linux-hardening - Re: [PATCH v4 03/26] x86/boot: Set cr0 to known state in trampoline

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Date:   Fri, 10 Mar 2023 15:48:09 +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 03/26] x86/boot: Set cr0 to known state in trampoline

On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@...ras.ru> wrote:
>
> Ensure WP bit to be set to prevent boot code from writing to
> non-writable memory pages.
>
> Tested-by: Mario Limonciello <mario.limonciello@....com>
> Tested-by: Peter Jones <pjones@...hat.com>
> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>

Acked-by: Ard Biesheuvel <ardb@...nel.org>

> ---
>  arch/x86/boot/compressed/head_64.S | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index a75712991df3..9f2e8f50fc71 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -660,9 +660,8 @@ SYM_CODE_START(trampoline_32bit_src)
>         pushl   $__KERNEL_CS
>         pushl   %eax
>
> -       /* Enable paging again. */
> -       movl    %cr0, %eax
> -       btsl    $X86_CR0_PG_BIT, %eax
> +       /* Enable paging and set CR0 to known state (this also sets WP flag) */
> +       movl    $CR0_STATE, %eax
>         movl    %eax, %cr0
>
>         lret
> --
> 2.37.4
>