Re: fsnotify events for overlayfs real file

[Amir Goldstein <amir73il@gmail.com>]

On Mon, May 31, 2021 at 6:18 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Tue, 18 May 2021 at 19:56, Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Tue, May 18, 2021 at 5:43 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> > >
> > > On Mon, 10 May 2021 at 18:32, Amir Goldstein <amir73il@gmail.com> wrote:
>
> > > > My thinking was that we can change d_real() to provide the real path:
> > > >
> > > > static inline struct path d_real_path(struct path *path,
> > > >                                     const struct inode *inode)
> > > > {
> > > >         struct realpath = {};
> > > >         if (!unlikely(dentry->d_flags & DCACHE_OP_REAL))
> > > >                return *path;
> > > >         dentry->d_op->d_real(path->dentry, inode, &realpath);
> > > >         return realpath;
> > > > }
>
> Real paths are internal, we can't pass them (as fd in permission
> events) to userspace.
>
> > > >
> > > >
> > > > Another option, instead of getting the realpath, just detect the
> > > > mismatch of file_inode(file) != d_inode(path->dentry) in
> > > > fanotify_file() and pass FSNOTIFY_EVENT_DENTRY data type
> > > > with d_real() dentry to backend instead of FSNOTIFY_EVENT_PATH.
> > > >
> > > > For inotify it should be enough and for fanotify it is enough for
> > > > FAN_REPORT_FID and legacy fanotify can report FAN_NOFD,
> > > > so at least permission events listeners can identify the situation and
> > > > be able to block access to unknown paths.
>
> That sounds like a good short term solution.
>

It may be a fine academic solution, but I don't think it solves any real
world problem.
I am not aware of any security oriented solutions that use permission
events on inode or directory (let alone sb).

The security oriented users of fanotify are anti-virus on-access
protection engines and those are using mount marks anyway
(dynamically adding them as far as I know).
[cc Marko who may be able to shed some light]

For those products, creating a bind mount inside a new mount ns
may actually escape the on-access policy or the new mount will
also be marked I am not sure. I suppose cloning mount ns may be
prohibited by another LSM or something(?).

>
> >
> > Is there a reason for the fake path besides the displayed path in
> > /proc/self/maps?
>
> I'm not aware of any.
>
> >
> > Does it make sense to keep one realfile with fake path for mmaped
> > files along side a realfile with private/detached path used for all the
> > other operations?
>
> This should work, but it would add more open files,

True, but only for the mmaped files.

> so needs some good justifications.
>

I'm afraid I don't have one yet..
Let's see what the AV vendors have to say.

Thanks,
Amir.