linux-kernel - Re: [PATCH v2 00/10] allow unprivileged overlay mounts

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Date:   Thu, 10 Dec 2020 10:39:44 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     John Johansen <john.johansen@...onical.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Miklos Szeredi <mszeredi@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/10] allow unprivileged overlay mounts

On Thu, Dec 10, 2020 at 10:00 AM John Johansen
<john.johansen@...onical.com> wrote:
>
> On 12/8/20 2:27 AM, Tetsuo Handa wrote:
> > On 2020/12/08 1:32, Miklos Szeredi wrote:
> >> A general observation is that overlayfs does not call security_path_*()
> >> hooks on the underlying fs.  I don't see this as a problem, because a
> >> simple bind mount done inside a private mount namespace also defeats the
> >> path based security checks.  Maybe I'm missing something here, so I'm
> >> interested in comments from AppArmor and Tomoyo developers.
> >
> > Regarding TOMOYO, I don't want overlayfs to call security_path_*() hooks on the
> > underlying fs, but the reason is different. It is not because a simple bind mount
> > done inside a private mount namespace defeats the path based security checks.
> > TOMOYO does want to check what device/filesystem is mounted on which location. But
> > currently TOMOYO is failing to check it due to fsopen()/fsmount()/move_mount() API.
> >
>
> Regardless of TOMOYO's approach I would say that overlays should call the
> security_path_*() hooks, making it possible for an LSM to do something based off of
> them when needed.
>
> The current state of private mounts with regard to path based mediation is broken.
> I just haven't had time to try and come up with an acceptable fix for it. overlayfs
> is actually broken under apparmor mediation, and accesses to the lower layer end up
> getting denied but there is no way to properly allow them. So policy that hits this
> needs a flag set that allows for it in a very hacky way (its on the list of things
> to fix).
>
> Path based mediation has to carefully control mounts otherwise policy can be
> circumvented as Miklos rightly points out. Ideally path based LSM wouldn't allow
> you to do the simple bind mount inside a private mount namespace (at least not
> unless policy allowed for it). AppArmor does mediate the mount hooks and bind
> mounts in a private mount namespace (if they go through the LSM mount hooks) will
> be denied. Again the problem is how to allow them, and this is broken.

Okay, so what does that mean for overlayfs?

AA can deny the overlay mount just as well as the bind mount, and it
can allow it just as well as the bind mount.  Policy could be the
same.

Also all the security_path_ hooks will still get called for each
access on overlayfs itself.  They won't be called for the accesses
which overlayfs does on underlying layers, but is that needed?

Overlay could call those hooks itself (since the vfs_ helpers don't)
but the big question is whether that makes any sense.  AFAICS it might
make sense, but only if AA would correctly handle bind mounts, and
especially detached bind mounts (which is what overlayfs technically
uses).

Thanks,
Miklos

Tja