linux-kernel - Re: [PATCH 3/3] capabilities: add cap userns sysctl mask

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Date: Tue, 21 May 2024 08:29:35 -0600
From: Tycho Andersen <tycho@...ho.pizza>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Jonathan Calmels <jcalmels@...0.net>, brauner@...nel.org,
	ebiederm@...ssion.com, Luis Chamberlain <mcgrof@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Joel Granados <j.granados@...sung.com>,
	Serge Hallyn <serge@...lyn.com>, Paul Moore <paul@...l-moore.com>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>, containers@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, keyrings@...r.kernel.org
Subject: Re: [PATCH 3/3] capabilities: add cap userns sysctl mask

On Tue, May 21, 2024 at 01:12:57AM +0300, Jarkko Sakkinen wrote:
> On Tue May 21, 2024 at 12:13 AM EEST, Tycho Andersen wrote:
> > On Mon, May 20, 2024 at 12:25:27PM -0700, Jonathan Calmels wrote:
> > > On Mon, May 20, 2024 at 07:30:14AM GMT, Tycho Andersen wrote:
> > > > there is an ongoing effort (started at [0]) to constify the first arg
> > > > here, since you're not supposed to write to it. Your usage looks
> > > > correct to me, so I think all it needs is a literal "const" here.
> > > 
> > > Will do, along with the suggestions from Jarkko
> > > 
> > > > > +	struct ctl_table t;
> > > > > +	unsigned long mask_array[2];
> > > > > +	kernel_cap_t new_mask, *mask;
> > > > > +	int err;
> > > > > +
> > > > > +	if (write && (!capable(CAP_SETPCAP) ||
> > > > > +		      !capable(CAP_SYS_ADMIN)))
> > > > > +		return -EPERM;
> > > > 
> > > > ...why CAP_SYS_ADMIN? You mention it in the changelog, but don't
> > > > explain why.
> > > 
> > > No reason really, I was hoping we could decide what we want here.
> > > UMH uses CAP_SYS_MODULE, Serge mentioned adding a new cap maybe.
> >
> > I don't have a strong preference between SETPCAP and a new capability,
> > but I do think it should be just one. SYS_ADMIN is already god mode
> > enough, IMO.
> 
> Sometimes I think would it make more sense to invent something
> completely new like capabilities but more modern and robust, instead of
> increasing complexity of a broken mechanism (especially thanks to
> CAP_MAC_ADMIN).
> 
> I kind of liked the idea of privilege tokens both in Symbian and Maemo
> (have been involved professionally in both). Emphasis on the idea not
> necessarily on implementation.
> 
> Not an LSM but like something that you could use in the place of POSIX
> caps. Probably quite tedious effort tho because you would need to pull
> the whole industry with the new thing...

And then we have LSM hooks, (ns_)capable(), __secure_computing() plus
a new set of hooks for this new thing sprinkled around. I guess
kernel developers wouldn't be excited about it, let alone the rest of
the industry :)

Thinking out loud: I wonder if fixing the seccomp TOCTOU against
pointers would help here. I guess you'd still have issues where your
policy engine resolves a path arg to open() and that inode changes
between the decision and the actual vfs access, you have just changed
the TOCTOU.

Or even scarier: what if you could change the return value at any
kprobe? :)

Tycho