Re: How should rlimits, suid exec, and capabilities interact?

From: Willy Tarreau <w@1wt.eu>
To: Andy Lutomirski <luto@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	linux-api@vger.kernel.org,
	"Etienne Dechamps" <etienne@edechamps.fr>,
	"Alexey Gladkov" <legion@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Christian Brauner" <brauner@kernel.org>,
	"Solar Designer" <solar@openwall.com>,
	"Ran Xiaokai" <ran.xiaokai@zte.com.cn>,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	"Linux Containers" <containers@lists.linux-foundation.org>,
	"Michal Koutný" <mkoutny@suse.com>,
	security@kernel.org, "Neil Brown" <neilb@cse.unsw.edu.au>,
	NeilBrown <neilb@suse.de>, "Serge E. Hallyn" <serge@hallyn.com>,
	"Jann Horn" <jannh@google.com>
Subject: Re: How should rlimits, suid exec, and capabilities interact?
Date: Wed, 23 Feb 2022 22:28:39 +0100	[thread overview]
Message-ID: <20220223212839.GA12121@1wt.eu> (raw)
In-Reply-To: <CALCETrVh8Xu3VJzseWEJZ+ryy5WANyJg+j4=hFaVFebzSu_TgQ@mail.gmail.com>

Hi Andy,

On Wed, Feb 23, 2022 at 11:44:51AM -0800, Andy Lutomirski wrote:
> On Wed, Feb 23, 2022 at 10:00 AM Eric W. Biederman
> <ebiederm@xmission.com> wrote:
> >
> >
> > [CC'd the security list because I really don't know who the right people
> >  are to drag into this discussion]
> >
> > While looking at some issues that have cropped up with making it so
> > that RLIMIT_NPROC cannot be escaped by creating a user namespace I have
> > stumbled upon a very old issue of how rlimits and suid exec interact
> > poorly.
> 
> Once upon a time, these resource limits were effectively the only way
> to control memory consumption and consumption of historically limited
> resources like processes.  (The scheduler used to have serious issues
> with too many processes -- this is not so true any more.  And without
> cgroups, too many processes could use too much CPU collectively.)
> This all worked pretty poorly.  Now we have cgroups, fancy memory
> accounting, etc.  So I'm wondering if NPROC is even useful anymore.  I
> don't have a brilliant idea of how to deprecate it, but I think it
> wouldn't be entirely nuts to take it much less seriously and maybe
> even eventually get rid of it.
> 
> I doubt there is much existing userspace that would break if a
> previously failing fork() started succeeding.

I strongly disagree. I've been using it for a long time as a security
measure. Setting NPROC to 0 after daemonizing remains a particularly
effective and portable method to mitigate the possible consequences of
an in-process intrusion. While I wouldn't care about approximate non-zero
values, for me it would be a significant security regression to drop the
inability to fork() when the limit is zero. Thus at least I do want to
keep that feature when NPROC is zero.

Willy