On 25 November 2016 at 12:03, David Howells <dhowells at redhat.com> wrote:
> How about the attached additional patch? Should I be checking the UEFI
> version number if such is available?
>
Yes. In pre-2.6, DeployedMode is not a reserved name, and so it may be
possible for someone to slip in a DeployedMode=0 on a secure boot
enabled system to trick the kernel into thinking lockdown should be
disabled.
> David
> ---
> commit 981110f45ba73798875af7639d0328dc2d6f9919
> Author: David Howells <dhowells at redhat.com>
> 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
> to engage lockdown mode.
>
> Reported-by: James Bottomley <James.Bottomley at HansenPartnership.com>
> Signed-off-by: David Howells <dhowells at redhat.com>
>
> diff --git a/drivers/firmware/efi/libstub/secureboot.c b/drivers/firmware/efi/libstub/secureboot.c
> index ca643eba5a4b..4c3bddef4fb3 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 +65,16 @@ int efi_get_secureboot(efi_system_table_t *sys_table_arg)
> if (val == 1)
> return 0;
>
> + status = get_efi_var(efi_DeployedMode_name, &efi_variable_guid,
> + NULL, &size, &val);
> + if (status != EFI_NOT_FOUND) {
> + if (status != EFI_SUCCESS)
> + goto out_efi_err;
> +
> + if (val == 1)
> + return 0;
I think the logic is the wrong way around here. Secure Boot is enabled
if SecureBoot=1 and SetupMode=0, unless DeployedMode=0. So you should
return 0 here if val == 0, but only when running on 2.6 or later.
--
Ard.