Re: [PATCH v2 2/3] efi/libstub: Add support for loading the initrd from a device path

[Ard Biesheuvel]

On Mon, 17 Feb 2020 at 10:15, Laszlo Ersek <lersek@xxxxxxxxxx> wrote:
>
> On 02/16/20 15:11, 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
> > 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.
> >
> > 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.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > ---
> >  drivers/firmware/efi/libstub/arm-stub.c        | 15 +++-
> >  drivers/firmware/efi/libstub/efi-stub-helper.c | 82 ++++++++++++++++++++
> >  drivers/firmware/efi/libstub/efistub.h         |  4 +
> >  drivers/firmware/efi/libstub/x86-stub.c        | 23 ++++++
> >  include/linux/efi.h                            |  1 +
> >  5 files changed, 122 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
> > index 2edc673ea06c..4bae620b95b9 100644
> > --- a/drivers/firmware/efi/libstub/arm-stub.c
> > +++ b/drivers/firmware/efi/libstub/arm-stub.c
> > @@ -160,6 +160,7 @@ 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;
> >
> > @@ -258,10 +259,18 @@ 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, &initrd_addr, &initrd_size, ULONG_MAX,
> > -                              efi_get_max_initrd_addr(dram_base, *image_addr));
> > +     max_addr = efi_get_max_initrd_addr(dram_base, *image_addr);
> > +     status = efi_load_initrd_dev_path(&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, &initrd_addr, &initrd_size,
> > +                                      ULONG_MAX, max_addr);
> > +             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 49008ac88b63..e37afe2c752e 100644
> > --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> > +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> > @@ -299,3 +299,85 @@ void efi_char16_printk(efi_char16_t *str)
> >       efi_call_proto(efi_table_attr(efi_system_table(), con_out),
> >                      output_string, str);
> >  }
> > +
> > +/*
> > + * The LINUX_EFI_INITRD_MEDIA_GUID vendor media device path below provides a way
> > + * for the firmware or bootloader to expose the initrd data directly to the stub
> > + * via the trivial LoadFile2 protocol, which is defined in the UEFI spec, and is
> > + * very easy to implement. It is a simple Linux initrd specific conduit between
> > + * kernel and firmware, allowing us to put the EFI stub (being part of the
> > + * kernel) in charge of where and when to load the initrd, while leaving it up
> > + * to the firmware to decide whether it needs to expose its filesystem hierarchy
> > + * via EFI protocols.
> > + */
> > +static const struct {
> > +     struct efi_vendor_dev_path      vendor;
> > +     struct efi_generic_dev_path     end;
> > +} __packed initrd_dev_path = {
> > +     {
> > +             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_load_initrd_dev_path - load the initrd from the Linux initrd device path
> > + * @load_addr:       pointer to store the address where the initrd was loaded
> > + * @load_size:       pointer to store the size of the loaded initrd
> > + * @max:     upper limit for the initrd memory allocation
> > + * @return:  %EFI_SUCCESS if the initrd was loaded successfully, in which case
> > + *           @load_addr and @load_size are assigned accordingly
> > + *           %EFI_NOT_FOUND if no LoadFile2 protocol exists on the initrd
> > + *           device path
> > + *           %EFI_LOAD_ERROR in all other cases
>
> [*]
>
> > + */
> > +efi_status_t efi_load_initrd_dev_path(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;
>
> Doesn't return EFI_LOAD_ERROR.
>
> > +
> > +     dp = (efi_device_path_protocol_t *)&initrd_dev_path;
> > +     status = efi_bs_call(locate_device_path, &lf2_proto_guid, &dp, &handle);
> > +     if (status != EFI_SUCCESS)
> > +             return status;
>
> Seems safe (the only plausible error could be EFI_NOT_FOUND).
>
> > +
> > +     status = efi_bs_call(handle_protocol, handle, &lf2_proto_guid,
> > +                          (void **)&lf2);
> > +     if (status != EFI_SUCCESS)
> > +             return status;
>
> Interesting case; this should never fail... but note, if it does, it
> returns EFI_UNSUPPORTED, not EFI_NOT_FOUND (if the protocol is missing
> from the handle).
>
> > +
> > +     status = efi_call_proto(lf2, load_file, dp, 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;
>
> Not sure about the efi_allocate_pages() wrapper (?); the UEFI service
> could return EFI_OUT_OF_RESOURCES.
>

Hmm, guess I was a bit sloppy with the return codes. The important
thing is that EFI_NOT_FOUND is only returned in the one specifically
defined case.

