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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1f3070d5-17df-4fc8-9700-bb8e37a3b0a1@intel.com>
Date: Fri, 31 Oct 2025 11:32:50 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, <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>, "Andy
 Lutomirski" <luto@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, "Peter
 Zijlstra" <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 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@...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 10/31/2025 10:11 AM, Dave Hansen wrote:
> One thing that's actually missing here is an explanation on how it's OK
> to munge CR bits here. Why are preemption and interrupts not a problem?
> 

This is called pretty early on from the BSP init flow.

start_kernel()
  arch_cpu_finalize_init()
    efi_enter_virtual_mode()
      __efi_enter_virtual_mode()

I had assumed we run with interrupts disabled. But, that's not true.
Interrupts are enabled midway during start_kernel(). So,
arch_cpu_finalize_init() is called with interrupts enabled.

We write to CR bits during FPU init which happens right before EFI
enters virtual mode. So I am probably missing something obvious that
makes it okay.