linux-hardening - Re: [PATCH 03/16] x86/boot: Set cr0 to known state in trampoline

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Date:   Wed, 19 Oct 2022 07:44:32 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Evgeniy Baskov <baskov@...ras.ru>, 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" <lvc-project@...uxtesting.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>
Subject: Re: [PATCH 03/16] x86/boot: Set cr0 to known state in trampoline

On 06/09/2022 11:41, Evgeniy Baskov wrote:
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index d33f060900d2..5273367283b7 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -619,9 +619,8 @@ SYM_CODE_START(trampoline_32bit_src)
>  	/* Set up new stack */
>  	leal	TRAMPOLINE_32BIT_STACK_END(%ecx), %esp
>  
> -	/* Disable paging */
> -	movl	%cr0, %eax
> -	btrl	$X86_CR0_PG_BIT, %eax
> +	/* Disable paging and setup CR0 */
> +	movl	$(CR0_STATE & ~X86_CR0_PG), %eax

Why here?  WP is ignored when PG is disabled.

~Andrew