libceph: add authorizer challenge

When a client authenticates with a service, an authorizer is sent with
a nonce to the service (ceph_x_authorize_[ab]) and the service responds
with a mutation of that nonce (ceph_x_authorize_reply). This lets the
client verify the service is who it says it is but it doesn't protect
against a replay: someone can trivially capture the exchange and reuse
the same authorizer to authenticate themselves.

Allow the service to reject an initial authorizer with a random
challenge (ceph_x_authorize_challenge). The client then has to respond
with an updated authorizer proving they are able to decrypt the
service's challenge and that the new authorizer was produced for this
specific connection instance.

The accepting side requires this challenge and response unconditionally
if the client side advertises they have CEPHX_V2 feature bit.

This addresses CVE-2018-1128.

Link: http://tracker.ceph.com/issues/24836
Signed-off-by: Ilya Dryomov <[e-mail]>
Reviewed-by: Sage Weil <[e-mail]>

fs/ceph/mds_client.c

···
 	return auth;
 }
 
+static int add_authorizer_challenge(struct ceph_connection *con,
+				    void *challenge_buf, int challenge_buf_len)
+{
+	struct ceph_mds_session *s = con->private;
+	struct ceph_mds_client *mdsc = s->s_mdsc;
+	struct ceph_auth_client *ac = mdsc->fsc->client->monc.auth;
+
+	return ceph_auth_add_authorizer_challenge(ac, s->s_auth.authorizer,
+					    challenge_buf, challenge_buf_len);
+}
 
 static int verify_authorizer_reply(struct ceph_connection *con)
 {
···
 	.put = con_put,
 	.dispatch = dispatch,
 	.get_authorizer = get_authorizer,
+	.add_authorizer_challenge = add_authorizer_challenge,
 	.verify_authorizer_reply = verify_authorizer_reply,
 	.invalidate_authorizer = invalidate_authorizer,
 	.peer_reset = peer_reset,

include/linux/ceph/auth.h

···
 	/* ensure that an existing authorizer is up to date */
 	int (*update_authorizer)(struct ceph_auth_client *ac, int peer_type,
 				 struct ceph_auth_handshake *auth);
+	int (*add_authorizer_challenge)(struct ceph_auth_client *ac,
+					struct ceph_authorizer *a,
+					void *challenge_buf,
+					int challenge_buf_len);
 	int (*verify_authorizer_reply)(struct ceph_auth_client *ac,
 				       struct ceph_authorizer *a);
 	void (*invalidate_authorizer)(struct ceph_auth_client *ac,
···
 extern int ceph_auth_update_authorizer(struct ceph_auth_client *ac,
 				       int peer_type,
 				       struct ceph_auth_handshake *a);
+int ceph_auth_add_authorizer_challenge(struct ceph_auth_client *ac,
+				       struct ceph_authorizer *a,
+				       void *challenge_buf,
+				       int challenge_buf_len);
 extern int ceph_auth_verify_authorizer_reply(struct ceph_auth_client *ac,
 					     struct ceph_authorizer *a);
 extern void ceph_auth_invalidate_authorizer(struct ceph_auth_client *ac,

include/linux/ceph/messenger.h

···
 	struct ceph_auth_handshake *(*get_authorizer) (
 				struct ceph_connection *con,
 			       int *proto, int force_new);
+	int (*add_authorizer_challenge)(struct ceph_connection *con,
+					void *challenge_buf,
+					int challenge_buf_len);
 	int (*verify_authorizer_reply) (struct ceph_connection *con);
 	int (*invalidate_authorizer)(struct ceph_connection *con);
 

include/linux/ceph/msgr.h

···
 #define CEPH_MSGR_TAG_SEQ           13 /* 64-bit int follows with seen seq number */
 #define CEPH_MSGR_TAG_KEEPALIVE2    14 /* keepalive2 byte + ceph_timespec */
 #define CEPH_MSGR_TAG_KEEPALIVE2_ACK 15 /* keepalive2 reply */
-
+#define CEPH_MSGR_TAG_CHALLENGE_AUTHORIZER 16  /* cephx v2 doing server challenge */
 
 /*
  * connection negotiation

net/ceph/auth.c

···
 }
 EXPORT_SYMBOL(ceph_auth_update_authorizer);
 
+int ceph_auth_add_authorizer_challenge(struct ceph_auth_client *ac,
+				       struct ceph_authorizer *a,
+				       void *challenge_buf,
+				       int challenge_buf_len)
+{
+	int ret = 0;
+
+	mutex_lock(&ac->mutex);
+	if (ac->ops && ac->ops->add_authorizer_challenge)
+		ret = ac->ops->add_authorizer_challenge(ac, a, challenge_buf,
+							challenge_buf_len);
+	mutex_unlock(&ac->mutex);
+	return ret;
+}
+EXPORT_SYMBOL(ceph_auth_add_authorizer_challenge);
+
 int ceph_auth_verify_authorizer_reply(struct ceph_auth_client *ac,
 				      struct ceph_authorizer *a)
 {

net/ceph/auth_x.c

···
  * authorizer.  The first part (ceph_x_authorize_a) should already be
  * encoded.
  */
-static int encrypt_authorizer(struct ceph_x_authorizer *au)
+static int encrypt_authorizer(struct ceph_x_authorizer *au,
+			      u64 *server_challenge)
 {
 	struct ceph_x_authorize_a *msg_a;
 	struct ceph_x_authorize_b *msg_b;
···
 	end = au->buf->vec.iov_base + au->buf->vec.iov_len;
 
