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

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Date:   Wed, 11 Jan 2023 16:27:49 -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 3/7] x86/cpu: Disable kernel LASS when patching kernel
 alternatives



On 1/10/2023 2:41 PM, Sohil Mehta wrote:
> On 1/9/2023 9:52 PM, Yian Chen wrote:
> 
>> LASS stops access to a lower half address in kernel,
>> and this can be deactivated if AC bit in EFLAGS
>> register is set. Hence use stac and clac instructions
>> around access to the address to avoid triggering a
>> LASS #GP fault.
>>
> 
> 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.

> I believe there needs to be a dependency between LASS and SMAP.
> 
Yes, In kernel mode, LASS depends on SMAP to work. And in user mode, it 
doesn't, so the dependency description in following may miss user space 
effect.

> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c 
> b/arch/x86/kernel/cpu/cpuid-deps.c
> index d95221117129..00bc7e4a65d2 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -79,6 +79,7 @@ static const struct cpuid_dep cpuid_deps[] = {
>       { X86_FEATURE_XFD,            X86_FEATURE_XSAVES    },
>       { X86_FEATURE_XFD,            X86_FEATURE_XGETBV1   },
>       { X86_FEATURE_AMX_TILE,            X86_FEATURE_XFD       },
> +    { X86_FEATURE_LASS,            X86_FEATURE_SMAP      },
>       {}
>   };

Thanks,
Yian