[eap] Add support for the MS-CHAPv2 authentication method

Add support for EAP-MSCHAPv2 (note that this is not the same as
PEAP-MSCHAPv2), controllable via the build configuration option
EAP_METHOD_MSCHAPV2 in config/general.h.

Our model for EAP does not encompass mutual authentication: we will
starting sending plaintext packets (e.g. DHCP requests) over the link
even before EAP completes, and our only use for an EAP success is to
mark the link as unblocked.

We therefore ignore the content of the EAP-MSCHAPv2 success request
(containing the MS-CHAPv2 authenticator response) and just send back
an EAP-MSCHAPv2 success response, so that the EAP authenticator will
complete the process and send through the real EAP success packet
(which will, in turn, cause us to unblock the link).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

5 files changed, 285 insertions, 0 deletions

diff --git a/src/config/config_eap.c b/src/config/config_eap.c
index d3fd77aa..e18c48ca 100644
--- a/src/config/config_eap.c
+++ b/src/config/config_eap.c
@@ -37,3 +37,6 @@ PROVIDE_REQUIRING_SYMBOL();
 #ifdef EAP_METHOD_MD5
 REQUIRE_OBJECT ( eap_md5 );
 #endif
+#ifdef EAP_METHOD_MSCHAPV2
+REQUIRE_OBJECT ( eap_mschapv2 );
+#endif
diff --git a/src/config/general.h b/src/config/general.h
index de009a87..c9cdb3dd 100644
--- a/src/config/general.h
+++ b/src/config/general.h
@@ -96,6 +96,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  *
  */
 #define EAP_METHOD_MD5		/* MD5-Challenge port authentication */
+//#define EAP_METHOD_MSCHAPV2	/* MS-CHAPv2 port authentication */
 
 /*
  * Name resolution modules
diff --git a/src/include/ipxe/eap.h b/src/include/ipxe/eap.h
index fe1bb528..a44f01e0 100644
--- a/src/include/ipxe/eap.h
+++ b/src/include/ipxe/eap.h
@@ -60,6 +60,35 @@ struct eap_md5 {
 	uint8_t value[0];
 } __attribute__ (( packed ));
 
+/** EAP MS-CHAPv2 request/response */
+#define EAP_TYPE_MSCHAPV2 26
+
+/** EAP MS-CHAPv2 request/response type data */
+struct eap_mschapv2 {
+	/** Code
+	 *
+	 * This is in the same namespace as the EAP header's code
+	 * field, but is used to extend the handshake by allowing for
+	 * "success request" and "success response" packets.
+	 */
+	uint8_t code;
+	/** Identifier
+	 *
+	 * This field serves no purposes: it always has the same value
+	 * as the EAP header's identifier field (located 5 bytes
+	 * earlier in the same packet).
+	 */
+	uint8_t id;
+	/** Length
+	 *
+	 * This field serves no purpose: it always has the same value
+	 * as the EAP header's length field (located 5 bytes earlier
+	 * in the same packet), minus the 5 byte length of the EAP
+	 * header.
+	 */
+	uint16_t len;
+} __attribute__ (( packed ));
+
 /** EAP success */
 #define EAP_CODE_SUCCESS 3
 
diff --git a/src/include/ipxe/errfile.h b/src/include/ipxe/errfile.h
index 1768748d..662f8496 100644
--- a/src/include/ipxe/errfile.h
+++ b/src/include/ipxe/errfile.h
@@ -298,6 +298,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_eap			( ERRFILE_NET | 0x004b0000 )
 #define ERRFILE_lldp			( ERRFILE_NET | 0x004c0000 )
 #define ERRFILE_eap_md5			( ERRFILE_NET | 0x004d0000 )
+#define ERRFILE_eap_mschapv2		( ERRFILE_NET | 0x004e0000 )
 
