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

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Message-ID: <CAMj1kXGK=z_taR2__RrCEDTE_F=+dbiXb_9HTCyG9R7iOp+yoA@mail.gmail.com>
Date:   Sun, 12 Mar 2023 14:09:28 +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 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.