[efi] Add support for executing images via a shim

Add support for using a shim as a helper to execute an EFI image.
When a shim has been specified via shim(), the shim image will be
passed to LoadImage() instead of the selected EFI image and the
command line will be prepended with the name of the selected EFI
image.  The selected EFI image will be accessible to the shim via the
virtual filesystem as a hidden file.

Reduce the Secure Boot attack surface by removing, where possible, the
spurious requirement for a third party second stage loader binary such
as GRUB to be used solely in order to call the "shim lock protocol"
entry point.

Do not install the EFI PXE APIs when using a shim, since if shim finds
EFI_PXE_BASE_CODE_PROTOCOL on the loaded image's device handle then it
will attempt to download files afresh instead of using the files
already downloaded by iPXE and exposed via the EFI_SIMPLE_FILE_SYSTEM
protocol.  (Experience shows that there is no point in trying to get a
fix for this upstreamed into shim.)

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

1 files changed, 251 insertions, 0 deletions

diff --git a/src/interface/efi/efi_shim.c b/src/interface/efi/efi_shim.c
new file mode 100644
index 0000000..9b1b69e
--- /dev/null
+++ b/src/interface/efi/efi_shim.c
@@ -0,0 +1,251 @@
+/*
+ * 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 <stdlib.h>
+#include <errno.h>
+#include <ipxe/image.h>
+#include <ipxe/efi/efi.h>
+#include <ipxe/efi/efi_strings.h>
+#include <ipxe/efi/efi_shim.h>
+#include <ipxe/efi/Protocol/PxeBaseCode.h>
+#include <ipxe/efi/Protocol/ShimLock.h>
+
+/** @file
+ *
+ * UEFI shim special handling
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * Require use of a third party loader binary
+ *
+ * The UEFI shim is gradually becoming less capable of directly
+ * executing a Linux kernel image, due to an ever increasing list of
+ * assumptions that it will only ever be used in conjunction with a
+ * second stage loader binary such as GRUB.
+ *
+ * For example: shim will erroneously complain if the image that it
+ * loads and executes does not in turn call in to the "shim lock
+ * protocol" to verify a separate newly loaded binary before calling
+ * ExitBootServices(), even if no such separate binary is used or
+ * required.
+ *
+ * Experience shows that there is unfortunately no point in trying to
+ * get a fix for this upstreamed into shim.  We therefore default to
+ * reducing the Secure Boot attack surface by removing, where
+ * possible, this spurious requirement for the use of an additional
+ * second stage loader.
+ *
+ * This option may be used to require the use of an additional second
+ * stage loader binary, in case this behaviour is ever desirable.
+ */
+int efi_shim_require_loader = 0;
+
+/**
+ * Allow use of PXE base code protocol
+ *
+ * We provide shim with access to all of the relevant downloaded files
+ * via our EFI_SIMPLE_FILE_SYSTEM_PROTOCOL interface.  However, shim
+ * will instead try to redownload the files via TFTP since it prefers
+ * to use the EFI_PXE_BASE_CODE_PROTOCOL installed on the same handle.
+ *
+ * Experience shows that there is unfortunately no point in trying to
+ * get a fix for this upstreamed into shim.  We therefore default to
+ * working around this undesirable behaviour by stopping the PXE base
+ * code protocol before invoking shim.
+ *
+ * This option may be used to allow shim to use the PXE base code
+ * protocol, in case this behaviour is ever desirable.
+ */
+int efi_shim_allow_pxe = 0;
+
+/** UEFI shim image */
+struct image_tag efi_shim __image_tag = {
+	.name = "SHIM",
+};
+
+/** Original GetMemoryMap() function */
+static EFI_GET_MEMORY_MAP efi_shim_orig_map;
+
+/**
+ * Unlock UEFI shim
+ *
+ * @v len		Memory map size
+ * @v map		Memory map