 #define ERRFILE_image		      ( ERRFILE_IMAGE | 0x00000000 )
 #define ERRFILE_elf		      ( ERRFILE_IMAGE | 0x00010000 )
diff --git a/src/net/eap_mschapv2.c b/src/net/eap_mschapv2.c
new file mode 100644
index 00000000..0be62ed5
--- /dev/null
+++ b/src/net/eap_mschapv2.c
@@ -0,0 +1,251 @@
+/*
+ * Copyright (C) 2024 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <byteswap.h>
+#include <ipxe/mschapv2.h>
+#include <ipxe/eap.h>
+
+/** @file
+ *
+ * EAP MS-CHAPv2 authentication method
+ *
+ * EAP-MSCHAPv2 was described in a draft RFC first published in 2002
+ * (draft-kamath-pppext-eap-mschapv2-02.txt).  The draft eventually
+ * expired in 2007 without becoming an official RFC, quite possibly
+ * because the protocol design was too ugly to be called an IETF
+ * standard.  It is, however, fairly widely used.
+ */
+
+/** An EAP MS-CHAPv2 request message */
+struct eap_mschapv2_request {
+	/** EAP-MSCHAPv2 header */
+	struct eap_mschapv2 hdr;
+	/** MS-CHAPv2 challenge length (fixed value) */
+	uint8_t len;
+	/** MS-CHAPv2 challenge */
+	struct mschapv2_challenge msg;
+} __attribute__ (( packed ));
+
+/** An EAP MS-CHAPv2 response message */
+struct eap_mschapv2_response {
+	/** EAP-MSCHAPv2 header */
+	struct eap_mschapv2 hdr;
+	/** MS-CHAPv2 response length (fixed value) */
+	uint8_t len;
+	/** MS-CHAPv2 response */
+	struct mschapv2_response msg;
+	/** User name */
+	char name[0];
+} __attribute__ (( packed ));
+
+/** An EAP MS-CHAPv2 success request message */
+struct eap_mschapv2_success_request {
+	/** EAP-MSCHAPv2 header */
+	struct eap_mschapv2 hdr;
+	/** Message */
+	char message[0];
+} __attribute__ (( packed ));
+
+/** An EAP MS-CHAPv2 success response message */
+struct eap_mschapv2_success_response {
+	/** Opcode */
+	uint8_t code;
+} __attribute__ (( packed ));
+
+/**
+ * Handle EAP MS-CHAPv2 request
+ *
+ * @v supplicant	EAP supplicant
+ * @v hdr		EAP-MSCHAPv2 header
+ * @v len		Message length
+ * @ret rc		Return status code
+ */
+static int eap_rx_mschapv2_request ( struct eap_supplicant *supplicant,
+				     const struct eap_mschapv2 *hdr,
+				     size_t len ) {
+	struct net_device *netdev = supplicant->netdev;
+	struct settings *settings = netdev_settings ( netdev );
+	const struct eap_mschapv2_request *msreq =
+		container_of ( hdr, struct eap_mschapv2_request, hdr );
+	struct eap_mschapv2_response *msrsp;
+	struct mschapv2_challenge peer;
+	char *username;
+	char *password;
+	int username_len;
+	int password_len;
+	size_t msrsp_len;
+	unsigned int i;
+	int rc;
+
+	/* Sanity check */
+	if ( len < sizeof ( *msreq ) ) {
+		DBGC ( netdev, "EAP %s underlength MS-CHAPv2 request\n",
+		       netdev->name );
+		DBGC_HDA ( netdev, 0, hdr, len );
+		rc = -EINVAL;
+		goto err_sanity;
+	}
+
+	/* Fetch username and password */
+	username_len = fetch_string_setting_copy ( settings, &username_setting,
+						   &username );
+	if ( username_len < 0 ) {
+		rc = username_len;
+		DBGC ( netdev, "EAP %s has no username: %s\n",
+		       netdev->name, strerror ( rc ) );
+		goto err_username;
+	}
+	password_len = fetch_string_setting_copy ( settings, &password_setting,
+						   &password );
+	if ( password_len < 0 ) {
+		rc = password_len;
+		DBGC ( netdev, "EAP %s has no password: %s\n",
+		       netdev->name, strerror ( rc ) );
+		goto err_password;
+	}
+
+	/* Construct a peer challenge.  We do not perform mutual
+	 * authentication, so this does not need to be strong.
+	 */
