linux-kernel - Re: [RFC PATCH 00/56] Dynamic mitigations

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Message-ID: <a5kvnas7cttg3pxqdxye7qhyesbtyulhhdiunl2hr5tfmecpbz@y4ggjfafqawn>
Date: Wed, 15 Oct 2025 09:02:19 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: "Kaplan, David" <David.Kaplan@....com>
Cc: Aaron Rainbolt <arraybolt3@...il.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	"x86@...nel.org" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, 
	Alexander Graf <graf@...zon.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/56] Dynamic mitigations

On Wed, Oct 15, 2025 at 03:51:01PM +0000, Kaplan, David wrote:
> > On Wed, Oct 15, 2025 at 01:53:31PM +0000, Kaplan, David wrote:
> > > > If `root` is capable of setting `mitigations=off` via this interface,
> > > > doesn't that somewhat defeat the purpose of denying `/proc/kcore`
> > > > access in lockdown confidentiality mode? Assuming one is running on a
> > > > CPU with some form of side-channel memory read vulnerability (which they
> > > > very likely are), they can turn off all mitigations, then read kernel
> > > > memory via one of those exploits.
> > > >
> > > > There should be a one-way switch to allow denying all further writes to
> > > > this interface, so that once the system's mitigations are set properly,
> > > > any further attempts to change them until the next reboot can be
> > > > prevented.
> > > >
> > >
> > > That's a good idea, there could be a separate mitigation_lock file
> > > perhaps that once written to 1 denies any further changes.
> >
> > Wouldn't the enablement of lockdown mode effectively function as that
> > one way switch?
> >
> 
> I'm not too familiar with lockdown mode, but that gets enabled (with
> right cmdline options) during boot right?  I guess the question is
> would we want to allow any window for userspace to reconfigure things
> and then lock things down, or say that if you enable lockdown then
> this interface is completely disabled and you need to specify your
> mitigation options on the cmdline only.

Yeah, I would say the latter, otherwise it defeats the point of lockdown
mode.  Note that lockdown mode can also be enabled at runtime.

-- 
Josh