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 7:46 PM, Ilias Apalodimas wrote:
> Hi Heinrich,
>
> I actually like the approach.
>
> On Thu, Feb 06, 2020 at 07:26:20PM +0100, Heinrich Schuchardt wrote:
> [...]
>>> 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.
>
> This patch isn't supposed to protect you against a bogus initrd and I don't
> think it should be the bootloaders job to verify that. The kernel already has
> tools to do that.

How do you expect a bootloader identify if the initrd is compatible with
the kernel?

>
>>
>> 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.
>>
>
> The problem with parsing the kernel command line is that at the moment,
> different options are supported by different architectures. A quick grep for
> CONFIG_CMDLINE_(FORCE/OVERWRITE/EXTEND/FROM_BOOTLOADER) will give you an idea.
>
> What you can do in U-Boot currently is not have an environment in any of the
> flashes and set the bootdelay to 0, in order to prevent the user from
> changing that command line.

If you don't have an environment or boot script how would
update-initramfs set the path of the initrd when it is updated?

Using a UEFI variable seems to be the natural choice.

>
>>>
>>> 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.
>
> What about booting the kernel directly?
>
>>
>> Please, indicate which software you expect to expose the initrd related
>> EFI_LOAD_FILE2_PROTOCOL.
>
> I have an implementation for this on U-Boot which works. The file and device are
> hardcoded at the moment, but the rest of the functionality works fine. I'll
> share it with you once I clean it up a bit.

Using a UEFI variable for passing the intird device path to Linux does
not require any change in U-Boot and is compatible with the UEFI
implementations of existing hardware like the laptop on which I am
writing this email.

Best regards

Heinrich

>
>>
>> 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.
>>
>
> Thanks
> /Ilias
>
>>>
>>> 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)
>>>
>>