[PATCH] efi_loader: Get rid of kaslr-seed

[Ard Biesheuvel]

On Thu, 16 Dec 2021 at 15:52, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> Right now we unconditionally pass a 'kaslr-seed' property to the kernel
> if the DTB we ended up in EFI includes the entry.  However the kernel
> EFI stub completely ignores it and only relies on EFI_RNG_PROTOCOL.
> So let's get rid of it unconditionally since it would mess up the
> (upcoming) DTB TPM measuring as well.
>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>

Acked-by: Ard Biesheuvel <ardb at kernel.org>

Note that the EFI stub itself does not consume the DTB /chosen entry
for its own randomness needs (i.e., the randomization of the physical
placement of the kernel, which also affects low order virtual
placement [bits 16-20]), and will blindly overwrite the seed with
whatever the EFI_RNG_PROTOCOL returns.


> ---
>  cmd/bootefi.c                 |  2 ++
>  include/efi_loader.h          |  2 ++
>  lib/efi_loader/efi_dt_fixup.c | 22 ++++++++++++++++++++++
>  3 files changed, 26 insertions(+)
>
> diff --git a/cmd/bootefi.c b/cmd/bootefi.c
> index d77d3b6e943d..25f9bfce9b84 100644
> --- a/cmd/bootefi.c
> +++ b/cmd/bootefi.c
> @@ -310,6 +310,8 @@ efi_status_t efi_install_fdt(void *fdt)
>         /* Create memory reservations as indicated by the device tree */
>         efi_carve_out_dt_rsv(fdt);
>
> +       efi_purge_kaslr_seed(fdt);
> +
>         /* Install device tree as UEFI table */
>         ret = efi_install_configuration_table(&efi_guid_fdt, fdt);
>         if (ret != EFI_SUCCESS) {
> diff --git a/include/efi_loader.h b/include/efi_loader.h
> index 9dd6c2033634..e560401ac54f 100644
> --- a/include/efi_loader.h
> +++ b/include/efi_loader.h
> @@ -519,6 +519,8 @@ efi_status_t EFIAPI efi_convert_pointer(efi_uintn_t debug_disposition,
>                                         void **address);
>  /* Carve out DT reserved memory ranges */
>  void efi_carve_out_dt_rsv(void *fdt);
> +/* Purge unused kaslr-seed */
> +void efi_purge_kaslr_seed(void *fdt);
>  /* Called by bootefi to make console interface available */
>  efi_status_t efi_console_register(void);
>  /* Called by bootefi to make all disk storage accessible as EFI objects */
> diff --git a/lib/efi_loader/efi_dt_fixup.c b/lib/efi_loader/efi_dt_fixup.c
> index b6fe5d2e5a34..02f7de73872e 100644
> --- a/lib/efi_loader/efi_dt_fixup.c
> +++ b/lib/efi_loader/efi_dt_fixup.c
> @@ -40,6 +40,28 @@ static void efi_reserve_memory(u64 addr, u64 size, bool nomap)
>                         addr, size);
>  }
>
> +/**
> + * efi_remove_kaslr_seed() - Removed unused kaslr-seed
> + *
> + * Kernel's EFI STUB only relies on EFI_RNG_PROTOCOL for randomization
> + * and completely ignores the kaslr-seed.  Weed it out from the DTB we
> + * hand over, which would mess up our DTB TPM measurements as well.
> + *
> + * @fdt: Pointer to device tree
> + */
> +void efi_purge_kaslr_seed(void *fdt)
> +{
> +       int nodeoff = fdt_path_offset(fdt, "/chosen");
> +       int err = 0;
> +
> +       if (nodeoff < 0)
> +               return;
> +
> +       err = fdt_delprop(fdt, nodeoff, "kaslr-seed");
> +       if (err < 0)
> +               log_err("Error deleting kaslr-seed\n");
> +}
> +
>  /**
>   * efi_carve_out_dt_rsv() - Carve out DT reserved memory ranges
>   *
> --
> 2.30.2
>