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

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Message-ID: <CAKv+Gu_d5+Kvo7Tptnpg7kfjpqxhQ3a3nsQ0Ufsm6rBOFry+mQ@mail.gmail.com>
Date:   Fri, 25 Nov 2016 12:24:00 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     David Howells <dhowells@...hat.com>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>, keyrings@...r.kernel.org,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/7] efi: Get the secure boot status [ver #3]

On 25 November 2016 at 12:03, David Howells <dhowells@...hat.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@...hat.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@...senPartnership.com>
>     Signed-off-by: David Howells <dhowells@...hat.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.