Re: [PATCH] cgroup: Fix crash with CLONE_INTO_CGROUP and v1 cgroups

From: Yosry Ahmed <yosryahmed@google.com>
To: Tejun Heo <tj@kernel.org>
Cc: "Christian A. Ehrhardt" <lk@c--e.de>,
	Christian Brauner <brauner@kernel.org>,
	syzbot <syzbot+534ee3d24c37c411f37f@syzkaller.appspotmail.com>,
	gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
	syzkaller-bugs@googlegroups.com,
	Martin KaFai Lau <martin.lau@linux.dev>
Subject: Re: [PATCH] cgroup: Fix crash with CLONE_INTO_CGROUP and v1 cgroups
Date: Mon, 10 Oct 2022 11:50:50 -0700	[thread overview]
Message-ID: <CAJD7tka2jwPnYyCmLu0T5Oq_X-LJCx7TVqLayLPS_6fOjqf93A@mail.gmail.com> (raw)
In-Reply-To: <Y0Rn5qzP6MjayoCz@slm.duckdns.org>

On Mon, Oct 10, 2022 at 11:44 AM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Sun, Oct 09, 2022 at 03:10:36PM +0200, Christian A. Ehrhardt wrote:
> >
> > Since commit f3a2aebdd6, Version 1 cgroups no longer cause an
> > error when used with CLONE_INTO_CGROUP. However, the permission
> > checks performed during clone assume a Version 2 cgroup.
> >
> > Restore the error check for V1 cgroups in the clone() path.
> >
> > Reported-by: syzbot+534ee3d24c37c411f37f@syzkaller.appspotmail.com
> > Link: https://lore.kernel.org/lkml/000000000000385cbf05ea3f1862@google.com/
> > Fixes: f3a2aebdd6 ("cgroup: enable cgroup_get_from_file() on cgroup1")
> > Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
>
> This feels too error prone. I'd rather revert the original commit. Yosry,
> imma revert f3a2aebdd6. Can you please add a separate function which allows
> looking up IDs for cgroup1 hierarchies if absolutely necessary? But,
> frankly, given how inherently confusing using IDs for cgroup1 hierarchies is
> (fd for cgroup1 identifies both the hierarchy and the cgroup, id is
> inherently partial which is super confusing), I'd rather just not do it.

The purpose of f3a2aebdd6 was to make cgroup_get_from_fd() support
cgroup1, which IIUC makes sense. It was unrelated to IDs.

There are currently two users of
cgroup_get_from_file()/cgroup_get_from_fd() AFAICT, one of which is
the fork code fixed by this commit, the second is BPF cgroup prog
attachment. I can send another patch to add explicit filtering in the
BPF attachment code as well.

Alternatively, we can have separate functions that do the filtering if
needed. For example:
cgroup_get_from_fd() / cgroup_get_from_file() -> support both v1 and v2
cgroup_get_dfl_from_fd() / cgroup_get_dfl_from_file() -> support only v2

We can then use the versions with filtering for all the current users
except cgroup_iter (that needs to support both v1 and v2). WDYT?

>
> Thanks.
>
> --
> tejun