linux-hardening - Re: [PATCH v4 15/26] efi/x86: Support extracting kernel from libstub

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Date:   Thu, 09 Mar 2023 20:05:53 +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 15/26] efi/x86: Support extracting kernel from libstub

On 2023-03-09 19:00, Ard Biesheuvel wrote:
> On Thu, 15 Dec 2022 at 13:40, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> 
>> Doing it that way allows setting up stricter memory attributes,
>> simplifies boot code path and removes potential relocation
>> of kernel image.
>> 
>> Wire up required interfaces and minimally initialize zero page
>> fields needed for it to function correctly.
>> 
>> Tested-by: Peter Jones <pjones@...hat.com>
>> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
>> ---
>>  arch/x86/boot/compressed/head_32.S            |  50 ++++-
>>  arch/x86/boot/compressed/head_64.S            |  58 ++++-
>>  drivers/firmware/efi/Kconfig                  |   2 +
>>  drivers/firmware/efi/libstub/Makefile         |   2 +-
>>  .../firmware/efi/libstub/x86-extract-direct.c | 208 
>> ++++++++++++++++++
>>  drivers/firmware/efi/libstub/x86-stub.c       | 119 +---------
>>  drivers/firmware/efi/libstub/x86-stub.h       |  14 ++
>>  7 files changed, 338 insertions(+), 115 deletions(-)
>>  create mode 100644 drivers/firmware/efi/libstub/x86-extract-direct.c
>>  create mode 100644 drivers/firmware/efi/libstub/x86-stub.h
>> 
>> diff --git a/arch/x86/boot/compressed/head_32.S 
>> b/arch/x86/boot/compressed/head_32.S
>> index ead6007df1e5..0be75e5072ae 100644
>> --- a/arch/x86/boot/compressed/head_32.S
>> +++ b/arch/x86/boot/compressed/head_32.S
>> @@ -152,11 +152,57 @@ SYM_FUNC_END(startup_32)
>> 
>>  #ifdef CONFIG_EFI_STUB
>>  SYM_FUNC_START(efi32_stub_entry)
>> +/*
>> + * Calculate the delta between where we were compiled to run
>> + * at and where we were actually loaded at.  This can only be done
>> + * with a short local call on x86.  Nothing  else will tell us what
>> + * address we are running at.  The reserved chunk of the real-mode
>> + * data at 0x1e4 (defined as a scratch field) are used as the stack
>> + * for this calculation. Only 4 bytes are needed.
>> + */
> 
> Please drop this comment

Will do.

> 
>> +       call    1f
>> +1:     popl    %ebx
>> +       addl    $_GLOBAL_OFFSET_TABLE_+(.-1b), %ebx
> 
> Please drop this and ...
> 
>> +
>> +       /* Clear BSS */
>> +       xorl    %eax, %eax
>> +       leal    _bss@...OFF(%ebx), %edi
>> +       leal    _ebss@...OFF(%ebx), %ecx
> 
> just use (_bss - 1b) here (etc)

I was trying to be consistent with the code below, but it will
indeed be better to do this like that. I guess, this will be
fine to stop putting GOT address to the %ebx, since the extraction
code does not use calls via PLT?

> 
>> +       subl    %edi, %ecx
>> +       shrl    $2, %ecx
>> +       rep     stosl
>> +
>>         add     $0x4, %esp
>>         movl    8(%esp), %esi   /* save boot_params pointer */
>> +       movl    %edx, %edi      /* save GOT address */
> 
> What does this do?

Hmm... It seems to be a remnant of the previous implementation
that I forgot to remove. I will remove that in the v5.

> 
>>         call    efi_main
>> -       /* efi_main returns the possibly relocated address of 
>> startup_32 */
>> -       jmp     *%eax
>> +       movl    %eax, %ecx
>> +
>> +       /*
>> +        * efi_main returns the possibly
>> +        * relocated address of extracted kernel entry point.
>> +        */
>> +
>> +       cli
>> +
>> +       /* Load new GDT */
>> +       leal    gdt@...OFF(%ebx), %eax
>> +       movl    %eax, 2(%eax)
>> +       lgdt    (%eax)
>> +
>> +       /* Load segment registers with our descriptors */
>> +       movl    $__BOOT_DS, %eax
>> +       movl    %eax, %ds
>> +       movl    %eax, %es
>> +       movl    %eax, %fs
>> +       movl    %eax, %gs
>> +       movl    %eax, %ss
>> +
>> +       /* Zero EFLAGS */
>> +       pushl   $0
>> +       popfl
>> +
>> +       jmp     *%ecx
>>  SYM_FUNC_END(efi32_stub_entry)
>>  SYM_FUNC_ALIAS(efi_stub_entry, efi32_stub_entry)
>>  #endif
> ...

Thanks,
Evgeniy Baskov