+	for ( i = 0 ; i < ( sizeof ( peer.byte ) /
+			    sizeof ( peer.byte[0] ) ) ; i++ ) {
+		peer.byte[i] = random();
+	}
+
+	/* Allocate response */
+	msrsp_len = ( sizeof ( *msrsp ) + username_len );
+	msrsp = malloc ( msrsp_len );
+	if ( ! msrsp ) {
+		rc = -ENOMEM;
+		goto err_alloc;
+	}
+
+	/* Construct response */
+	msrsp->hdr.code = EAP_CODE_RESPONSE;
+	msrsp->hdr.id = msreq->hdr.id;
+	msrsp->hdr.len = htons ( msrsp_len );
+	msrsp->len = sizeof ( msrsp->msg );
+	mschapv2_response ( username, password, &msreq->msg, &peer,
+			    &msrsp->msg );
+	memcpy ( msrsp->name, username, username_len );
+
+	/* Send response */
+	if ( ( rc = eap_tx_response ( supplicant, msrsp, msrsp_len ) ) != 0 )
+		goto err_tx;
+
+ err_tx:
+	free ( msrsp );
+ err_alloc:
+	free ( password );
+ err_password:
+	free ( username );
+ err_username:
+ err_sanity:
+	return rc;
+}
+
+/**
+ * Handle EAP MS-CHAPv2 success request
+ *
+ * @v supplicant	EAP supplicant
+ * @v hdr		EAP-MSCHAPv2 header
+ * @v len		Message length
+ * @ret rc		Return status code
+ */
+static int eap_rx_mschapv2_success ( struct eap_supplicant *supplicant,
+				     const struct eap_mschapv2 *hdr,
+				     size_t len ) {
+	const struct eap_mschapv2_success_request *msreq =
+		container_of ( hdr, struct eap_mschapv2_success_request, hdr );
+	static const struct eap_mschapv2_success_response msrsp = {
+		.code = EAP_CODE_SUCCESS,
+	};
+
+	/* Sanity check */
+	assert ( len >= sizeof ( *msreq ) );
+
+	/* The success request contains the MS-CHAPv2 authenticator
+	 * response, which could potentially be used to verify that
+	 * the EAP authenticator also knew the password (or, at least,
+	 * the MD4 hash of the password).
+	 *
+	 * Our model for EAP does not encompass mutual authentication:
+	 * we will starting sending plaintext packets (e.g. DHCP
+	 * requests) over the link even before EAP completes, and our
+	 * only use for an EAP success is to mark the link as
+	 * unblocked.
+	 *
+	 * We therefore ignore the content of the success request and
+	 * just send back a success response, so that the EAP
+	 * authenticator will complete the process and send through
+	 * the real EAP success packet (which will, in turn, cause us
+	 * to unblock the link).
+	 */
+	return eap_tx_response ( supplicant, &msrsp, sizeof ( msrsp ) );
+}
+
+/**
+ * Handle EAP MS-CHAPv2
+ *
+ * @v supplicant	EAP supplicant
+ * @v req		Request type data
+ * @v req_len		Length of request type data
+ * @ret rc		Return status code
+ */
+static int eap_rx_mschapv2 ( struct eap_supplicant *supplicant,
+			     const void *req, size_t req_len ) {
+	struct net_device *netdev = supplicant->netdev;
+	const struct eap_mschapv2 *hdr = req;
+
+	/* Sanity check */
+	if ( req_len < sizeof ( *hdr ) ) {
+		DBGC ( netdev, "EAP %s underlength MS-CHAPv2:\n",
+		       netdev->name );
+		DBGC_HDA ( netdev, 0, req, req_len );
+		return -EINVAL;
+	}
+
+	/* Handle according to opcode */
+	switch ( hdr->code ) {
+	case EAP_CODE_REQUEST:
+		return eap_rx_mschapv2_request ( supplicant, hdr, req_len );
+	case EAP_CODE_SUCCESS:
+		return eap_rx_mschapv2_success ( supplicant, hdr, req_len );
+	default:
+		DBGC ( netdev, "EAP %s unsupported MS-CHAPv2 opcode %d\n",
+		       netdev->name, hdr->code );
+		DBGC_HDA ( netdev, 0, req, req_len );
+		return -ENOTSUP;
+	}
+}
+
+/** EAP MS-CHAPv2 method */
+struct eap_method eap_mschapv2_method __eap_method = {
+	.type = EAP_TYPE_MSCHAPV2,
+	.rx = eap_rx_mschapv2,
+};