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

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Date:   Sat, 11 Mar 2023 18:31:15 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Evgeniy Baskov <baskov@...ras.ru>
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 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.

> Also, why .reloc is at offset 0x2000 and not just 0x1000, is there
> anything important I am missing? I understand that is cannot be 0
> and should be aligned on page size, but nothing else comes to my
> mind...
>

That was just arbitrary, because the raw allocations of reloc and
compat are also allocated towards the end. But I guess starting at
0x1000 for .reloc makes more sense so feel free to change that.