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

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Date:   Wed, 23 Feb 2022 22:28:39 +0100
From:   Willy Tarreau <w@....eu>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-api@...r.kernel.org, Etienne Dechamps <etienne@...champs.fr>,
        Alexey Gladkov <legion@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Solar Designer <solar@...nwall.com>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Michal Koutný <mkoutny@...e.com>,
        security@...nel.org, Neil Brown <neilb@....unsw.edu.au>,
        NeilBrown <neilb@...e.de>, "Serge E. Hallyn" <serge@...lyn.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: How should rlimits, suid exec, and capabilities interact?

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@...ssion.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