> Looks OK to me otherwise.
>

Thanks.

> (... I'm a bit doubtful of passing and End node to LF2 rather than a
> filepath node with "" for pathname, but it's an LF2 on our own vendor
> path, so I guess we dictate what we accept.)
>

It seems to me that the whole point of advancing the
pointer-to-pointer-to-device path protocol past the matched part of
the device path is to make it straight-forward for the caller to pass
the remainder into the protocol implementation. So if the matched part
fully defines the target, pointing to an end node is the only correct
thing to do imo, as the empty file did not appear in the device path
that was used to locate the protocol.


> > +
> > +     status = efi_call_proto(lf2, load_file, dp, false, &initrd_size,
> > +                             (void *)initrd_addr);
> > +     if (status != EFI_SUCCESS) {
> > +             efi_free(initrd_size, initrd_addr);
> > +             return EFI_LOAD_ERROR;
> > +     }
> > +
> > +     *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 34fe3fad042f..b58cb2c4474e 100644
> > --- a/drivers/firmware/efi/libstub/efistub.h
> > +++ b/drivers/firmware/efi/libstub/efistub.h
> > @@ -640,4 +640,8 @@ efi_status_t efi_load_initrd(efi_loaded_image_t *image,
> >                            unsigned long soft_limit,
> >                            unsigned long hard_limit);
> >
> > +efi_status_t efi_load_initrd_dev_path(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 681b620d8d40..16bf4ed21f1f 100644
> > --- a/drivers/firmware/efi/libstub/x86-stub.c
> > +++ b/drivers/firmware/efi/libstub/x86-stub.c
> > @@ -699,9 +699,14 @@ struct boot_params *efi_main(efi_handle_t handle,
> >  {
> >       unsigned long bzimage_addr = (unsigned long)startup_32;
> >       struct setup_header *hdr = &boot_params->hdr;
> > +     unsigned long max_addr = hdr->initrd_addr_max;
> > +     unsigned long initrd_addr, initrd_size;
> >       efi_status_t status;
> >       unsigned long cmdline_paddr;
> >
> > +     if (hdr->xloadflags & XLF_CAN_BE_LOADED_ABOVE_4G)
> > +             max_addr = ULONG_MAX;
> > +
> >       sys_table = sys_table_arg;
> >
> >       /* Check if we were booted by the EFI firmware */
> > @@ -734,6 +739,24 @@ struct boot_params *efi_main(efi_handle_t handle,
> >                        ((u64)boot_params->ext_cmd_line_ptr << 32));
> >       efi_parse_options((char *)cmdline_paddr);
> >
> > +     /*
> > +      * At this point, an initrd may already have been loaded, either by
> > +      * the bootloader and passed via bootparams, or loaded from a initrd=
> > +      * command line option by efi_pe_entry() above. In either case, we
> > +      * permit an initrd loaded from the LINUX_EFI_INITRD_MEDIA_GUID device
> > +      * path to supersede it.
> > +      */
> > +     status = efi_load_initrd_dev_path(&initrd_addr, &initrd_size, max_addr);
> > +     if (status == EFI_SUCCESS) {
> > +             hdr->ramdisk_image              = (u32)initrd_addr;
> > +             hdr->ramdisk_size               = (u32)initrd_size;
> > +             boot_params->ext_ramdisk_image  = (u64)initrd_addr >> 32;
> > +             boot_params->ext_ramdisk_size   = (u64)initrd_size >> 32;
> > +     } else if (status != EFI_NOT_FOUND) {
> > +             efi_printk("efi_load_initrd_dev_path() 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 0976e57b4caa..1bf482daa22d 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)
> >
>