linux-kernel - Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during context switching

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Date: Sat, 9 Mar 2024 13:37:06 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...el.com>, Andy Lutomirski <luto@...nel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	"Peter Zijlstra (Intel)" <peterz@...radead.org>, "the arch/x86 maintainers" <x86@...nel.org>, linux-mm@...ck.org, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/3] x86/mm: make sure LAM is up-to-date during
 context switching

On Sat, Mar 9, 2024 at 8:34 AM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Sat, Mar 09, 2024 at 02:19:19AM +0000, Yosry Ahmed wrote:
> > I don't see how skipping set_tlbstate_lam_mode() for kthreads fixes this
> > problem. Do you mind elaborating?
>
> Define what problem is.
>
> Yes, in this scenario kthread gets more permissive LAM mode than it needs.
> But nothing breaks.

The problem here is not how the kthread runs at all. It is the fact
that if that kthread context switches into the user process that has
enabled LAM, it may not update CR3 because the mm doesn't change.
switch_mm_irqs_off() will only update CR3 in this case if there is a
pending TLB flush. Otherwise, we just return, even if the LAM for this
mm has changed.

This can cause the process that has enabled LAM to run with LAM
disabled and fault on tagged addresses, right? Did I miss something?