[PATCH v3] efi_loader: Get rid of kaslr-seed if EFI_RNG_PROTOCOL is installed

[Mark Kettenis]

> From: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> Date: Mon,  3 Jan 2022 14:07:37 +0200
> 
> U-Boot, in some occasions, injects a 'kaslr-seed' property on the /chosen
> node.  That would be problematic in case we want to measure the DTB we
> install in the config table, since it would change across reboots.
> 
> The Linux kernel EFI-stub completely ignores it and only relies on
> EFI_RNG_PROTOCOL for it's own randomness needs (i.e the randomization
> of the physical placement of the kernel).  In fact it (blindly) overwrites
> the existing seed if the protocol is installed.  However it still uses it
> for randomizing it's virtual placement.
> So let's get rid of it in the presence of the RNG protocol.
> 
> It's worth noting that TPMs also provide an RNG.  So if we tweak our
> EFI_RNG_PROTOCOL slightly and install the protocol when a TPM device
> is present the 'kaslr-seed' property will always be removed, allowing
> us to reliably measure our DTB.
> 
> Acked-by: Ard Biesheuvel <ardb at kernel.org>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
> changes since v2:
> - Mark proposed a better commit message description
> changes since v1:
> - Only removing the property if EFI_RNG_PROTOCOL is installed, since some
>   OS'es rely on kaslr-seed
> - Only display an error message if the kaslr-seed entry was found but not
>   removed
>  cmd/bootefi.c                 |  2 ++
>  include/efi_loader.h          |  2 ++
>  lib/efi_loader/efi_dt_fixup.c | 33 +++++++++++++++++++++++++++++++++
>  3 files changed, 37 insertions(+)

Reviewed-by: Mark Kettenis <kettenis at openbsd.org>

> diff --git a/cmd/bootefi.c b/cmd/bootefi.c
> index d77d3b6e943d..57f13ce701ec 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_try_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..1fe003db69e0 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_try_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..d3923e5dba1b 100644
> --- a/lib/efi_loader/efi_dt_fixup.c
> +++ b/lib/efi_loader/efi_dt_fixup.c
> @@ -8,6 +8,7 @@
>  #include <common.h>
>  #include <efi_dt_fixup.h>
>  #include <efi_loader.h>
> +#include <efi_rng.h>
>  #include <fdtdec.h>
>  #include <mapmem.h>
>  
> @@ -40,6 +41,38 @@ static void efi_reserve_memory(u64 addr, u64 size, bool nomap)
>  			addr, size);
>  }
>  
> +/**
> + * efi_try_purge_kaslr_seed() - Remove unused kaslr-seed
> + *
> + * Kernel's EFI STUB only relies on EFI_RNG_PROTOCOL for randomization
> + * and completely ignores the kaslr-seed for its own randomness needs
> + * (i.e the randomization of the physical placement of the kernel).
> + * 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_try_purge_kaslr_seed(void *fdt)
> +{
> +	const efi_guid_t efi_guid_rng_protocol = EFI_RNG_PROTOCOL_GUID;
> +	struct efi_handler *handler;
> +	efi_status_t ret;
> +	int nodeoff = 0;
> +	int err = 0;
> +
> +	ret = efi_search_protocol(efi_root, &efi_guid_rng_protocol, &handler);
> +	if (ret != EFI_SUCCESS)
> +		return;
> +
> +	nodeoff = fdt_path_offset(fdt, "/chosen");
> +	if (nodeoff < 0)
> +		return;
> +
> +	err = fdt_delprop(fdt, nodeoff, "kaslr-seed");
> +	if (err < 0 && err != -FDT_ERR_NOTFOUND)
> +		log_err("Error deleting kaslr-seed\n");
> +}
> +
>  /**
>   * efi_carve_out_dt_rsv() - Carve out DT reserved memory ranges
>   *
> -- 
> 2.30.2
> 
>