linux-hardening - Re: [PATCH v5 03/27] x86/boot: Set cr0 to known state in trampoline

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Date:   Wed, 5 Apr 2023 19:54:41 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Evgeniy Baskov <baskov@...ras.ru>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        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>,
        Gerd Hoffmann <kraxel@...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 v5 03/27] x86/boot: Set cr0 to known state in trampoline

On Tue, Mar 14, 2023 at 01:13:30PM +0300, Evgeniy Baskov 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>
> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
> ---
>  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 03c4328a88cb..01fa42d31648 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

This sets a lot more than WP. Why?

-- 
Regards/Gruss,
    Boris.

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