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

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Message-ID: <ZeokMhiB2MhTlIXP@google.com>
Date: Thu, 7 Mar 2024 20:31:46 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 07:22:36PM +0200, Kirill A. Shutemov wrote:
> 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.

Right, but a kthread may run with that single-threaded process's mm
IIUC. I think this can happen via kthread_use_mm() or if we context
switch directly from the user process to the kthread (context_switch()
doesn't seem to update the mm in this case).

> 
> > 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.