linux-kernel - Re: [PATCH 0/2] Allow individual features to be locked down

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Message-ID: <68226ad551afd_29032945b@dwillia2-xfh.jf.intel.com.notmuch>
Date: Mon, 12 May 2025 14:40:37 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Dan Williams <dan.j.williams@...el.com>, Paul Moore <paul@...l-moore.com>,
	Nikolay Borisov <nik.borisov@...e.com>
CC: <linux-security-module@...r.kernel.org>, <serge@...lyn.com>,
	<kees@...nel.org>, <linux-kernel@...r.kernel.org>,
	<kirill.shutemov@...ux.intel.com>, <linux-coco@...ts.linux.dev>
Subject: Re: [PATCH 0/2] Allow individual features to be locked down

Dan Williams wrote:
> Paul Moore wrote:
> > On Fri, Mar 21, 2025 at 6:24 AM Nikolay Borisov <nik.borisov@...e.com> wrote:
> > >
> > > This simple change allows usecases where someone might want to  lock only specific
> > > feature at a finer granularity than integrity/confidentiality levels allows.
> > > The first likely user of this is the CoCo subsystem where certain features will be
> > > disabled.
> > >
> > > Nikolay Borisov (2):
> > >   lockdown: Switch implementation to using bitmap
> > >   lockdown/kunit: Introduce kunit tests
> > 
> > Hi Nikolay,
> > 
> > Thanks for the patches!  With the merge window opening in a few days,
> > it is too late to consider this for the upcoming merge window so
> > realistically this patchset is two weeks out and I'm hopeful we'll
> > have a dedicated Lockdown maintainer by then so I'm going to defer the
> > ultimate decision on acceptance to them.
> 
> The patches in this thread proposed to selectively disable /dev/mem
> independent of all the other lockdown mitigations. That goal can be
> achieved with more precision with this proposed patch:
> 
> http://lore.kernel.org/67f5b75c37143_71fe2949b@dwillia2-xfh.jf.intel.com.notmuch

Just wanted to circle back here and repair the damage I caused to the
momentum of this "lockdown feature bitmap" proposal. It turns out that
devmem maintainers are not looking to add yet more arch-specific hacks
[1].

    "Restricting /dev/mem further is a good idea, but it would be nice
     if that could be done without adding yet another special case."

security_locked_down() is already plumbed into all the places that
confidential VMs may need to manage userspace access to confidential /
private memory.

I considered registering a new "coco-LSM" to hook
security_locked_down(), but that immediately raises the next question of
how does userspace discover what is currently locked_down. So just teach
the native lockdown LSM how to be more fine-grained rather than
complicate the situation with a new LSM.

[1]: http://lore.kernel.org/0bdb1876-0cb3-4632-910b-2dc191902e3e@app.fastmail.com