systemd/src/boot/efi/linux.c

/* SPDX-License-Identifier: LGPL-2.1-or-later */

/*
 * Generic Linux boot protocol using the EFI/PE entry point of the kernel. Passes
 * initrd with the LINUX_INITRD_MEDIA_GUID DevicePath and cmdline with
 * EFI LoadedImageProtocol.
 *
 * This method works for Linux 5.8 and newer on ARM/Aarch64, x86/x68_64 and RISC-V.
 */

#include <efi.h>
#include <efilib.h>

#include "initrd.h"
#include "linux.h"
#include "pe.h"
#include "secure-boot.h"
#include "util.h"

#define STUB_PAYLOAD_GUID \
        { 0x55c5d1f8, 0x04cd, 0x46b5, { 0x8a, 0x20, 0xe5, 0x6c, 0xbb, 0x30, 0x52, 0xd0 } }

typedef struct {
        const void *addr;
        size_t len;
        const EFI_DEVICE_PATH *device_path;
} ValidationContext;

static bool validate_payload(
                const void *ctx, const EFI_DEVICE_PATH *device_path, const void *file_buffer, size_t file_size) {

        const ValidationContext *payload = ASSERT_PTR(ctx);

        if (device_path != payload->device_path)
                return false;

        /* Security arch (1) protocol does not provide a file buffer. Instead we are supposed to fetch the payload
         * ourselves, which is not needed as we already have everything in memory and the device paths match. */
        if (file_buffer && (file_buffer != payload->addr || file_size != payload->len))
                return false;

        return true;
}

static EFI_STATUS load_image(EFI_HANDLE parent, const void *source, size_t len, EFI_HANDLE *ret_image) {
        assert(parent);
        assert(source);
        assert(ret_image);

        /* We could pass a NULL device path, but it's nicer to provide something and it allows us to identify
         * the loaded image from within the security hooks. */
        struct {
                VENDOR_DEVICE_PATH payload;
                EFI_DEVICE_PATH end;
        } _packed_ payload_device_path = {
                .payload = {
                        .Header = {
                                .Type = MEDIA_DEVICE_PATH,
                                .SubType = MEDIA_VENDOR_DP,
                                .Length = { sizeof(payload_device_path.payload), 0 },
                        },
                        .Guid = STUB_PAYLOAD_GUID,
                },
                .end = {
                        .Type = END_DEVICE_PATH_TYPE,
                        .SubType = END_ENTIRE_DEVICE_PATH_SUBTYPE,
                        .Length = { sizeof(payload_device_path.end), 0 },
                },
        };

        /* We want to support unsigned kernel images as payload, which is safe to do under secure boot
         * because it is embedded in this stub loader (and since it is already running it must be trusted). */
        install_security_override(
                        validate_payload,
                        &(ValidationContext) {
                                .addr = source,
                                .len = len,
                                .device_path = &payload_device_path.payload.Header,
                        });

        EFI_STATUS ret = BS->LoadImage(
                        /*BootPolicy=*/false,
                        parent,
                        &payload_device_path.payload.Header,
                        (void *) source,
                        len,
                        ret_image);

        uninstall_security_override();

        return ret;
}

EFI_STATUS linux_exec(
                EFI_HANDLE parent,
                const char16_t *cmdline,
                const void *linux_buffer,
                size_t linux_length,
                const void *initrd_buffer,
                size_t initrd_length) {

        uint32_t compat_address;
        EFI_STATUS err;

        assert(parent);
        assert(linux_buffer && linux_length > 0);
        assert(initrd_buffer || initrd_length == 0);

        err = pe_kernel_info(linux_buffer, &compat_address);
#if defined(__i386__) || defined(__x86_64__)
        if (err == EFI_UNSUPPORTED)
                /* Kernel is too old to support LINUX_INITRD_MEDIA_GUID, try the deprecated EFI handover
                 * protocol. */
                return linux_exec_efi_handover(
                                parent,
                                cmdline,
                                linux_buffer,
                                linux_length,
                                initrd_buffer,
                                initrd_length);
#endif
        if (err != EFI_SUCCESS)
                return log_error_status_stall(err, u"Bad kernel image: %r", err);

        _cleanup_(unload_imagep) EFI_HANDLE kernel_image = NULL;
        err = load_image(parent, linux_buffer, linux_length, &kernel_image);
        if (err != EFI_SUCCESS)
                return log_error_status_stall(err, u"Error loading kernel image: %r", err);

        EFI_LOADED_IMAGE_PROTOCOL *loaded_image;
        err = BS->HandleProtocol(kernel_image, &LoadedImageProtocol, (void **) &loaded_image);
        if (err != EFI_SUCCESS)
                return log_error_status_stall(err, u"Error getting kernel loaded image protocol: %r", err);

        if (cmdline) {
                loaded_image->LoadOptions = (void *) cmdline;
                loaded_image->LoadOptionsSize = strsize16(loaded_image->LoadOptions);
        }

        _cleanup_(cleanup_initrd) EFI_HANDLE initrd_handle = NULL;
        err = initrd_register(initrd_buffer, initrd_length, &initrd_handle);
        if (err != EFI_SUCCESS)
                return log_error_status_stall(err, u"Error registering initrd: %r", err);

        err = BS->StartImage(kernel_image, NULL, NULL);

        /* Try calling the kernel compat entry point if one exists. */
        if (err == EFI_UNSUPPORTED && compat_address > 0) {
                EFI_IMAGE_ENTRY_POINT compat_entry =
                                (EFI_IMAGE_ENTRY_POINT) ((uint8_t *) loaded_image->ImageBase + compat_address);
                err = compat_entry(kernel_image, ST);
        }

        return log_error_status_stall(err, u"Error starting kernel image: %r", err);
}