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// SPDX-License-Identifier: BSD-2-Clause-Patent
/*
* shim - trivial UEFI first-stage bootloader
*
* Copyright Red Hat, Inc
*/
/* Chemical agents lend themselves to covert use in sabotage against
* which it is exceedingly difficult to visualize any really effective
* defense... I will not dwell upon this use of CBW because, as one
* pursues the possibilities of such covert uses, one discovers that the
* scenarios resemble that in which the components of a nuclear weapon
* are smuggled into New York City and assembled in the basement of the
* Empire State Building.
* In other words, once the possibility is recognized to exist, about
* all that one can do is worry about it.
* -- Dr. Ivan L Bennett, Jr., testifying before the Subcommittee on
* National Security Policy and Scientific Developments, November 20,
* 1969.
*/
#include "shim.h"
static EFI_SYSTEM_TABLE *systab;
EFI_SYSTEM_TABLE *
get_active_systab(void)
{
if (systab)
return systab;
return ST;
}
static typeof(systab->BootServices->LoadImage) system_load_image;
static typeof(systab->BootServices->StartImage) system_start_image;
static typeof(systab->BootServices->Exit) system_exit;
#if !defined(DISABLE_EBS_PROTECTION)
static typeof(systab->BootServices->ExitBootServices) system_exit_boot_services;
#endif /* !defined(DISABLE_EBS_PROTECTION) */
static EFI_HANDLE last_loaded_image;
void
unhook_system_services(void)
{
if (!systab)
return;
systab->BootServices->LoadImage = system_load_image;
systab->BootServices->StartImage = system_start_image;
#if !defined(DISABLE_EBS_PROTECTION)
systab->BootServices->ExitBootServices = system_exit_boot_services;
#endif /* !defined(DISABLE_EBS_PROTECTION) */
BS = systab->BootServices;
}
static EFI_STATUS EFIAPI
load_image(BOOLEAN BootPolicy, EFI_HANDLE ParentImageHandle,
EFI_DEVICE_PATH *DevicePath, VOID *SourceBuffer,
UINTN SourceSize, EFI_HANDLE *ImageHandle)
{
EFI_STATUS efi_status;
unhook_system_services();
efi_status = BS->LoadImage(BootPolicy, ParentImageHandle, DevicePath,
SourceBuffer, SourceSize, ImageHandle);
hook_system_services(systab);
if (EFI_ERROR(efi_status))
last_loaded_image = NULL;
else
last_loaded_image = *ImageHandle;
return efi_status;
}
static EFI_STATUS EFIAPI
replacement_start_image(EFI_HANDLE image_handle, UINTN *exit_data_size, CHAR16 **exit_data)
{
EFI_STATUS efi_status;
unhook_system_services();
if (image_handle == last_loaded_image) {
UINT8 retain_protocol = 0;
UINTN retain_protocol_size = sizeof(retain_protocol);
UINT32 retain_protocol_attrs = 0;
loader_is_participating = 1;
/* If a boot component asks us, keep our protocol around - it will be used to
* validate further PE payloads (e.g.: by the UKI stub, before the kernel is booted).
