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.
>
> 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.
> >
> > 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 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@xxxxxxxxxx>
> > ---
> > 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)
> >
>
