pem: cap accepted KDF rounds in pem_parse_data_openssh()

To 1^20, which ensures the function does complete eventually.

The limit mirrors the one introduced in OpenSSH 10.6. (Release Notes
say 1M, the actual patch uses 1^20; we go with the latter.)

Refs:
https://www.openssh.org/txt/release-10.6
f938c78d49
OpenBSD-Commit-ID: 843ff37b066b2879f4579a784e42a3c9d22b2ddc

Reported-by: QcN3ep
Fixes GHSA-vc97-vgww-qggv

Closes #2657

src/pem.c

@@ -383,6 +383,8 @@ out:
 #define OPENSSH_PRIVKEY_FOOTER     "-----END OPENSSH PRIVATE KEY-----"
 #define OPENSSH_PRIVKEY_AUTH_MAGIC "openssh-key-v1"
 
+#define SSH2_KDF_ROUNDS_MAX  (1 << 20)
+
 static int pem_parse_data_openssh(LIBSSH2_SESSION *session,
                                   const char *passphrase,
                                   const char *b64data, size_t b64datalen,
@@ -546,6 +548,12 @@ static int pem_parse_data_openssh(LIBSSH2_SESSION *session,
                 goto out;
             }
 
+            if(rounds > SSH2_KDF_ROUNDS_MAX) {
+                ret = ssh2_err(session, LIBSSH2_ERROR_PROTO,
+                               "Too many KDF rounds");
+                goto out;
+            }
+
             if(ssh2_bcrypt_pbkdf(passphrase, strlen(passphrase),
                                  salt, salt_len, key,
                                  keylen + ivlen, rounds) < 0) {