Re: [RFC][PATCH] fanotify: introduce filesystem view mark

From: Jan Kara <jack@suse.cz>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Jan Kara <jack@suse.cz>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Miklos Szeredi <miklos@szeredi.hu>
Subject: Re: [RFC][PATCH] fanotify: introduce filesystem view mark
Date: Mon, 10 May 2021 17:27:58 +0200	[thread overview]
Message-ID: <20210510152758.GC24154@quack2.suse.cz> (raw)
In-Reply-To: <CAOQ4uxhKk3oJdWF8YxYRPyomimg9xQaHnMo3ggALOhTuwWxYBw@mail.gmail.com>

On Mon 10-05-21 18:08:31, Amir Goldstein wrote:
> > > > OK, so this feature would effectively allow sb-wide watching of events that
> > > > are generated from within the container (or its descendants). That sounds
> > > > useful. Just one question: If there's some part of a filesystem, that is
> > > > accesible by multiple containers (and thus multiple namespaces), or if
> > > > there's some change done to the filesystem say by container management SW,
> > > > then event for this change won't be visible inside the container (despite
> > > > that the fs change itself will be visible).
> > >
> > > That is correct.
> > > FYI, a privileged user can already mount an overlayfs in order to indirectly
> > > open and write to a file.
> > >
> > > Because overlayfs opens the underlying file FMODE_NONOTIFY this will
> > > hide OPEN/ACCESS/MODIFY/CLOSE events also for inode/sb marks.
> > > Since 459c7c565ac3 ("ovl: unprivieged mounts"), so can unprivileged users.
> > >
> > > I wonder if that is a problem that we need to fix...
> >
> > I assume you are speaking of the filesystem that is absorbing the changes?
> > AFAIU usually you are not supposed to access that filesystem alone but
> > always access it only through overlayfs and in that case you won't see the
> > problem?
> >
> 
> Yes I am talking about the "backend" store for overlayfs.
> Normally, that would be a subtree where changes are not expected
> except through overlayfs and indeed it is documented that:
> "If the underlying filesystem is changed, the behavior of the overlay
>  is undefined, though it will not result in a crash or deadlock."
> Not reporting events falls well under "undefined".
> 
> But that is not the problem.
> The problem is that if user A is watching a directory D for changes, then
> an adversary user B which has read/write access to D can:
> - Clone a userns wherein user B id is 0
> - Mount a private overlayfs instance using D as upperdir
> - Open file in D indirectly via private overlayfs and edit it
> 
> So it does not require any special privileges to circumvent generating
> events. Unless I am missing something.

I see, right. I agree that is unfortunate especially for stuff like audit
or fanotify permission events so we should fix that.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR