linux-kernel - Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI runtime services

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Message-ID: <ee2fce64-91ce-4b78-b2f9-33364ea0c52f@intel.com>
Date: Fri, 31 Oct 2025 11:12:53 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, Andy Lutomirski <luto@...nel.org>,
 the arch/x86 maintainers <x86@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>
Cc: Jonathan Corbet <corbet@....net>, "H. Peter Anvin" <hpa@...or.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Ard Biesheuvel <ardb@...nel.org>, "Kirill A . Shutemov" <kas@...nel.org>,
 Xin Li <xin@...or.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Sean Christopherson <seanjc@...gle.com>,
 Rick P Edgecombe <rick.p.edgecombe@...el.com>,
 Vegard Nossum <vegard.nossum@...cle.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 Randy Dunlap <rdunlap@...radead.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Kees Cook <kees@...nel.org>,
 Tony Luck <tony.luck@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 linux-doc@...r.kernel.org,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 linux-efi@...r.kernel.org
Subject: Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI
 runtime services

On 10/31/25 11:03, Sohil Mehta wrote:
>> Deferring is a good idea. I was just asking for the same thing for the
>> CR pinning enforcement. The earlier we try to do these things, the more
>> we just trip over ourselves.
> I had suggested deferring as well to Kirill when I was reviewing the
> series. He preferred to enable LASS with other similar features such as
> SMAP, SMEP.
> 
> One other thing to consider:
> 
> Doing it in identify_cpu() makes it easy for all the APs to program
> their CR4.LASS bit. If we were to defer it, we would need some
> additional work to setup all the APs.

That's true. We'd need an smp_call_function() of some kind. *But*, once
that is in place, it's hopefully just a matter of moving that one line
of code per feature from identify_cpu() over to the new function.

> Do we already do this for something else? That would make it easier to
> tag along.

We don't do it for anything else that I can think of.

But there's a pretty broad set of things that are for "security" that
aren't necessary while you're just running trusted ring0 code:

 * SMAP/SMEP
 * CR pinning itself
 * MSR_IA32_SPEC_CTRL
 * MSR_IA32_TSX_CTRL

They just haven't mattered until now because they don't have any
practical effect until you actually have code running on _PAGE_USER
mappings trying to attack the kernel.