linux-kernel - Re: [PATCH] fanotify: remove redundant capable(CAP_SYS

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Date:   Thu, 23 May 2019 11:55:07 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s

On Wed, May 22, 2019 at 11:00:22PM +0300, Amir Goldstein wrote:
> On Wed, May 22, 2019 at 9:57 PM Christian Brauner <christian@...uner.io> wrote:
> >
> > On May 22, 2019 8:29:37 PM GMT+02:00, Amir Goldstein <amir73il@...il.com> wrote:
> > >On Wed, May 22, 2019 at 7:32 PM Christian Brauner
> > ><christian@...uner.io> wrote:
> > >>
> > >> This removes two redundant capable(CAP_SYS_ADMIN) checks from
> > >> fanotify_init().
> > >> fanotify_init() guards the whole syscall with capable(CAP_SYS_ADMIN)
> > >at the
> > >> beginning. So the other two capable(CAP_SYS_ADMIN) checks are not
> > >needed.
> > >
> > >It's intentional:
> > >
> > >commit e7099d8a5a34d2876908a9fab4952dabdcfc5909
> > >Author: Eric Paris <eparis@...hat.com>
> > >Date:   Thu Oct 28 17:21:57 2010 -0400
> > >
> > >    fanotify: limit the number of marks in a single fanotify group
> > >
> > >There is currently no limit on the number of marks a given fanotify
> > >group
> > >can have.  Since fanotify is gated on CAP_SYS_ADMIN this was not seen
> > >as
> > >a serious DoS threat.  This patch implements a default of 8192, the
> > >same as
> > >inotify to work towards removing the CAP_SYS_ADMIN gating and
> > >eliminating
> > >    the default DoS'able status.
> > >
> > >    Signed-off-by: Eric Paris <eparis@...hat.com>
> > >
> > >There idea is to eventually remove the gated CAP_SYS_ADMIN.
> > >There is no reason that fanotify could not be used by unprivileged
> > >users
> > >to setup inotify style watch on an inode or directories children, see:
> > >https://patchwork.kernel.org/patch/10668299/
> > >
> > >>
> > >> Fixes: 5dd03f55fd2 ("fanotify: allow userspace to override max queue
> > >depth")
> > >> Fixes: ac7e22dcfaf ("fanotify: allow userspace to override max
> > >marks")
> > >
> > >Fixes is used to tag bug fixes for stable.
> > >There is no bug.
> > >
> > >Thanks,
> > >Amir.
> >
> > Interesting. When do you think the gate can be removed?
> 
> Nobody is working on this AFAIK.
> What I posted was a simple POC, but I have no use case for this.
> In the patchwork link above, Jan has listed the prerequisites for
> removing the gate.
> 
> One of the prerequisites is FAN_REPORT_FID, which is now merged.
> When events gets reported with fid instead of fd, unprivileged user
> (hopefully) cannot use fid for privilege escalation.
> 
> > I was looking into switching from inotify to fanotify but since it's not usable from
> > non-initial userns it's a no-no
> > since we support nested workloads.
> 
> One of Jan's questions was what is the benefit of using inotify-compatible
> fanotify vs. using inotify.
> So what was the reason you were looking into switching from inotify to fanotify?
> Is it because of mount/filesystem watch? Because making those available for

Yeah. Well, I would need to look but you could probably do it safely for
filesystems mountable in user namespaces (which are few).
Can you do a bind-mount and then place a watch on the bind-mount or is
this superblock based?

Thanks!
Christian