linux-kernel - Re: [PATCH 2/7] x86: Add CONFIG option X86

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Message-ID: <130a0ad6-31fd-dfe4-7b9a-cabb2e726349@intel.com>
Date:   Tue, 10 Jan 2023 13:05:33 -0800
From:   Sohil Mehta <sohil.mehta@...el.com>
To:     Yian Chen <yian.chen@...el.com>, <linux-kernel@...r.kernel.org>,
        <x86@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        "Tony Luck" <tony.luck@...el.com>, Paul Lai <paul.c.lai@...el.com>
Subject: Re: [PATCH 2/7] x86: Add CONFIG option X86_LASS

The subject can be better stated as:

x86/Kconfig: Add config option for LASS

On 1/9/2023 9:51 PM, Yian Chen wrote:
> LASS is an Intel x86-64 only feature. 

> Add CONFIG
> option X86_LASS and flag DISABLE_LASS to choose
> opt-in/out the feature from kernel binary.
> The second sentence is unnecessary.

> CONFIG_X86_LASS is enabled by default because it
> is a security feature which should have little
> to no overhead or side effects. 

Doesn't it have a side effect that it modifies default vsyscall behavior?

I am guessing the impact of the vsyscall change would be minimal. 
However, should LASS be disabled by default at least initially to 
minimize the impact on userspace?

A follow-up patch can then enable this by default once the overall 
impact is clearly known.

> If any issues are
> found with specific use cases, the CONFIG option
> makes it easy to disable.
>This sentence is unnecessary.

>   
> +config X86_LASS
> +	def_bool y
> +	prompt "Linear Address Space Separation"
> +	depends on X86_64 && CPU_SUP_INTEL
> +	help
> +	  Linear Address Space Separation (LASS) is a processor
> +	  feature that mitigates address space layout probes.
> +

Let's try to be consistent about what LASS is expected to do. This 
definition is very different from the one in patch 1/7.

Also, we should include some information here on how enabling the LASS 
config option can impact vsyscall behavior.

> +	  if unsure, say Y.
> +