linux-kernel - Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during context switch

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <updn6j7jcqdru73vwt3fvxlx4t73rjrlk7h6i6js3lizeueoov@tz7fyrd3a4yi>
Date: Thu, 7 Mar 2024 19:22:36 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...el.com>, Peter Zijlstra <peterz@...radead.org>, 
	Andy Lutomirski <luto@...nel.org>, x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/3] x86/mm: fix LAM cr3 mask inconsistency during
 context switch

On Thu, Mar 07, 2024 at 01:39:14PM +0000, Yosry Ahmed wrote:
> In switch_mm_irqs_off(), we read the 'mm->context.lam_cr3_mask' into
> 'new_lam', which is later passed to load_new_mm_cr3(). However, there is
> a call to set_tlbstate_lam_mode() in between which will read
> 'mm->context.lam_cr3_mask' again and set 'cpu_tlbstate.lam' accordingly.
> If we race with another thread updating 'mm->context.lam_cr3_mask', the
> value in 'cpu_tlbstate.lam' could end up being different from CR3.

What other thread? LAM can only be enabled when the process has single
thread. And cannot be disabled. See MM_CONTEXT_LOCK_LAM.

> While we are at it, remove the misguiding comment that states that
> 'new_lam' may not match tlbstate_lam_cr3_mask() if a race occurs.

The comment is indeed misguiding, but for different reason. It is leftover
from the earlier version of LAM patchset.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov