ssh: limit bcrypt KDF rounds in OpenSSH private key decryption

passphraseProtectedOpenSSHKey decodes the bcrypt round count from
the key file and passes it directly to bcrypt_pbkdf.Key, whose
running time is linear in that count. Files specifying very large
round values cause the decryption to consume excessive CPU time:
a uint32 maximum is several months of work on commodity hardware.

Cap the accepted round count at 2048 (128x the default of 16, a
few seconds of CPU on a modern core). OpenSSH itself does not
impose an upper bound, but accepting arbitrary values turns key
loading into a resource-exhaustion footgun for any code that
processes files supplied by end users.

This issue was found during a security audit by NCC Group
Cryptography Services, sponsored by Teleport, and was assessed
and is being fixed as a non-security bug.

Change-Id: I01112bdf1b484ae4fab5dc8841d1a272f112df74
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/782422
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/keys.go

@@ -1466,6 +1466,17 @@ func passphraseProtectedOpenSSHKey(passphrase []byte) openSSHDecryptFunc {
 			return nil, err
 		}
 
+		// OpenSSH does not impose an upper bound on the bcrypt round count
+		// stored in the key file, but bcrypt_pbkdf cost is linear in rounds:
+		// the default is 16, ssh-keygen lets users pick anything up to
+		// INT_MAX. Cap at 2048 (128x the default, a few seconds of CPU) so
+		// that an oversized value in the file cannot tie up the caller for
+		// months.
+		const maxRounds = 1 << 11
+		if opts.Rounds > maxRounds {
+			return nil, fmt.Errorf("ssh: bcrypt KDF rounds %d exceed maximum %d", opts.Rounds, maxRounds)
+		}
+
 		k, err := bcrypt_pbkdf.Key(passphrase, []byte(opts.Salt), int(opts.Rounds), 32+16)
 		if err != nil {
 			return nil, err

ssh/keys_test.go

@@ -506,6 +506,38 @@ func TestParseEncryptedPrivateKeysWithIncorrectPassphrase(t *testing.T) {
 	}
 }
 
+func TestParseEncryptedPrivateKeyExcessiveBcryptRounds(t *testing.T) {
+	// Craft a minimal openssh-key-v1 blob whose KdfOpts declares a bcrypt
+	// round count above the accepted maximum. The check must reject the file
+	// before bcrypt_pbkdf is invoked, so the rest of the blob (public key,
+	// encrypted body) can be empty.
+	kdfOpts := Marshal(struct {
+		Salt   []byte
+		Rounds uint32
+	}{
+		Salt:   []byte("salt-not-used"),
+		Rounds: (1 << 11) + 1,
+	})
+	header := Marshal(openSSHEncryptedPrivateKey{
+		CipherName: "aes256-ctr",
+		KdfName:    "bcrypt",
+		KdfOpts:    string(kdfOpts),
+		NumKeys:    1,
+	})
+	pemBytes := pem.EncodeToMemory(&pem.Block{
+		Type:  "OPENSSH PRIVATE KEY",
+		Bytes: append([]byte(privateKeyAuthMagic), header...),
+	})
+
+	_, err := ParseRawPrivateKeyWithPassphrase(pemBytes, []byte("password"))
+	if err == nil {
+		t.Fatal("expected error for excessive bcrypt rounds, got nil")
+	}
+	if !strings.Contains(err.Error(), "bcrypt KDF rounds") {
+		t.Errorf("got error %q, want substring %q", err.Error(), "bcrypt KDF rounds")
+	}
+}
+
 func TestParseDSA(t *testing.T) {
 	// We actually exercise the ParsePrivateKey codepath here, as opposed to
 	// using the ParseRawPrivateKey+NewSignerFromKey path that testdata_test.go