 	msg_b = p + ceph_x_encrypt_offset();
-	msg_b->struct_v = 1;
+	msg_b->struct_v = 2;
 	msg_b->nonce = cpu_to_le64(au->nonce);
+	if (server_challenge) {
+		msg_b->have_challenge = 1;
+		msg_b->server_challenge_plus_one =
+		    cpu_to_le64(*server_challenge + 1);
+	} else {
+		msg_b->have_challenge = 0;
+		msg_b->server_challenge_plus_one = 0;
+	}
 
 	ret = ceph_x_encrypt(&au->session_key, p, end - p, sizeof(*msg_b));
 	if (ret < 0)
 		return ret;
 
 	p += ret;
-	WARN_ON(p > end);
-	au->buf->vec.iov_len = p - au->buf->vec.iov_base;
+	if (server_challenge) {
+		WARN_ON(p != end);
+	} else {
+		WARN_ON(p > end);
+		au->buf->vec.iov_len = p - au->buf->vec.iov_base;
+	}
 
 	return 0;
 }
···
 	     le64_to_cpu(msg_a->ticket_blob.secret_id));
 
 	get_random_bytes(&au->nonce, sizeof(au->nonce));
-	ret = encrypt_authorizer(au);
+	ret = encrypt_authorizer(au, NULL);
 	if (ret) {
 		pr_err("failed to encrypt authorizer: %d", ret);
 		goto out_au;
···
 	return 0;
 }
 
+static int decrypt_authorize_challenge(struct ceph_x_authorizer *au,
+				       void *challenge_buf,
+				       int challenge_buf_len,
+				       u64 *server_challenge)
+{
+	struct ceph_x_authorize_challenge *ch =
+	    challenge_buf + sizeof(struct ceph_x_encrypt_header);
+	int ret;
+
+	/* no leading len */
+	ret = __ceph_x_decrypt(&au->session_key, challenge_buf,
+			       challenge_buf_len);
+	if (ret < 0)
+		return ret;
+	if (ret < sizeof(*ch)) {
+		pr_err("bad size %d for ceph_x_authorize_challenge\n", ret);
+		return -EINVAL;
+	}
+
+	*server_challenge = le64_to_cpu(ch->server_challenge);
+	return 0;
+}
+
+static int ceph_x_add_authorizer_challenge(struct ceph_auth_client *ac,
+					   struct ceph_authorizer *a,
+					   void *challenge_buf,
+					   int challenge_buf_len)
+{
+	struct ceph_x_authorizer *au = (void *)a;
+	u64 server_challenge;
+	int ret;
+
+	ret = decrypt_authorize_challenge(au, challenge_buf, challenge_buf_len,
+					  &server_challenge);
+	if (ret) {
+		pr_err("failed to decrypt authorize challenge: %d", ret);
+		return ret;
+	}
+
+	ret = encrypt_authorizer(au, &server_challenge);
+	if (ret) {
+		pr_err("failed to encrypt authorizer w/ challenge: %d", ret);
+		return ret;
+	}
+
+	return 0;
+}
+
 static int ceph_x_verify_authorizer_reply(struct ceph_auth_client *ac,
 					  struct ceph_authorizer *a)
 {
···
 	.handle_reply = ceph_x_handle_reply,
 	.create_authorizer = ceph_x_create_authorizer,
 	.update_authorizer = ceph_x_update_authorizer,
+	.add_authorizer_challenge = ceph_x_add_authorizer_challenge,
 	.verify_authorizer_reply = ceph_x_verify_authorizer_reply,
 	.invalidate_authorizer = ceph_x_invalidate_authorizer,
 	.reset =  ceph_x_reset,

net/ceph/auth_x_protocol.h

···
 struct ceph_x_authorize_b {
 	__u8 struct_v;
 	__le64 nonce;
+	__u8 have_challenge;
+	__le64 server_challenge_plus_one;
+} __attribute__ ((packed));
+
+struct ceph_x_authorize_challenge {
+	__u8 struct_v;
+	__le64 server_challenge;
 } __attribute__ ((packed));
 
 struct ceph_x_authorize_reply {

net/ceph/messenger.c

···
 	if (con->auth) {
 		/*
 		 * Any connection that defines ->get_authorizer()
-		 * should also define ->verify_authorizer_reply().
+		 * should also define ->add_authorizer_challenge() and
+		 * ->verify_authorizer_reply().
+		 *
 		 * See get_connect_authorizer().
 		 */
+		if (con->in_reply.tag == CEPH_MSGR_TAG_CHALLENGE_AUTHORIZER) {
+			ret = con->ops->add_authorizer_challenge(
+				    con, con->auth->authorizer_reply_buf,
+				    le32_to_cpu(con->in_reply.authorizer_len));
+			if (ret < 0)
+				return ret;
+
+			con_out_kvec_reset(con);
+			__prepare_write_connect(con);
+			prepare_read_connect(con);
+			return 0;
+		}
+
 		ret = con->ops->verify_authorizer_reply(con);
 		if (ret < 0) {
 			con->error_msg = "bad authorize reply";

net/ceph/osd_client.c

···
 	return auth;
 }
 
+static int add_authorizer_challenge(struct ceph_connection *con,
+				    void *challenge_buf, int challenge_buf_len)
+{
+	struct ceph_osd *o = con->private;
+	struct ceph_osd_client *osdc = o->o_osdc;
+	struct ceph_auth_client *ac = osdc->client->monc.auth;
+
+	return ceph_auth_add_authorizer_challenge(ac, o->o_auth.authorizer,
+					    challenge_buf, challenge_buf_len);
+}
 
 static int verify_authorizer_reply(struct ceph_connection *con)
 {
···
 	.put = put_osd_con,
 	.dispatch = dispatch,
 	.get_authorizer = get_authorizer,
+	.add_authorizer_challenge = add_authorizer_challenge,
 	.verify_authorizer_reply = verify_authorizer_reply,
 	.invalidate_authorizer = invalidate_authorizer,
 	.alloc_msg = alloc_msg,