Re: [PATCH 5/7] efi: Get the secure boot status [ver #3]

[Ard Biesheuvel]

On 25 November 2016 at 13:00, David Howells <dhowells@xxxxxxxxxx> wrote:
> Okay, how about the attached?
>
> Can these variables every be anything other than 1 or 0?  E.g. should the
> check on SetupMode be that it isn't 0 rather than it is 1?
>

The firmware will ensure that these variables only ever assume the
documented values.

> David
> ---
> commit 6d4bb08e376045e27706c2740c0fdff0a6ec43f7
> Author: David Howells <dhowells@xxxxxxxxxx>
> Date:   Fri Nov 25 11:52:05 2016 +0000
>
>     efi: Handle secure boot from UEFI-2.6
>
>     UEFI-2.6 adds a new variable, DeployedMode.  If it exists, this must be 1
>     if we're to engage lockdown mode.
>
>     Reported-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
>     Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
>
> diff --git a/drivers/firmware/efi/libstub/secureboot.c b/drivers/firmware/efi/libstub/secureboot.c
> index ca643eba5a4b..157782d1c552 100644
> --- a/drivers/firmware/efi/libstub/secureboot.c
> +++ b/drivers/firmware/efi/libstub/secureboot.c
> @@ -22,6 +22,9 @@ static const efi_char16_t const efi_SecureBoot_name[] = {
>  static const efi_char16_t const efi_SetupMode_name[] = {
>         'S', 'e', 't', 'u', 'p', 'M', 'o', 'd', 'e', 0
>  };
> +static const efi_char16_t const efi_DeployedMode_name[] = {
> +       'D', 'e', 'p', 'l', 'o', 'y', 'e', 'd', 'M', 'o', 'd', 'e', 0
> +};
>
>  /* SHIM variables */
>  static const efi_guid_t shim_guid = EFI_SHIM_LOCK_GUID;
> @@ -62,6 +65,17 @@ int efi_get_secureboot(efi_system_table_t *sys_table_arg)
>         if (val == 1)
>                 return 0;
>
> +       /* UEFI-2.6 requires DeployedMode to be 1. */
> +       if (sys_table_arg->hdr.revision == EFI_2_60_SYSTEM_TABLE_REVISION) {

>=

> +               status = get_efi_var(efi_DeployedMode_name, &efi_variable_guid,
> +                                    NULL, &size, &val);
> +               if (status != EFI_SUCCESS)
> +                       goto out_efi_err;
> +
> +               if (val != 1)
> +                       return 0;

val == 0 is better imo, since that will prevent unexpected values from
being interpreted as 'secure boot disabled'

> +       }
> +
>         /* See if a user has put shim into insecure mode.  If so, and if the
>          * variable doesn't have the runtime attribute set, we might as well
>          * honor that.
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 333d31bf55bf..563abb37f03f 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -645,6 +645,10 @@ typedef struct {
>
>  #define EFI_SYSTEM_TABLE_SIGNATURE ((u64)0x5453595320494249ULL)
>
> +#define EFI_2_60_SYSTEM_TABLE_REVISION  ((2 << 16) | (60))
> +#define EFI_2_50_SYSTEM_TABLE_REVISION  ((2 << 16) | (50))
> +#define EFI_2_40_SYSTEM_TABLE_REVISION  ((2 << 16) | (40))
> +#define EFI_2_31_SYSTEM_TABLE_REVISION  ((2 << 16) | (31))
>  #define EFI_2_30_SYSTEM_TABLE_REVISION  ((2 << 16) | (30))
>  #define EFI_2_20_SYSTEM_TABLE_REVISION  ((2 << 16) | (20))
>  #define EFI_2_10_SYSTEM_TABLE_REVISION  ((2 << 16) | (10))