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/*
* 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 );
/** UEFI shim image */
struct image_tag efi_shim __image_tag = {
.name = "SHIM",
};
/** UEFI shim crutch image */
struct image_tag efi_shim_crutch __image_tag = {
.name = "SHIMCRUTCH",
};
/** Original ExitBootServices() function */
static EFI_EXIT_BOOT_SERVICES efi_shim_orig_ebs;
/**
* Unlock UEFI shim
*
* @v image Image handle
* @v key Map key
* @ret efirc EFI status code
*
* 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 EFI_STATUS efi_shim_unlock ( EFI_HANDLE image, UINTN key ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
uint8_t empty[0];
union {
EFI_SHIM_LOCK_PROTOCOL *lock;
void *interface;
} u;
EFI_STATUS efirc;
//
DBG ( "******** called\n" );
/* Locate shim lock protocol */
if ( ( efirc = bs->LocateProtocol ( &efi_shim_lock_protocol_guid,
NULL, &u.interface ) ) == 0 ) {
u.lock->Verify ( empty, sizeof ( empty ) );
DBGC ( u.lock, "SHIM unlocked %p\n", u.lock );
}
/* Hand off to original ExitBootServices() */
return efi_shim_orig_ebs ( image, key );
}
/**
* Install UEFI shim unlocker
*
* @ret rc Return status code
*/
int efi_shim_install ( ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
/* Intercept ExitBootServices() via boot services table */
efi_shim_orig_ebs = bs->ExitBootServices;
bs->ExitBootServices = efi_shim_unlock;
//
DBG ( "******** hooked in systab %p\n", efi_systab );
return 0;
}
/**
* Uninstall UEFI shim unlocker
*
*/
void efi_shim_uninstall ( void ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
/* Restore original ExitBootServices() */
bs->ExitBootServices = efi_shim_orig_ebs;
}
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