linux-hardening - Re: [PATCH 06/16] x86/boot: Setup memory protection for bzImage code

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Date:   Wed, 19 Oct 2022 07:57:57 +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 06/16] x86/boot: Setup memory protection for bzImage code

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 5273367283b7..889ca7176aa7 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -602,6 +603,28 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
>  	jmp	*%rax
>  SYM_FUNC_END(.Lrelocated)
>  
> +SYM_FUNC_START_LOCAL_NOALIGN(startup32_enable_nx_if_supported)
> +	pushq	%rbx
> +
> +	leaq	has_nx(%rip), %rcx
> +
> +	mov	$0x80000001, %eax
> +	cpuid
> +	btl	$20, %edx

btl $(X86_FEATURE_NX & 31), %edx

But also need to check for the availability of the extended leaf in the
first place.

> +	jnc	.Lnonx
> +
> +	movl	$1, (%rcx)

Your pointer has been clobbered with some feature flags.

movl $1, has_nx(%rip)

will work fine without needing the intermediary lea.

~Andrew