linux-hardening - Re: [PATCH v4 20/26] x86/build: Make generated PE more spec compliant

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Message-ID: <f069d71d9e1f3e7dc253a16386befa5d@ispras.ru>
Date:   Mon, 13 Mar 2023 12:11:09 +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 20/26] x86/build: Make generated PE more spec compliant

On 2023-03-12 16:09, Ard Biesheuvel wrote:
> On Sun, 12 Mar 2023 at 13:02, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> 
>> On 2023-03-11 20:31, Ard Biesheuvel wrote:
>> > On Sat, 11 Mar 2023 at 16:02, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> >>
>> >> On 2023-03-10 18:17, Ard Biesheuvel wrote:
>> >> > On Thu, 15 Dec 2022 at 13:42, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> >> >>
>> >> >> Currently kernel image is not fully compliant PE image, so it may
>> >> >> fail to boot with stricter implementations of UEFI PE loaders.
>> >> >>
>> >> >> Set minimal alignments and sizes specified by PE documentation [1]
>> >> >> referenced by UEFI specification [2]. Align PE header to 8 bytes.
>> >> >>
>> >> >> Generate PE sections dynamically. This simplifies code, since with
>> >> >> current implementation all of the sections needs to be defined in
>> >> >> header.S, where most section header fields do not hold valid values,
>> >> >> except for their names. Before the change, it also held flags,
>> >> >> but now flags depend on kernel configuration and it is simpler
>> >> >> to set them from build.c too.
>> >> >>
>> >> >> Setup sections protection. Since we cannot fit every needed section,
>> >> >> set a part of protection flags dynamically during initialization.
>> >> >> This step is omitted if CONFIG_EFI_DXE_MEM_ATTRIBUTES is not set.
>> >> >>
>> >> >> [1]
>> >> >> https://download.microsoft.com/download/9/c/5/9c5b2167-8017-4bae-9fde-d599bac8184a/pecoff_v83.docx
>> >> >> [2]
>> >> >> https://uefi.org/sites/default/files/resources/UEFI_Spec_2_9_2021_03_18.pdf
>> >> >>
>> >> >> Tested-by: Peter Jones <pjones@...hat.com>
>> >> >> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
>> >> >
>> >> > I would prefer it if we didn't rewrite the build tool this way.
>> >> >
>> >> > Having the sections in header.S in the order they appear in the binary
>> >> > is rather useful, and I don't think we should manipulate the section
>> >> > flags based on whether CONFIG_DXE_MEM_ATTRIBUTES is set. I also don't
>> >> > think we need more than .text / .,data (as discussed in the other
>> >> > thread on linux-efi@)
>> >> >
>> >> > Furthermore, I had a look at the audk PE loader [0], and I think it is
>> >> > being overly pedantic.
>> >> >
>> >> > The PE/COFF spec does not require that all sections are virtually
>> >> > contiguous, and it does not require that the file content is
>> >> > completely covered by either the header or by a section.
>> >> >
>> >> > So what I would prefer to do is the following:
>> >> >
>> >> > Sections:
>> >> > Idx Name          Size     VMA              Type
>> >> >   0 .reloc        00000200 0000000000002000 DATA
>> >> >   1 .compat       00000200 0000000000003000 DATA
>> >> >   2 .text         00bee000 0000000000004000 TEXT
>> >> >   3 .data         00002200 0000000000bf2000 DATA
>> >> >
>> >> > using 4k section alignment and 512 byte file alignment, and a header
>> >> > size of 0x200 as before (This requires my patch that allows the setup
>> >> > header to remain unmapped when running the stub [1])
>> >> >
>> >> > The reloc and compat payloads are placed at the end of the setup data
>> >> > as before, but increased in size to 512 bytes each, and then mapped
>> >> > non-1:1 into the RVA space.
>> >> >
>> >> > This works happily with both the existing PE loader as well as the
>> >> > audk one, but with the pedantic flags disabled.
>> >> >
>> >>
>> >> This makes sense. I'll change this patch to use this layout and
>> >> to keep sections in headers.S before sending v5. (and I guess I'll
>> >> make the compressed kernel a part of .text). I have a few questions
>> >> though:
>> >>
>> >> This layout assumes having the local copy of the bootparams as
>> >> in your RFC patches, right?
>> >>
>> >
>> > Indeed. Otherwise, the setup header may not have been copied to memory
>> > by the loader.
>> >
>> >> Can I keep the .rodata -- 5th section fits in the section table
>> >> without much work?
>> >>
>> >
>> > You could, but at least the current PE/COFF loader in EDK2 will map it
>> > read/write, as it only distinguishes between executable sections and
>> > non-executable sections.
>> >
>> 
>> At least it will slightly improve security for some implementations
>> (e.g. audk, while being overly strict support RO sections)
>> 
> 
> Yeah, but more common loaders will put the compressed data in a
> writable region. I'd prefer to have a simple and common baseline where
> we always just use R-X for all text and rodata, and RW- for everything
> else.

Hmm... I'll remove the .rodata for now then. If anything changes I can
always submit it as a separate patch later anyways.