* But also check that the variable was set by a boot component, to ensure that
* nobody at runtime can attempt to change shim's behaviour. */
efi_status = RT->GetVariable(SHIM_RETAIN_PROTOCOL_VAR_NAME,
&SHIM_LOCK_GUID,
&retain_protocol_attrs,
&retain_protocol_size,
&retain_protocol);
if (EFI_ERROR(efi_status) ||
(retain_protocol_attrs & EFI_VARIABLE_NON_VOLATILE) ||
!(retain_protocol_attrs & EFI_VARIABLE_BOOTSERVICE_ACCESS) ||
retain_protocol_size != sizeof(retain_protocol) ||
retain_protocol == 0)
uninstall_shim_protocols();
}
efi_status = BS->StartImage(image_handle, exit_data_size, exit_data);
if (EFI_ERROR(efi_status)) {
if (image_handle == last_loaded_image) {
EFI_STATUS efi_status2 = install_shim_protocols();
if (EFI_ERROR(efi_status2)) {
console_print(L"Something has gone seriously wrong: %r\n",
efi_status2);
console_print(L"shim cannot continue, sorry.\n");
usleep(5000000);
RT->ResetSystem(EfiResetShutdown,
EFI_SECURITY_VIOLATION,
0, NULL);
}
}
hook_system_services(systab);
loader_is_participating = 0;
}
return efi_status;
}
#if !defined(DISABLE_EBS_PROTECTION)
static EFI_STATUS EFIAPI
exit_boot_services(EFI_HANDLE image_key, UINTN map_key)
{
if (loader_is_participating ||
verification_method == VERIFIED_BY_HASH) {
unhook_system_services();
EFI_STATUS efi_status;
efi_status = BS->ExitBootServices(image_key, map_key);
if (EFI_ERROR(efi_status))
hook_system_services(systab);
return efi_status;
}
console_print(L"Bootloader has not verified loaded image.\n");
console_print(L"System is compromised. halting.\n");
usleep(5000000);
RT->ResetSystem(EfiResetShutdown, EFI_SECURITY_VIOLATION, 0, NULL);
return EFI_SECURITY_VIOLATION;
}
#endif /* !defined(DISABLE_EBS_PROTECTION) */
static EFI_STATUS EFIAPI
do_exit(EFI_HANDLE ImageHandle, EFI_STATUS ExitStatus,
UINTN ExitDataSize, CHAR16 *ExitData)
{
EFI_STATUS efi_status;
shim_fini();
restore_loaded_image();
efi_status = BS->Exit(ImageHandle, ExitStatus,
ExitDataSize, ExitData);
if (EFI_ERROR(efi_status)) {
EFI_STATUS efi_status2 = shim_init();
if (EFI_ERROR(efi_status2)) {
console_print(L"Something has gone seriously wrong: %r\n",
efi_status2);
console_print(L"shim cannot continue, sorry.\n");
usleep(5000000);
RT->ResetSystem(EfiResetShutdown,
EFI_SECURITY_VIOLATION, 0, NULL);
}
}
return efi_status;
}
void
hook_system_services(EFI_SYSTEM_TABLE *local_systab)
{
systab = local_systab;
BS = systab->BootServices;
/* We need to hook various calls to make this work... */
/* We need LoadImage() hooked so that fallback.c can load shim
* without having to fake LoadImage as well. This allows it
* to call the system LoadImage(), and have us track the output
* and mark loader_is_participating in replacement_start_image. This
* means anything added by fallback has to be verified by the system
* db, which we want to preserve anyway, since that's all launching
* through BDS gives us. */
system_load_image = systab->BootServices->LoadImage;
systab->BootServices->LoadImage = load_image;
/* we need StartImage() so that we can allow chain booting to an
* image trusted by the firmware */
system_start_image = systab->BootServices->StartImage;
systab->BootServices->StartImage = replacement_start_image;
#if !defined(DISABLE_EBS_PROTECTION)
/* we need to hook ExitBootServices() so a) we can enforce the policy
* and b) we can unwrap when we're done. */
system_exit_boot_services = systab->BootServices->ExitBootServices;
systab->BootServices->ExitBootServices = exit_boot_services;
#endif /* defined(DISABLE_EBS_PROTECTION) */
}
void
unhook_exit(void)
{
systab->BootServices->Exit = system_exit;
BS = systab->BootServices;
}
void
hook_exit(EFI_SYSTEM_TABLE *local_systab)
{
systab = local_systab;
BS = local_systab->BootServices;
/* we need to hook Exit() so that we can allow users to quit the
* bootloader and still e.g. start a new one or run an internal
* shell. */
system_exit = systab->BootServices->Exit;
systab->BootServices->Exit = do_exit;
}
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