Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s

[Amir Goldstein]

On Wed, May 22, 2019 at 9:57 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
>
> On May 22, 2019 8:29:37 PM GMT+02:00, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> >On Wed, May 22, 2019 at 7:32 PM Christian Brauner
> ><christian@xxxxxxxxxx> 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@xxxxxxxxxx>
> >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@xxxxxxxxxx>
> >
> >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
unprivileged user sounds risky...

Thanks,
Amir.