WIP - shim unlocker

src/image/efi_image.c

@@ -32,6 +32,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <ipxe/efi/efi_pxe.h>
 #include <ipxe/efi/efi_driver.h>
 #include <ipxe/efi/efi_image.h>
+#include <ipxe/efi/efi_shim.h>
 #include <ipxe/image.h>
 #include <ipxe/init.h>
 #include <ipxe/features.h>
@@ -56,16 +57,6 @@ FEATURE ( FEATURE_IMAGE, "EFI", DHCP_EB_FEATURE_EFI, 1 );
 			  "Could not start image" )
 #define EEFI_START( efirc ) EPLATFORM ( EINFO_EEFI_START, efirc )
 
-/** EFI shim image */
-struct image_tag efi_shim __image_tag = {
-	.name = "SHIM",
-};
-
-/** EIF shim crutch image */
-struct image_tag efi_shim_crutch __image_tag = {
-	.name = "SHIMCRUTCH",
-};
-
 /**
  * Create device path for image
  *
@@ -165,6 +156,7 @@ static int efi_image_exec ( struct image *image ) {
 	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 	struct efi_snp_device *snpdev;
 	EFI_DEVICE_PATH_PROTOCOL *path;
+	struct efi_shim_unlocker unlocker;
 	union {
 		EFI_LOADED_IMAGE_PROTOCOL *image;
 		void *interface;
@@ -242,6 +234,14 @@ static int efi_image_exec ( struct image *image ) {
 		goto err_cmdline;
 	}
 
+	/* Install shim unlocker (if using a shim) */
+	if ( shim &&
+	     ( ( rc = efi_shim_install ( &unlocker ) ) != 0 ) ) {
+		DBGC ( image, "EFIIMAGE %s could not install shim unlocker: "
+		       "%s\n", image->name, strerror ( rc ) );
+		goto err_shim_install;
+	}
+
 	/* Attempt loading image */
 	handle = NULL;
 	if ( ( efirc = bs->LoadImage ( FALSE, efi_image_handle, path,
@@ -336,6 +336,9 @@ static int efi_image_exec ( struct image *image ) {
 	if ( rc != 0 )
 		bs->UnloadImage ( handle );
  err_load_image:
+	if ( shim )
+		efi_shim_uninstall ( &unlocker );
+ err_shim_install:
 	free ( cmdline );
  err_cmdline:
 	free ( path );

src/include/ipxe/efi/Protocol/ShimLock.h

@@ -0,0 +1,29 @@
+#ifndef _IPXE_EFI_SHIM_LOCK_PROTOCOL_H
+#define _IPXE_EFI_SHIM_LOCK_PROTOCOL_H
+
+/** @file
+ *
+ * EFI "shim lock" protocol
+ *
+ */
+
+FILE_LICENCE ( BSD3 );
+
+#define EFI_SHIM_LOCK_PROTOCOL_GUID					\
+	{ 0x605dab50, 0xe046, 0x4300,					\
+	{ 0xab, 0xb6, 0x3d, 0xd8, 0x10, 0xdd, 0x8b, 0x23 } }
+
+typedef
+EFI_STATUS
+(*EFI_SHIM_LOCK_VERIFY) (
+  IN VOID *buffer,
+  IN UINT32 size
+  );
+
+typedef struct _EFI_SHIM_LOCK_PROTOCOL {
+  EFI_SHIM_LOCK_VERIFY Verify;
+  VOID *Reserved1;
+  VOID *Reserved2;
+} EFI_SHIM_LOCK_PROTOCOL;
+
+#endif /*_IPXE_EFI_SHIM_LOCK_PROTOCOL_H */

src/include/ipxe/efi/efi.h

@@ -197,6 +197,7 @@ extern EFI_GUID efi_pci_io_protocol_guid;
 extern EFI_GUID efi_pci_root_bridge_io_protocol_guid;
 extern EFI_GUID efi_pxe_base_code_protocol_guid;
 extern EFI_GUID efi_serial_io_protocol_guid;
+extern EFI_GUID efi_shim_lock_protocol_guid;
 extern EFI_GUID efi_simple_file_system_protocol_guid;
 extern EFI_GUID efi_simple_network_protocol_guid;
 extern EFI_GUID efi_simple_pointer_protocol_guid;

src/include/ipxe/efi/efi_image.h

@@ -11,9 +11,6 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <ipxe/image.h>
 
