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

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Message-ID: <CAKv+Gu9c-C2uXZ0fVatCM8ffXJZDgjBcDb_T0nsFecGqgrF11Q@mail.gmail.com>
Date:   Fri, 25 Nov 2016 12:43:52 +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 12:35, David Howells <dhowells@...hat.com> wrote:
> Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
>> 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.
>
> How does one get the version number?  Unfortunately, searching the document
> for 'version' doesn't help as every page has that in the footer:-/
>

There is a 'revision' field in the header ('hdr') of the EFI system
table, so something like

(sys_table_arg->hdr.revision >> 16) > 2 ||
((sys_table_arg->hdr.revision >> 16) == 2 &&
(sys_table_arg->hdr.revision & 0xffff) >= 6)

should do the trick I think


>> > +               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.
>
> Good point.
>
> David