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

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, May 12, 2021 at 03:34:15PM +0200, Jan Kara wrote:
> On Wed 12-05-21 15:07:05, Christian Brauner wrote:
> > On Mon, May 10, 2021 at 06:08:31PM +0300, 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.
> > 
> > No, I think you're right. That should work. I don't think that's
> > necessarily a problem though. It's a bit unexpected and slightly
> > unpleasant but it's documented already and it's not a security issue
> > afaict.
> 
> fanotify(7) is used in applications (such as virus scanners or anti-malware
> products) where they expect to see all filesystem changes. There are
> products which implement access mediation policy based on fanotify
> permission events. So a way for unpriviledged application to escape
> notification is a "security" issue (not a kernel one but it defeats
> protections userspace implements).

Ah, good point. I assumed since this has always been the case although
restricted to privileged users on the host, i.e. creating an overlayfs
mount would always have that affect iiuc.