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

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Date:   Thu, 23 May 2019 13:25:08 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matthew Bobrowski <mbobrowski@...browski.org>
Subject: Re: [PATCH] fanotify: remove redundant capable(CAP_SYS_ADMIN)s

On Thu, May 23, 2019 at 12:55 PM Christian Brauner <christian@...uner.io> wrote:
>
> 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?
>

Either.
FAN_MARK_MOUNT was there from day 1 of fanotify.
FAN_MARK_FILESYSTEM was merged to Linux Linux 4.20.

But directory modification events that are supported since v5.1 are
not available
with FAN_MARK_MOUNT, see:
https://github.com/amir73il/man-pages/blob/fanotify_fid/man2/fanotify_init.2#L97

Matthew,

Perhaps this fact is worth a mention in the linked entry for FAN_REPORT_FID
in fanotify_init.2 in addition to the comment on the entry for FAN_MARK_MOUNT
in fanotify_mark.2.

Thanks,
Amir.