linux-hardening - Re: [PATCH v4 17/26] x86/boot: Reduce size of the DOS stub

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Date:   Sat, 11 Mar 2023 17:49:19 +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 17/26] x86/boot: Reduce size of the DOS stub

On 2023-03-10 17:59, Ard Biesheuvel wrote:
> On Thu, 15 Dec 2022 at 13:40, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> 
>> This is required to fit more sections in PE section tables,
>> since its size is restricted by zero page located at specific offset
>> after the PE header.
>> 
>> Tested-by: Mario Limonciello <mario.limonciello@....com>
>> Tested-by: Peter Jones <pjones@...hat.com>
>> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
> 
> I'd prefer to rip this out altogether.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/commit/?id=9510f6f04f579b9a3f54ad762c75ab2d905e37d8

Sounds great! Can I replace this patch with yours in v5?

> 
> (and refer to the other thread in linux-efi@)

Which thread exactly? The one about the removal of
real-mode code?

> 
>> ---
>>  arch/x86/boot/header.S | 14 ++++++--------
>>  1 file changed, 6 insertions(+), 8 deletions(-)
>> 
>> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
>> index 9338c68e7413..9fec80bc504b 100644
>> --- a/arch/x86/boot/header.S
>> +++ b/arch/x86/boot/header.S
>> @@ -59,17 +59,16 @@ start2:
>>         cld
>> 
>>         movw    $bugger_off_msg, %si
>> +       movw    $bugger_off_msg_size, %cx
>> 
>>  msg_loop:
>>         lodsb
>> -       andb    %al, %al
>> -       jz      bs_die
>>         movb    $0xe, %ah
>>         movw    $7, %bx
>>         int     $0x10
>> -       jmp     msg_loop
>> +       decw    %cx
>> +       jnz     msg_loop
>> 
>> -bs_die:
>>         # Allow the user to press a key, then reboot
>>         xorw    %ax, %ax
>>         int     $0x16
>> @@ -90,10 +89,9 @@ bs_die:
>> 
>>         .section ".bsdata", "a"
>>  bugger_off_msg:
>> -       .ascii  "Use a boot loader.\r\n"
>> -       .ascii  "\n"
>> -       .ascii  "Remove disk and press any key to reboot...\r\n"
>> -       .byte   0
>> +       .ascii  "Use a boot loader. "
>> +       .ascii  "Press a key to reboot"
>> +       .set    bugger_off_msg_size, . - bugger_off_msg
>> 
>>  #ifdef CONFIG_EFI_STUB
>>  pe_header:
>> --
>> 2.37.4
>>