linux-kernel - Re: [RFC PATCH v2] x86/boot: add .sbat section to the bzImage

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Message-ID: <186450BA-9C7F-41C7-9F97-BA1277AEC9FD@oracle.com>
Date:   Thu, 20 Jul 2023 18:10:36 +0000
From:   Eric Snowberg <eric.snowberg@...cle.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
CC:     Ard Biesheuvel <ardb@...nel.org>,
        Daniel P. Berrangé <berrange@...hat.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "bluca@...ian.org" <bluca@...ian.org>,
        "lennart@...ttering.net" <lennart@...ttering.net>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        Jarkko Sakkinen <jarkko@...nel.org>
Subject: Re: [RFC PATCH v2] x86/boot: add .sbat section to the bzImage



> On Jul 20, 2023, at 11:07 AM, James Bottomley <James.Bottomley@...senPartnership.com> wrote:
> 
> On Thu, 2023-07-20 at 16:46 +0000, Eric Snowberg wrote:
>> If a distro adds a SBAT section to either their UKI, or if kernel
>> SBAT enforcement is turned on from GRUB2 by default, there is one
>> piece missing that would need  to be handled by the mainline kernel
>> which is SBAT enforcement for kexec. This  would mean the revocations
>> SBAT protect against would need to be referenced  before doing the
>> signature validation in kexec. If this is not added, any distro that 
>> allows kexec really doesn’t have a SBAT protected kernel.
> 
> Um, actually, this is actually one of the misunderstandings of the
> whole thread: sbat is a revocation mechanism for protecting EFI boot
> security.  It's design is to prevent malicious actors exploiting buggy
> code to get into the EFI boot system before ExitBootServices is called
> and nothing more.  The kernel's intrusion into EFI boot security is
> tiny: it's basically the EFI stub up to ExitBootServices, so even if
> the kernel were to have an sbat number it would obviously be under the
> control of the maintainers of only that code (i.e. Ard) and it would
> only rev if we actually found a usable exploit in the efi stub.
> 
> As far as kexec is concerned, ExitBootServices is long gone and nothing
> a future kexec'd kernel can do can alter that, so there's no EFI
> security benefit to making kexec sbat aware, and thus it seems there's
> no need to do anything about it for kexec.  Now if we're interested in
> sbat as a more general revocation mechanism, that might change, but I
> think sbat is too tightly designed for the problems of EFI variables to
> be more generally useful.

If the line of protection SBAT provides ends at EBS then I agree, kexec 
support would not be needed.  While reading the SBAT spec, I got the 
impression the revocation mechanism it provides would go beyond the 
EBS line.  I guess that needs to be clarified.