linux-kernel - Re: [PATCHv7 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <20250626134921.GK1613200@noisy.programming.kicks-ass.net>
Date: Thu, 26 Jun 2025 15:49:21 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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Subject: Re: [PATCHv7 03/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Wed, Jun 25, 2025 at 03:50:56PM +0300, Kirill A. Shutemov wrote:

> +/*
> + * The CLAC/STAC instructions toggle the enforcement of X86_FEATURE_SMAP and
> + * X86_FEATURE_LASS.
> + *
> + * SMAP enforcement is based on the _PAGE_BIT_USER bit in the page tables: the
> + * kernel is not allowed to touch pages with the bit set unless the AC bit is
> + * set.
> + *
> + * LASS enforcement is based on bit 63 of the virtual address. The kernel is
> + * not allowed to touch memory in the lower half of the virtual address space
> + * unless the AC bit is set.
> + *
> + * Note: a barrier is implicit in alternative().
> + */
> +
>  static __always_inline void clac(void)
>  {
> -	/* Note: a barrier is implicit in alternative() */
>  	alternative("", "clac", X86_FEATURE_SMAP);
>  }
>  
>  static __always_inline void stac(void)
>  {
> -	/* Note: a barrier is implicit in alternative() */
>  	alternative("", "stac", X86_FEATURE_SMAP);
>  }
>  
> +static __always_inline void lass_enable_enforcement(void)
> +{
> +	alternative("", "clac", X86_FEATURE_LASS);
> +}
> +
> +static __always_inline void lass_disable_enforcement(void)
> +{
> +	alternative("", "stac", X86_FEATURE_LASS);
> +}

Much hate for this naming. WTH was wrong with lass_{clac,stac}()?

We're not calling those other functions smap_{en,dis}able_enforcement()
either (and please don't take that as a suggestion, its terrible
naming).