-extern struct image_tag efi_shim __image_tag;
-extern struct image_tag efi_shim_crutch __image_tag;
-
 extern struct image_type efi_image_type[] __image_type ( PROBE_NORMAL );
 
 /**

src/include/ipxe/efi/efi_shim.h

@@ -0,0 +1,29 @@
+#ifndef _IPXE_EFI_SHIM_H
+#define _IPXE_EFI_SHIM_H
+
+/** @file
+ *
+ * UEFI shim handling
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <ipxe/image.h>
+#include <ipxe/efi/efi.h>
+
+/** A shim unlocker */
+struct efi_shim_unlocker {
+	/** Protocol installation event */
+	EFI_EVENT event;
+	/** Protocol notification registration token */
+	void *token;
+};
+
+extern struct image_tag efi_shim __image_tag;
+extern struct image_tag efi_shim_crutch __image_tag;
+
+extern int efi_shim_install ( struct efi_shim_unlocker *unlocker );
+extern void efi_shim_uninstall ( struct efi_shim_unlocker *unlocker );
+
+#endif /* _IPXE_EFI_SHIM_H */

src/include/ipxe/errfile.h

@@ -404,6 +404,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_dhe		      ( ERRFILE_OTHER | 0x005a0000 )
 #define ERRFILE_efi_cmdline	      ( ERRFILE_OTHER | 0x005b0000 )
 #define ERRFILE_efi_rng		      ( ERRFILE_OTHER | 0x005c0000 )
+#define ERRFILE_efi_shim	      ( ERRFILE_OTHER | 0x005d0000 )
 
 /** @} */
 

src/interface/efi/efi_debug.c

@@ -143,6 +143,8 @@ static struct efi_well_known_guid efi_well_known_guids[] = {
 	  "PxeBaseCode" },
 	{ &efi_serial_io_protocol_guid,
 	  "SerialIo" },
+	{ &efi_shim_lock_protocol_guid,
+	  "ShimLock" },
 	{ &efi_simple_file_system_protocol_guid,
 	  "SimpleFileSystem" },
 	{ &efi_simple_network_protocol_guid,

src/interface/efi/efi_guid.c

@@ -54,6 +54,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #include <ipxe/efi/Protocol/PciRootBridgeIo.h>
 #include <ipxe/efi/Protocol/PxeBaseCode.h>
 #include <ipxe/efi/Protocol/SerialIo.h>
+#include <ipxe/efi/Protocol/ShimLock.h>
 #include <ipxe/efi/Protocol/SimpleFileSystem.h>
 #include <ipxe/efi/Protocol/SimpleNetwork.h>
 #include <ipxe/efi/Protocol/SimplePointer.h>
@@ -227,6 +228,10 @@ EFI_GUID efi_pxe_base_code_protocol_guid
 EFI_GUID efi_serial_io_protocol_guid
 	= EFI_SERIAL_IO_PROTOCOL_GUID;
 
+/** Shim lock protocol GUID */
+EFI_GUID efi_shim_lock_protocol_guid
+	= EFI_SHIM_LOCK_PROTOCOL_GUID;
+
 /** Simple file system protocol GUID */
 EFI_GUID efi_simple_file_system_protocol_guid
 	= EFI_SIMPLE_FILE_SYSTEM_PROTOCOL_GUID;

