Re: [PATCH 1/2] efi/libstub: add support for loading the initrd from a device path

[Heinrich Schuchardt <xypron.glpk@gmx.de>]

On 2/6/20 3:03 PM, Ard Biesheuvel wrote:
> There are currently two ways to specify the initrd to be passed to the
> Linux kernel when booting via the EFI stub:
> - it can be passed as a initrd= command line option when doing a pure PE
>    boot (as opposed to the EFI handover protocol that exists for x86)
> - otherwise, the bootloader or firmware can load the initrd into memory,
>    and pass the address and size via the bootparams struct (x86) or
>    device tree (ARM)
>
> In the first case, we are limited to loading from the same file system
> that the kernel was loaded from, and it is also problematic in a trusted

Hello Ard,

"same file system" is not a limitation of using a command line
parameter. Any device path can be passed as a string.

> boot context, given that we cannot easily protect the command line from
> tampering without either adding complicated white/blacklisting of boot
> arguments or locking down the command line altogether.

Not relying on the command line for finding the initrd image does not
secure the integrity and the validity of the initrd image.

A signature on the initrd image could solve the security problem you
describe. It would not require non-Linux software to be changed, and
would provide much better security.

>
> In the second case, we force the bootloader to duplicate knowledge about
> the boot protocol which is already encoded in the stub, and which may be
> subject to change over time, e.g., bootparams struct definitions, memory
> allocation/alignment requirements for the placement of the initrd etc etc.
> In the ARM case, it also requires the bootloader to modify the hardware
> description provided by the firmware, as it is passed in the same file.
> On systems where the initrd is measured after loading, it creates a time
> window where the initrd contents might be manipulated in memory before
> handing over to the kernel.
>
> Address these concerns by adding support for loading the initrd into
> memory by invoking the EFI LoadFile2 protocol installed on a vendor
> GUIDed device path that specifically designates a Linux initrd.
> This addresses the above concerns, by putting the EFI stub in charge of
> placement in memory and of passing the base and size to the kernel proper
> (via whatever means it desires) while still leaving it up to the firmware
> or bootloader to obtain the file contents, potentially from other file
> systems than the one the kernel itself was loaded from. On platforms that
> implement measured boot, it permits the firmware to take the measurement
> right before the kernel actually consumes the contents.

A firmware implementing the UEFI standard will not be aware of any
initrd image as such an object does not exist in the standard. It was a
wise decision that the UEFI standard is operating system agnostic
(accomodating BSD, Linux, Windows, etc.). So the firmware (EDK2, U-Boot,
etc.) seems to be out of scope for providing a Linux specific
EFI_LOAD_FILE2_PROTOCOL.

When booting via GRUB it will be GRUB knowing which initrd to load.

Please, indicate which software you expect to expose the initrd related
EFI_LOAD_FILE2_PROTOCOL.

Using an UEFI variable for passing the initrd device path would be a
leaner solution on the bootloader side than requiring an extra
EFI_LOAD_FILE2_PROTOCOL implementation.

