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

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Message-ID: <20251107101003.GB1618871@noisy.programming.kicks-ass.net>
Date: Fri, 7 Nov 2025 11:10:03 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Dave Hansen <dave.hansen@...el.com>,
	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>,
	Jonathan Corbet <corbet@....net>, "H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...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 Fri, Nov 07, 2025 at 10:22:30AM +0100, Ard Biesheuvel wrote:

> There is also PRM, which is much worse, as it permits devices in the
> ACPI namespace to call firmware routines that are mapped privileged in
> the OS address space in the same way. I objected to this at the time,
> and asked for a facility where we could at least mark such code as
> unprivileged (and run it as such) but this was ignored, as Intel and
> MS had already sealed the deal and put this into production. This is
> much worse than typical EFI routines, as the PRM code is ODM/OEM code
> rather than something that comes from the upstream EFI implementation.
> It is basically a dumping ground for code that used to run in SMM
> because it was too ugly to run anywhere else. </rant>

'https://uefi.org/sites/default/files/resources/Platform Runtime Mechanism - with legal notice.pdf'

Has on page 16, section 3.1:

  8. PRM handlers must not contain any privileged instructions.

So we should be able to actually run this crap in ring3, right?