linux-hardening - Re: [PATCH 01/16] x86/boot: Align vmlinuz sections on page size

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Date:   Thu, 20 Oct 2022 14:13:04 +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>,
        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 01/16] x86/boot: Align vmlinuz sections on page size

On 2022-10-19 10:01, Ard Biesheuvel wrote:
> On Tue, 6 Sept 2022 at 12:41, 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.
>> 
>> 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 = .;
> 
> Why are you adding these?

It is used for address compressed kernel blob during memory protection 
setup.
Although it can be addressed via different symbols, I though that 
addressing
sections data in a common way (through linker generated symbols) would 
be better.
I can remove or mention the change in commit message (for now I will do 
the latter).

> 
>>                 *(.rodata..compressed)
>> +               _ecompressed = .;
>>         }
>> +       . = ALIGN(PAGE_SIZE);
> 
> On other EFI architectures, we only distinguish between R-X and RW-
> regions, and alignment between .rodata and .text is unnecessary. Do we
> really need to deviate from that here?

I though that leaving a huge compressed kernel blob executable is
undesirable, so I decided to split it out. I can make it either RW- or 
R-X
if it would be more acceptable.

> 
> 
>>         .text : {
>>                 _text = .;      /* Text */
>>                 *(.text)
>>                 *(.text.*)
>>                 _etext = . ;
>>         }
>> +       . = ALIGN(PAGE_SIZE);
>>         .rodata : {
>>                 _rodata = . ;
>>                 *(.rodata)       /* read-only data */
>>                 *(.rodata.*)
>>                 _erodata = . ;
>>         }
>> +       . = ALIGN(PAGE_SIZE);
>>         .data : {
>>                 _data = . ;
>>                 *(.data)
>> --
>> 2.35.1
>>