linux-hardening - Re: [PATCH v4 21/26] efi/x86: Explicitly set sections memory attributes

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Date:   Sun, 12 Mar 2023 15:10:04 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 21/26] efi/x86: Explicitly set sections memory
 attributes

On 2023-03-11 20:39, Ard Biesheuvel wrote:
> On Sat, 11 Mar 2023 at 16:09, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> 
>> On 2023-03-10 18:20, Ard Biesheuvel wrote:
>> > On Thu, 15 Dec 2022 at 13:42, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> >>
>> >> Explicitly change sections memory attributes in efi_pe_entry in case
>> >> of incorrect EFI implementations and to reduce access rights to
>> >> compressed kernel blob. By default it is set executable due to
>> >> restriction in maximum number of sections that can fit before zero
>> >> page.
>> >>
>> >> Tested-by: Peter Jones <pjones@...hat.com>
>> >> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
>> >
>> > I don't think we need this patch. Firmware that cares about W^X will
>> > map the PE image with R-X for text/rodata and RW- for data/bss, which
>> > is sufficient, and firmware that doesn't is a lost cause anyway.
>> 
>> This patch were here mainly here to make .rodata non-executable and 
>> for
>> the UEFI handover protocol, for which attributes are usually not 
>> getting
>> applied.
>> 
>> Since the UEFI handover protocol is deprecated, I'll exclude patches
>> from
>> v5 and maybe submit it separately modified to apply attributes only 
>> when
>> booting via this protocol.
>> 
> 
> I think the issue here is that loaders that use the UEFI handover
> protocol use their own implementations of LoadImage/StartImage as
> well, and some of those tend to do little more than copy the image
> into memory and jump to the EFI handover protocol entry point, without
> even accounting for the image size in memory or clearing the bss.
> 

AFAIK this patch does not break loaders that load PE image as a flat
binary, since it only operates on ELF sections that are mapped 1-to-1.
But that's just the note for a future.

> To be honest, even though I understand the reason these had to be
> implemented, I'm a bit reluctant to cater for the needs of such
> loaders, given that these are all downstream distro forks of GRUB
> (with shim) with varying levels of adherence to the PE/COFF spec.
> 
> I'm happy to revisit this later if others feel this is important, but
> for the moment, I'd prefer it if we could focus on making the x86
> image work better with compliant loaders, which is what this series is
> primarily about.

That's very reasonable. I'll put this patch aside for now then.