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

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Message-ID: <47981665-0a97-7919-8eb0-e772e4035f39@intel.com>
Date:   Wed, 11 Jan 2023 16:13:17 -0800
From:   "Chen, Yian" <yian.chen@...el.com>
To:     Sohil Mehta <sohil.mehta@...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



On 1/10/2023 1:05 PM, Sohil Mehta wrote:
> 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.
> 
Sure, It makes sense to remove unnecessary sentence here.
>> 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.
> 
sure, I will remove this state too.

>> +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.
> 
Sure, I will rewrite this help message and explain the impact to legacy 
vsyscall as well.

>> +      if unsure, say Y.
>> +
> 

thanks,
Yian