Re: [RFC PATCH 0/6] RLIMIT_NPROC in ucounts fixups

From: "Eric W. Biederman" <ebiederm@xmission.com>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: 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>
Subject: Re: [RFC PATCH 0/6] RLIMIT_NPROC in ucounts fixups
Date: Tue, 08 Feb 2022 07:54:00 -0600	[thread overview]
Message-ID: <87ee4dihvr.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <20220207121800.5079-1-mkoutny@suse.com> ("Michal =?utf-8?Q?K?= =?utf-8?Q?outn=C3=BD=22's?= message of "Mon, 7 Feb 2022 13:17:54 +0100")

Michal Koutný <mkoutny@suse.com> writes:

> This series is a result of looking deeper into breakage of
> tools/testing/selftests/rlimits/rlimits-per-userns.c after 
> https://lore.kernel.org/r/20220204181144.24462-1-mkoutny@suse.com/
> is applied.
>
> The description of the original problem that lead to RLIMIT_NPROC et al.
> ucounts rewrite could be ambiguously interpretted as supporting either
> the case of:
> - never-fork service or
> - fork (RLIMIT_NPROC-1) times service.
>
> The scenario is weird anyway given existence of pids controller.
>
> The realization of that scenario relies not only on tracking number of
> processes per user_ns but also newly allows the root to override limit through
> set*uid. The commit message didn't mention that, so it's unclear if it
> was the intention too.
>
> I also noticed that the RLIMIT_NPROC enforcing in fork seems subject to TOCTOU
> race (check(nr_tasks),...,nr_tasks++) so the limit is rather advisory (but
> that's not a new thing related to ucounts rewrite).
>
> This series is RFC to discuss relevance of the subtle changes RLIMIT_NPROC to
> ucounts rewrite introduced.

A quick reply (because I don't have a lot of time at the moment).

I agree with the issues your first patch before this series addresses
and the issues the first 3 patches address.

I have not looked at the tests.

I actually disagree with most of your fixes.  Both because of
intrusiveness and because of awkwardness.  My basic problem with
your fixes is I don't think they leave the code in a more maintainable
state.

Hopefully later today I can propose some alternative fixes and we can
continue the discussion.

One thing I think you misunderstood is the capability checks in set_user
have always been there.  There is a very good argument they are badly
placed so are not exactly checking the correct credentials.  Especially
now.

Your patch 4/6 I don't think makes sense.  It has always been the
case that root without capabilities is subject to the rlimit.  If  you
are in a user namespace you are root without capabilities.

Eric