RE: [RFC PATCH 00/56] Dynamic mitigations

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Sent: Wednesday, October 15, 2025 11:02 AM
> To: Kaplan, David <David.Kaplan@xxxxxxx>
> Cc: Aaron Rainbolt <arraybolt3@xxxxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Peter Zijlstra
> <peterz@xxxxxxxxxxxxx>; Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>;
> Ingo Molnar <mingo@xxxxxxxxxx>; Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>;
> x86@xxxxxxxxxx; H . Peter Anvin <hpa@xxxxxxxxx>; Alexander Graf
> <graf@xxxxxxxxxx>; Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH 00/56] Dynamic mitigations
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> 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.
>

Ok.  So that's using the security_locked_down() API presumably.  And what reason would we want to check for?  Should it be LOCKDOWN_DEV_MEM (since we're potentially leaking arbitrary memory)?  Or a new lockdown reason?

--David Kaplan