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

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Message-ID: <693d8332-3b86-3dcf-fc87-5c3a08a752db@intel.com>
Date:   Tue, 10 Jan 2023 14:41:25 -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 3/7] x86/cpu: Disable kernel LASS when patching kernel
 alternatives

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"?

I believe there needs to be a dependency between LASS and SMAP.

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      },
  	{}
  };