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From: Amir Goldstein <amir73il@gmail.com>
To: Jan Kara <jack@suse.cz>
Cc: 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 18:08:31 +0300
Message-ID: <CAOQ4uxhKk3oJdWF8YxYRPyomimg9xQaHnMo3ggALOhTuwWxYBw@mail.gmail.com> (raw)
In-Reply-To: <20210510142107.GA24154@quack2.suse.cz>

> > > 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.

Thanks,
Amir.

  reply index

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-09 18:00 Amir Goldstein
2020-11-10  5:07 ` Amir Goldstein
2020-11-17  7:09 ` [fanotify] a23a7dc576: unixbench.score -3.7% regression kernel test robot
2020-11-24 13:49 ` [RFC][PATCH] fanotify: introduce filesystem view mark Jan Kara
2020-11-24 14:47   ` Amir Goldstein
2020-11-25 11:01     ` Jan Kara
2020-11-25 12:34       ` Amir Goldstein
2020-11-26 11:10         ` Jan Kara
2020-11-26 11:50           ` Amir Goldstein
2020-11-26  3:42       ` Amir Goldstein
2020-11-26 11:17         ` Jan Kara
2021-04-28 18:28           ` Amir Goldstein
2021-05-03 16:53             ` Jan Kara
2021-05-03 18:44               ` Amir Goldstein
2021-05-05 12:28                 ` Jan Kara
2021-05-05 14:24                   ` Christian Brauner
2021-05-05 14:42                     ` Amir Goldstein
2021-05-05 14:56                       ` Christian Brauner
2021-05-10 10:13                     ` Jan Kara
2021-05-10 11:37                       ` Amir Goldstein
2021-05-10 14:21                         ` Jan Kara
2021-05-10 15:08                           ` Amir Goldstein [this message]
2021-05-10 15:27                             ` Jan Kara
2021-05-12 13:07                             ` Christian Brauner
2021-05-12 13:34                               ` Jan Kara
2021-05-12 16:15                                 ` Christian Brauner
2021-05-12 15:26                         ` Christian Brauner
2021-05-13 10:55                           ` Jan Kara
2021-05-14 13:56                             ` Christian Brauner
2021-05-15 14:28                               ` Amir Goldstein
2021-05-17  9:09                                 ` Jan Kara
2021-05-17 12:45                                   ` Amir Goldstein
2021-05-17 13:07                                     ` Jan Kara
2021-05-18 10:11                                 ` Christian Brauner
2021-05-18 16:02                                   ` Amir Goldstein
2021-05-19  9:31                                     ` Christian Brauner
2021-05-12 16:11                         ` Christian Brauner
2021-05-05 13:25               ` Christian Brauner

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