify signatures") |
| 261 | } |
| 262 | |
| 263 | func (p *HostKeyPolicy) loadCallback() (ssh.HostKeyCallback, string, error) { |
| 264 | files := p.systemFiles() |
| 265 | managed := p.managedPath() |
| 266 | if managed != "" { |
| 267 | if err := os.MkdirAll(filepath.Dir(managed), 0o700); err != nil { |
| 268 | return nil, "", err |
| 269 | } |
| 270 | // knownhosts.New requires each file to exist; create an empty managed |
| 271 | // file on first use. |
| 272 | if _, err := os.Stat(managed); os.IsNotExist(err) { |
| 273 | if err := os.WriteFile(managed, nil, 0o600); err != nil { |
| 274 | return nil, "", err |
| 275 | } |
| 276 | } |
| 277 | files = append(files, managed) |
| 278 | } |
| 279 | |
| 280 | var base ssh.HostKeyCallback |
| 281 | if len(files) > 0 { |
| 282 | var err error |
| 283 | base, err = knownhosts.New(files...) |
| 284 | if err != nil { |
| 285 | return nil, "", fmt.Errorf("load known_hosts: %w", err) |
| 286 | } |
| 287 | } |
| 288 | return base, managed, nil |
| 289 | } |
| 290 | |
| 291 | func (p *HostKeyPolicy) tofu(ctx context.Context, host, hostname string, remote net.Addr, key ssh.PublicKey, managed string) error { |
| 292 | if p.Prompt == nil { |
| 293 | return fmt.Errorf("%w for %s: unknown host key %s (no confirmation available)", |
| 294 | ErrHostKeyRejected, host, ssh.FingerprintSHA256(key)) |
| 295 | } |
| 296 | accept, err := p.Prompt(ctx, HostKeyQuestion{ |
| 297 | Host: host, |
| 298 | Address: remote.String(), |
| 299 | KeyType: key.Type(), |
| 300 | Fingerprint: ssh.FingerprintSHA256(key), |
| 301 | }) |
| 302 | if err != nil { |
| 303 | return err |
| 304 | } |
| 305 | if !accept { |
| 306 | return fmt.Errorf("%w for %s", ErrHostKeyRejected, host) |
| 307 | } |
| 308 | if managed == "" { |
| 309 | return nil // accepted for this session only |
| 310 | } |
| 311 | return p.appendManaged(managed, hostname, remote, key) |
| 312 | } |
| 313 | |
| 314 | func (p *HostKeyPolicy) appendManaged(managed, hostname string, remote net.Addr, key ssh.PublicKey) error { |
| 315 | p.mu.Lock() |
| 316 | defer p.mu.Unlock() |
| 317 | addrs := []string{knownhosts.Normalize(hostname)} |
| 318 | if remote != nil { |
| 319 | if norm := knownhosts.Normalize(remote.String()); norm != addrs[0] { |
| 320 | addrs = append(addrs, norm) |
| 321 | } |
| 322 | } |
| 323 | line := knownhosts.Line(addrs, key) |
| 324 | f, err := os.OpenFile(managed, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0o600) |
| 325 | if err != nil { |
| 326 | return err |
| 327 | } |
| 328 | defer f.Close() |
| 329 | if _, err := f.WriteString(strings.TrimRight(line, "\n") + "\n"); err != nil { |
| 330 | return err |
| 331 | } |
| 332 | return nil |
| 333 | } |
| 334 | |
| 335 | func (p *HostKeyPolicy) systemFiles() []string { |
| 336 | if len(p.SystemKnownHosts) > 0 { |
| 337 | out := make([]string, 0, len(p.SystemKnownHosts)) |
| 338 | for _, f := range p.SystemKnownHosts { |
| 339 | if f = expandHome(f); fileExists(f) { |
| 340 | out = append(out, f) |
| 341 | } |
| 342 | } |
| 343 | return out |
| 344 | } |
| 345 | home, err := os.UserHomeDir() |
| 346 | if err != nil { |
| 347 | return nil |
| 348 | } |
| 349 | var out []string |
| 350 | for _, name := range []string{"known_hosts", "known_hosts2"} { |
| 351 | p := filepath.Join(home, ".ssh", name) |
| 352 | if fileExists(p) { |
| 353 | out = append(out, p) |
| 354 | } |
| 355 | } |
| 356 | return out |
| 357 | } |
| 358 | |
| 359 | func (p *HostKeyPolicy) managedPath() string { |
| 360 | if p.ManagedPath != "" { |
| 361 | return p.ManagedPath |
| 362 | } |
| 363 | return defaultManagedKnownHosts() |
| 364 | } |
| 365 | |
| 366 | func newHostKeyMismatchError(host, presented string, e *knownhosts.KeyError) error { |
| 367 | locations := make([]KnownHostLocation, 0, len(e.Want)) |
| 368 | for _, k := range e.Want { |
| 369 | locations = append(locations, KnownHostLocation{Filename: k.Filename, Line: k.Line}) |
| 370 | } |
| 371 | return &HostKeyMismatchError{Host: host, PresentedFingerprint: presented, Locations: locations} |
| 372 | } |
| 373 | |
| 374 | func fileExists(path string) bool { |
| 375 | fi, err := os.Stat(path) |
| 376 | return err == nil && !fi.IsDir() |
| 377 | } |
| 378 |