linux-kernel - Re: [PATCH 3/7] x86/cpu: Disable kernel LASS when patching kernel alternatives

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Message-ID: <9e0a8b20-cb76-b06d-67fb-f8942df5a2f7@intel.com>
Date:   Thu, 12 Jan 2023 10:36:39 -0800
From:   "Chen, Yian" <yian.chen@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        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 3/7] x86/cpu: Disable kernel LASS when patching kernel
 alternatives



On 1/11/2023 4:37 PM, Dave Hansen wrote:
> On 1/11/23 16:27, Chen, Yian wrote:
>>> It seems we are implicitly relying on the on stac() and clac()
>>> calls that are added for SMAP. Have you tried running with SMAP
>>> disabled i.e "clearcpuid=smap"?
>>>
>> Yes, I tested with clearcpuid=smap.
> It works by accident, then.
> 
> clearcpuid=smap means that the kernel should be running as if
> CPUID.(EAX=07H,ECX=0H):EBX.SMAP[bit 20]==0.  STAC/CLAC should #UD in
> that case.
> 
It could be something wrong in my Simics simulation environment.

> The only reason that it happens to work is that STAC/CLAC apparently
> actually continue to work even if CR4.SMAP==0.
> 
> I'm actually a _bit_ surprised by this, but I bet there's a good reason
> for it.
> 
> In any case, please just make LASS dependent on SMAP.  It's the right
> thing to do on several levels.
Sure, I will add the dependency.