+ * @v key		Memory map key
+ * @v desclen		Descriptor size
+ * @v descver		Descriptor version
+ * @ret efirc		EFI status code
+ *
+ */
+static EFIAPI EFI_STATUS efi_shim_unlock ( UINTN *len,
+					   EFI_MEMORY_DESCRIPTOR *map,
+					   UINTN *key, UINTN *desclen,
+					   UINT32 *descver ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	uint8_t empty[0];
+	union {
+		EFI_SHIM_LOCK_PROTOCOL *lock;
+		void *interface;
+	} u;
+	EFI_STATUS efirc;
+
+	/* Locate shim lock protocol */
+	if ( ( efirc = bs->LocateProtocol ( &efi_shim_lock_protocol_guid,
+					    NULL, &u.interface ) ) == 0 ) {
+		u.lock->Verify ( empty, sizeof ( empty ) );
+		DBGC ( &efi_shim, "SHIM unlocked via %p\n", u.lock );
+	}
+
+	/* Hand off to original GetMemoryMap() */
+	return efi_shim_orig_map ( len, map, key, desclen, descver );
+}
+
+/**
+ * Inhibit use of PXE base code
+ *
+ * @v handle		EFI handle
+ * @ret rc		Return status code
+ */
+static int efi_shim_inhibit_pxe ( EFI_HANDLE handle ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	union {
+		EFI_PXE_BASE_CODE_PROTOCOL *pxe;
+		void *interface;
+	} u;
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Locate PXE base code */
+	if ( ( efirc = bs->OpenProtocol ( handle,
+					  &efi_pxe_base_code_protocol_guid,
+					  &u.interface, efi_image_handle, NULL,
+					  EFI_OPEN_PROTOCOL_GET_PROTOCOL ))!=0){
+		rc = -EEFI ( efirc );
+		DBGC ( &efi_shim, "SHIM could not open PXE base code: %s\n",
+		       strerror ( rc ) );
+		goto err_no_base;
+	}
+
+	/* Stop PXE base code */
+	if ( ( efirc = u.pxe->Stop ( u.pxe ) ) != 0 ) {
+		rc = -EEFI ( efirc );
+		DBGC ( &efi_shim, "SHIM could not stop PXE base code: %s\n",
+		       strerror ( rc ) );
+		goto err_stop;
+	}
+
+	/* Success */
+	rc = 0;
+	DBGC ( &efi_shim, "SHIM stopped PXE base code\n" );
+
+ err_stop:
+	bs->CloseProtocol ( handle, &efi_pxe_base_code_protocol_guid,
+			    efi_image_handle, NULL );
+ err_no_base:
+	return rc;
+}
+
+/**
+ * Update command line
+ *
+ * @v shim		Shim image
+ * @v cmdline		Command line to update
+ * @ret rc		Return status code
+ */
+static int efi_shim_cmdline ( struct image *shim, wchar_t **cmdline ) {
+	wchar_t *shimcmdline;
+	int len;
+	int rc;
+
+	/* Construct new command line */
+	len = ( shim->cmdline ?
+		efi_asprintf ( &shimcmdline, "%s %s", shim->name,
+			       shim->cmdline ) :
+		efi_asprintf ( &shimcmdline, "%s %ls", shim->name,
+			       *cmdline ) );
+	if ( len < 0 ) {
+		rc = len;
+		DBGC ( &efi_shim, "SHIM could not construct command line: "
+		       "%s\n", strerror ( rc ) );
+		return rc;
+	}
+
+	/* Replace command line */
+	free ( *cmdline );
+	*cmdline = shimcmdline;
+
+	return 0;
+}
+
+/**
+ * Install UEFI shim special handling
+ *
+ * @v shim		Shim image
+ * @v handle		EFI device handle
+ * @v cmdline		Command line to update
+ * @ret rc		Return status code
+ */
+int efi_shim_install ( struct image *shim, EFI_HANDLE handle,
+		       wchar_t **cmdline ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+	int rc;
+
+	/* Intercept GetMemoryMap() via boot services table */
+	efi_shim_orig_map = bs->GetMemoryMap;
+	if ( ! efi_shim_require_loader )
+		bs->GetMemoryMap = efi_shim_unlock;
+
+	/* Stop PXE base code */
+	if ( ( ! efi_shim_allow_pxe ) &&
+	     ( ( rc = efi_shim_inhibit_pxe ( handle ) ) != 0 ) ) {
+		goto err_inhibit_pxe;
+	}
+
+	/* Update command line */
+	if ( ( rc = efi_shim_cmdline ( shim, cmdline ) ) != 0 )
+		goto err_cmdline;
+
+	return 0;
+
+ err_cmdline:
+ err_inhibit_pxe:
+	bs->GetMemoryMap = efi_shim_orig_map;
+	return rc;
+}
+
+/**
+ * Uninstall UEFI shim special handling
+ *
+ */
+void efi_shim_uninstall ( void ) {
+	EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
+
+	/* Restore original GetMemoryMap() */
+	bs->GetMemoryMap = efi_shim_orig_map;
+}