src/interface/efi/efi_shim.c

@@ -0,0 +1,144 @@
+/*
+ * Copyright (C) 2022 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 (at your option) 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 <string.h>
+#include <errno.h>
+#include <ipxe/image.h>
+#include <ipxe/efi/efi.h>
+#include <ipxe/efi/efi_shim.h>
+#include <ipxe/efi/Protocol/ShimLock.h>
+
+/** @file
+ *
+ * UEFI shim handling
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/** EFI shim image */
+struct image_tag efi_shim __image_tag = {
+	.name = "SHIM",
+};
+
+/** EFI shim crutch image */
+struct image_tag efi_shim_crutch __image_tag = {
+	.name = "SHIMCRUTCH",
+};
+
+/**
+ * Unlock UEFI shim
+ *
+ * @v event		Event
+ * @v context		Event context
+ *
+ * The UEFI shim is gradually becoming less capable of directly
+ * executing a kernel image, due to an ever increasing list of
+ * assumptions that it will only ever be used in conjunction with a
+ * second stage loader such as GRUB.
+ *
+ * For example: shim will erroneously complain if the image that it
+ * loads and executes does not call in to the "shim lock protocol"
+ * before calling ExitBootServices(), even if there is no valid reason
+ * for it to have done so.
+ *
+ * Reduce the Secure Boot attack surface by removing, where possible,
+ * this spurious requirement for the use of an additional second stage
+ * loader.
+ */
+static EFIAPI void efi_shim_unlock ( EFI_EVENT event __unused, void *context ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	EFI_GUID *protocol = &efi_shim_lock_protocol_guid;
+	struct efi_shim_unlocker *unlocker = context;
+	union {
+		EFI_SHIM_LOCK_PROTOCOL *lock;
+		void *interface;
+	} u;
+	uint8_t empty[0];
+	EFI_STATUS efirc;
+
+	/* Process all new instances of the shim lock protocol */
+	while ( 1 ) {
+
+		/* Get next instance */
+		if ( ( efirc = bs->LocateProtocol ( protocol, unlocker->token,
+						    &u.interface ) ) != 0 )
+			break;
+
+		/* Call shim lock protocol with empty buffer */
+		u.lock->Verify ( empty, sizeof ( empty ) );
+		DBGC ( unlocker, "SHIM unlocked %p\n", u.interface );
+	}
+}
+
+/**
+ * Install UEFI shim unlocker
+ *
+ * @v unlocker		Shim unlocker
+ * @ret rc		Return status code
+ */
+int efi_shim_install ( struct efi_shim_unlocker *unlocker ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	EFI_GUID *protocol = &efi_shim_lock_protocol_guid;
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Create event */
+	if ( ( efirc = bs->CreateEvent ( EVT_NOTIFY_SIGNAL, TPL_CALLBACK,
+					 efi_shim_unlock, unlocker,
+					 &unlocker->event ) ) != 0 ) {
+		rc = -EEFI ( efirc );
+		DBGC ( unlocker, "SHIM could not create event: %s\n",
+		       strerror ( rc ) );
+		goto err_create_event;
+	}
+
+	/* Register for protocol installations */
+	if ( ( efirc = bs->RegisterProtocolNotify ( protocol, unlocker->event,
+						    &unlocker->token ) ) != 0){
+		rc = -EEFI ( efirc );
+		DBGC ( unlocker, "SHIM could not register for protocols: %s\n",
+		       strerror ( rc ) );
+		goto err_register_notify;
+	}
+
+	return 0;
+
+ err_register_notify:
+	bs->CloseEvent ( unlocker->event );
+ err_create_event:
+	return rc;
+}
+
+/**
+ * Uninstall UEFI shim unlocker
+ *
+ * @v unlocker		Shim unlocker
+ */
+void efi_shim_uninstall ( struct efi_shim_unlocker *unlocker ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+
+	bs->CloseEvent ( unlocker->event );
+}

src/usr/shimmgmt.c

@@ -24,7 +24,7 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <ipxe/efi/efi.h>
-#include <ipxe/efi/efi_image.h>
+#include <ipxe/efi/efi_shim.h>
 #include <usr/shimmgmt.h>
 
 /** @file