linux-hardening - Re: [PATCH v5 00/27] x86_64: Improvements at compressed kernel stage

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Message-Id: <d575db7f-bad3-477e-a501-19d2d84527cd@app.fastmail.com>
Date:   Tue, 14 Mar 2023 14:23:39 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Evgeniy Baskov" <baskov@...ras.ru>,
        "Ard Biesheuvel" <ardb@...nel.org>
Cc:     "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Alexey Khoroshilov" <khoroshilov@...ras.ru>,
        "Peter Jones" <pjones@...hat.com>,
        "Gerd Hoffmann" <kraxel@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-efi@...r.kernel.org,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v5 00/27] x86_64: Improvements at compressed kernel stage

On Tue, Mar 14, 2023, at 3:13 AM, Evgeniy Baskov wrote:
> This patchset is aimed
> * to improve UEFI compatibility of compressed kernel code for x86_64
> * to setup proper memory access attributes for code and rodata sections
> * to implement W^X protection policy throughout the whole execution 
>   of compressed kernel for EFISTUB code path. 

The overall code quality seems okay, but I have some questions as to what this is for.  The early boot environment is not exposed to most sorts of attacks -- there's no userspace, there's no network, and there is not a whole lot of input that isn't implicitly completely trusted.

What parts of this series are actually needed to get these fancy new bootloaders to boot Linux?  And why?

>
> Kernel is made to be more compatible with PE image specification [3],
> allowing it to be successfully loaded by stricter PE loader
> implementations like the one from [2]. There is at least one
> known implementation that uses that loader in production [4].
> There are also ongoing efforts to upstream these changes.

Can you clarify 

>
> Also the patchset adds EFI_MEMORY_ATTTRIBUTE_PROTOCOL, included into
> EFI specification since version 2.10, as a better alternative to
> using DXE services for memory protection attributes manipulation,
> since it is defined by the UEFI specification itself and not UEFI PI
> specification. This protocol is not widely available so the code
> using DXE services is kept in place as a fallback in case specific
> implementation does not support the new protocol.
> One of EFI implementations that already support
> EFI_MEMORY_ATTTRIBUTE_PROTOCOL is Microsoft Project Mu [5].

Maybe make this a separate series?

> 
> Kernel image generation tool (tools/build.c) is refactored as a part
> of changes that makes PE image more compatible.
>   
> The patchset implements memory protection for compressed kernel
> code while executing both inside EFI boot services and outside of
> them. For EFISTUB code path W^X protection policy is maintained
> throughout the whole execution of compressed kernel. The latter
> is achieved by extracting the kernel directly from EFI environment
> and jumping to it's head immediately after exiting EFI boot services.
> As a side effect of this change one page table rebuild and a copy of
> the kernel image is removed.

I have no problem with this, but what's it needed for?

>
> Memory protection inside EFI environment is controlled by the
> CONFIG_DXE_MEM_ATTRIBUTES option, although with these patches this
> option also control the use EFI_MEMORY_ATTTRIBUTE_PROTOCOL and memory
> protection attributes of PE sections and not only DXE services as the
> name might suggest.
>

> [1] 
> https://lore.kernel.org/lkml/893da11995f93a7ea8f7485d17bf356a@ispras.ru/
> [2] https://github.com/acidanthera/audk/tree/secure_pe

Link is broken

> [3] 
> https://download.microsoft.com/download/9/c/5/9c5b2167-8017-4bae-9fde-d599bac8184a/pecoff_v83.docx

I skimmed this very briefly, and I have no idea what I'm supposed to look at.  This is the entire PE spec!