>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>   drivers/firmware/efi/libstub/arm-stub.c        | 16 +++--
>   drivers/firmware/efi/libstub/efi-stub-helper.c | 65 ++++++++++++++++++++
>   drivers/firmware/efi/libstub/efistub.h         | 12 ++++
>   drivers/firmware/efi/libstub/x86-stub.c        | 36 ++++++++++-
>   include/linux/efi.h                            |  1 +
>   5 files changed, 123 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
> index c7b091f50e55..1db943c1ba2b 100644
> --- a/drivers/firmware/efi/libstub/arm-stub.c
> +++ b/drivers/firmware/efi/libstub/arm-stub.c
> @@ -157,6 +157,7 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table_arg,
>   	enum efi_secureboot_mode secure_boot;
>   	struct screen_info *si;
>   	efi_properties_table_t *prop_tbl;
> +	unsigned long max_addr;
>
>   	sys_table = sys_table_arg;
>
> @@ -255,11 +256,18 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table_arg,
>   	if (!fdt_addr)
>   		pr_efi("Generating empty DTB\n");
>
> -	status = efi_load_initrd(image, ULONG_MAX,
> -				 efi_get_max_initrd_addr(dram_base, *image_addr),
> -				 &initrd_addr, &initrd_size);
> +	max_addr = efi_get_max_initrd_addr(dram_base, *image_addr);
> +	status = efi_load_initrd_devpath(&initrd_addr, &initrd_size, max_addr);
> +	if (status == EFI_SUCCESS)
> +		pr_efi("Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path\n");
> +	else if (status == EFI_NOT_FOUND) {
> +		status = efi_load_initrd(image, ULONG_MAX, max_addr,
> +					 &initrd_addr, &initrd_size);
> +		if (status == EFI_SUCCESS)
> +			pr_efi("Loaded initrd from command line option\n");
> +	}
>   	if (status != EFI_SUCCESS)
> -		pr_efi_err("Failed initrd from command line!\n");
> +		pr_efi_err("Failed to load initrd!\n");
>
>   	efi_random_get_seed();
>
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index 8e60a39df3c5..eaf45ea749b3 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -323,3 +323,68 @@ void efi_char16_printk(efi_char16_t *str)
>   	efi_call_proto(efi_table_attr(efi_system_table(), con_out),
>   		       output_string, str);
>   }
> +
> +static const struct {
> +	struct efi_vendor_dev_path	vendor;
> +	struct efi_generic_dev_path	end;
> +} __packed initrd_devpath = {
> +	{
> +		EFI_DEV_MEDIA,
> +		EFI_DEV_MEDIA_VENDOR,
> +		sizeof(struct efi_vendor_dev_path),
> +		LINUX_EFI_INITRD_MEDIA_GUID
> +	}, {
> +		EFI_DEV_END_PATH,
> +		EFI_DEV_END_ENTIRE,
> +		sizeof(struct efi_generic_dev_path)
> +	}
> +};
> +
> +efi_status_t efi_load_initrd_devpath(unsigned long *load_addr,
> +				     unsigned long *load_size,
> +				     unsigned long max)
> +{
> +	efi_guid_t lf2_proto_guid = EFI_LOAD_FILE2_PROTOCOL_GUID;
> +	efi_device_path_protocol_t *dp;
> +	efi_load_file2_protocol_t *lf2;
> +	unsigned long initrd_addr;
> +	unsigned long initrd_size;
> +	efi_handle_t handle;
> +	efi_status_t status;
> +
> +	if (!load_addr || !load_size)
> +		return EFI_INVALID_PARAMETER;
> +
> +	dp = (efi_device_path_protocol_t *)&initrd_devpath;
> +	status = efi_bs_call(locate_device_path, &lf2_proto_guid, &dp, &handle);
> +	if (status != EFI_SUCCESS)
> +		return status;
> +
> +	status = efi_bs_call(handle_protocol, handle, &lf2_proto_guid,
> +			     (void **)&lf2);
> +	if (status != EFI_SUCCESS)
> +		return status;

You require here that there is a handle exposing the device path
protocol with the initrd specific device path. On the same handle the
EFI_LOAD_FILE2_PROTOCOL must be installed which will load the initrd
file when called with the same device path.

An alternative implementation would simple loop over all instances of
the EFI_LOAD_FILE2_PROTOCOL and try to load the initrd.

It would be worthwhile to describe the requirements on the
implementation of the EFI_LOAD_FILE2_PROTOCOL in a code comment and in
the documentation.

Unfortunately the documentation of UEFI has been duplicated into two files:

