linux-hardening - Re: [PATCH v4 01/26] x86/boot: Align vmlinuz sections on page size

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Date:   Sat, 11 Mar 2023 15:42:31 +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 01/26] x86/boot: Align vmlinuz sections on page size

On Sat, 11 Mar 2023 at 15:30, Evgeniy Baskov <baskov@...ras.ru> wrote:
>
> On 2023-03-10 17:43, Ard Biesheuvel wrote:
> > On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@...ras.ru> wrote:
> >>
> >> To protect sections on page table level each section
> >> needs to be aligned on page size (4KB).
> >>
> >> Set sections alignment in linker script.
> >>
> >> Tested-by: Mario Limonciello <mario.limonciello@....com>
> >> Tested-by: Peter Jones <pjones@...hat.com>
> >> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
> >> ---
> >>  arch/x86/boot/compressed/vmlinux.lds.S | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S
> >> b/arch/x86/boot/compressed/vmlinux.lds.S
> >> index 112b2375d021..6be90f1a1198 100644
> >> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> >> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> >> @@ -27,21 +27,27 @@ SECTIONS
> >>                 HEAD_TEXT
> >>                 _ehead = . ;
> >>         }
> >> +       . = ALIGN(PAGE_SIZE);
> >>         .rodata..compressed : {
> >> +               _compressed = .;
> >>                 *(.rodata..compressed)
> >
> > Can you just move this bit into the rodata section below?
>
> I don't think that easily possible, as the layout need
> to stay compatible with in-place extraction for non-UEFI boot.
> For that execution path the code in .head.text moves everything
> behind it to the end of the extraction buffer and extraction
> code overwrites compressed kernel blob progressively during
> extraction. And that is why we have effectively have two code
> sections...
>

A right - thanks for explaining that to me.

So in the end, I think it doesn't matter in any case if we just stick
to a single .text section with R-X attributes and a single .data
section with RW- attributes.


> >
> >> +               _ecompressed = .;
> >>         }
> >> +       . = ALIGN(PAGE_SIZE);
> >>         .text : {
> >
> > Please use
> >
> > .text : ALIGN(PAGE_SIZE) {
> >
> > which marks the section as being page aligned, rather than just being
> > placed on a 4k boundary.
>
> Will fix in v5.
>
> >
> >>                 _text = .;      /* Text */
> >>                 *(.text)
> >>                 *(.text.*)
> >>                 _etext = . ;
> >>         }
> >> +       . = ALIGN(PAGE_SIZE);
> >>         .rodata : {
> >>                 _rodata = . ;
> >>                 *(.rodata)       /* read-only data */
> >>                 *(.rodata.*)
> >>                 _erodata = . ;
> >>         }
> >> +       . = ALIGN(PAGE_SIZE);
> >>         .data : {
> >>                 _data = . ;
> >>                 *(.data)
> >> --
> >> 2.37.4
> >>