Documentation/arm/uefi.rst
Documentation/x86/x86_64/uefi.rst

Best regards

Heinrich

> +
> +	initrd_size = 0;
> +	status = efi_call_proto(lf2, load_file,
> +				(efi_device_path_protocol_t *)&initrd_devpath,
> +				false, &initrd_size, NULL);
> +	if (status != EFI_BUFFER_TOO_SMALL)
> +		return EFI_LOAD_ERROR;
> +
> +	status = efi_allocate_pages(initrd_size, &initrd_addr, max);
> +	if (status != EFI_SUCCESS)
> +		return status;
> +
> +	status = efi_call_proto(lf2, load_file,
> +				(efi_device_path_protocol_t *)&initrd_devpath,
> +				false, &initrd_size, (void *)initrd_addr);
> +	if (status != EFI_SUCCESS) {
> +		efi_free(initrd_size, initrd_addr);
> +		return status;
> +	}
> +
> +	*load_addr = initrd_addr;
> +	*load_size = initrd_size;
> +	return EFI_SUCCESS;
> +}
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index 99e93fd76ec5..fbf9f9442eed 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -566,6 +566,14 @@ union efi_load_file_protocol {
>   	} mixed_mode;
>   };
>
> +struct efi_vendor_dev_path {
> +	u8		type;
> +	u8		sub_type;
> +	u16		length;
> +	efi_guid_t	vendorguid;
> +	u8		vendordata[];
> +} __packed;
> +
>   void efi_pci_disable_bridge_busmaster(void);
>
>   typedef efi_status_t (*efi_exit_boot_map_processing)(
> @@ -651,4 +659,8 @@ efi_status_t efi_load_initrd(efi_loaded_image_t *image,
>   			     unsigned long *load_addr,
>   			     unsigned long *load_size);
>
> +efi_status_t efi_load_initrd_devpath(unsigned long *load_addr,
> +				     unsigned long *load_size,
> +				     unsigned long max);
> +
>   #endif
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index f3e2ff31b624..7f38f95676dd 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -419,9 +419,20 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
>   	if (status != EFI_SUCCESS)
>   		goto fail2;
>
> -	status = efi_load_initrd(image, hdr->initrd_addr_max,
> -				 above4g ? ULONG_MAX : hdr->initrd_addr_max,
> -				 &ramdisk_addr, &ramdisk_size);
> +	/*
> +	 * The initrd loaded from the Linux initrd vendor device
> +	 * path should take precedence, as we don't want the
> +	 * [unverified] command line to override the initrd
> +	 * supplied by the [potentially verified] firmware.
> +	 */
> +	status = efi_load_initrd_devpath(&ramdisk_addr, &ramdisk_size,
> +					 above4g ? ULONG_MAX
> +						 : hdr->initrd_addr_max);
> +	if (status == EFI_NOT_FOUND)
> +		status = efi_load_initrd(image, hdr->initrd_addr_max,
> +					 above4g ? ULONG_MAX
> +						 : hdr->initrd_addr_max,
> +					 &ramdisk_addr, &ramdisk_size);
>   	if (status != EFI_SUCCESS)
>   		goto fail2;
>   	hdr->ramdisk_image = ramdisk_addr & 0xffffffff;
> @@ -732,6 +743,25 @@ struct boot_params *efi_main(efi_handle_t handle,
>   			 ((u64)boot_params->ext_cmd_line_ptr << 32));
>   	efi_parse_options((char *)cmdline_paddr);
>
> +	if (!hdr->ramdisk_size && !boot_params->ext_ramdisk_size) {
> +		unsigned long max = hdr->initrd_addr_max;
> +		unsigned long addr, size;
> +
> +		if (hdr->xloadflags & XLF_CAN_BE_LOADED_ABOVE_4G)
> +			max = ULONG_MAX;
> +
> +		status = efi_load_initrd_devpath(&addr, &size, max);
> +		if (status == EFI_SUCCESS) {
> +			hdr->ramdisk_image		= (u32)addr;
> +			hdr->ramdisk_size 		= (u32)size;
> +			boot_params->ext_ramdisk_image	= (u64)addr >> 32;
> +			boot_params->ext_ramdisk_size 	= (u64)size >> 32;
> +		} else if (status != EFI_NOT_FOUND) {
> +			efi_printk("efi_load_initrd_devpath() failed!\n");
> +			goto fail;
> +		}
> +	}
> +
>   	/*
>   	 * If the boot loader gave us a value for secure_boot then we use that,
>   	 * otherwise we ask the BIOS.
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 9ccf313fe9de..75c83c322c40 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -353,6 +353,7 @@ void efi_native_runtime_setup(void);
>   #define LINUX_EFI_TPM_EVENT_LOG_GUID		EFI_GUID(0xb7799cb0, 0xeca2, 0x4943,  0x96, 0x67, 0x1f, 0xae, 0x07, 0xb7, 0x47, 0xfa)
>   #define LINUX_EFI_TPM_FINAL_LOG_GUID		EFI_GUID(0x1e2ed096, 0x30e2, 0x4254,  0xbd, 0x89, 0x86, 0x3b, 0xbe, 0xf8, 0x23, 0x25)
>   #define LINUX_EFI_MEMRESERVE_TABLE_GUID		EFI_GUID(0x888eb0c6, 0x8ede, 0x4ff5,  0xa8, 0xf0, 0x9a, 0xee, 0x5c, 0xb9, 0x77, 0xc2)
> +#define LINUX_EFI_INITRD_MEDIA_GUID		EFI_GUID(0x5568e427, 0x68fc, 0x4f3d,  0xac, 0x74, 0xca, 0x55, 0x52, 0x31, 0xcc, 0x68)
>
>   /* OEM GUIDs */
>   #define DELLEMC_EFI_RCI2_TABLE_GUID		EFI_GUID(0x2d9f28a2, 0xa886, 0x456a,  0x97, 0xa8, 0xf1, 0x1e, 0xf2, 0x4f, 0xf4, 0x55)
>