Re: [RFC][PATCH 2/2] fanotify: support limited functionality for unprivileged users

[Jan Kara <jack@suse.cz>]

On Tue 23-02-21 19:16:40, Amir Goldstein wrote:
> On Fri, Feb 19, 2021 at 6:16 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Tue, Feb 16, 2021 at 8:12 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Tue, Feb 16, 2021 at 7:01 PM Jan Kara <jack@suse.cz> wrote:
> > > >
> > > > On Sun 24-01-21 20:42:04, Amir Goldstein wrote:
> > > > > Add limited support for unprivileged fanotify event listener.
> > > > > An unprivileged event listener does not get an open file descriptor in
> > > > > the event nor the process pid of another process.  An unprivileged event
> > > > > listener cannot request permission events, cannot set mount/filesystem
> > > > > marks and cannot request unlimited queue/marks.
> > > > >
> > > > > This enables the limited functionality similar to inotify when watching a
> > > > > set of files and directories for OPEN/ACCESS/MODIFY/CLOSE events, without
> > > > > requiring SYS_CAP_ADMIN privileges.
> > > > >
> > > > > The FAN_REPORT_DFID_NAME init flag, provide a method for an unprivileged
> > > > > event listener watching a set of directories (with FAN_EVENT_ON_CHILD)
> > > > > to monitor all changes inside those directories.
> > > > >
> > > > > This typically requires that the listener keeps a map of watched directory
> > > > > fid to dirfd (O_PATH), where fid is obtained with name_to_handle_at()
> > > > > before starting to watch for changes.
> > > > >
> > > > > When getting an event, the reported fid of the parent should be resolved
> > > > > to dirfd and fstatsat(2) with dirfd and name should be used to query the
> > > > > state of the filesystem entry.
> > > > >
> > > > > Note that even though events do not report the event creator pid,
> > > > > fanotify does not merge similar events on the same object that were
> > > > > generated by different processes. This is aligned with exiting behavior
> > > > > when generating processes are outside of the listener pidns (which
> > > > > results in reporting 0 pid to listener).
> > > > >
> > > > > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > >
> > > > The patch looks mostly good to me. Just two questions:
> > > >
> > > > a) Remind me please, why did we decide pid isn't safe to report to
> > > > unpriviledged listeners?
> > >
> > > Just because the information that process X modified file Y is not an
> > > information that user can generally obtain without extra capabilities(?)
> > > I can add a flag FAN_REPORT_OWN_PID to make that behavior
> > > explicit and then we can relax reporting pids later.
> > >
> >
> > FYI a patch for flag FAN_REPORT_SELF_PID is pushed to branch
> > fanotify_unpriv.
> >
> > The UAPI feels a bit awkward with this flag, but that is the easiest way
> > to start without worrying about disclosing pids.
> >
> > I guess we can require that unprivileged listener has pid 1 in its own
> > pid ns. The outcome is similar to FAN_REPORT_SELF_PID, except
> > it can also get pids of its children which is probably fine.
> >
> 
> Jan,
> 
> WRT your comment in github:
> "So maybe we can just require that this flag is already set by userspace
> instead of silently setting it? Like:
> 
> if (!(flags & FAN_REPORT_SELF_PID)) return -EPERM;
> 
> I'd say that variant is more futureproof and the difference for user
> is minimal."
> 
> I started with this approach and then I wrote the tests and imagined
> the man page
> requiring this flag would be a bit awkward, so I changed it to auto-enable.
> 
> I am not strongly against the more implicit flag requirement, but in
> favor of the
> auto-enable approach I would like to argue that with current fanotify you CAN
> get zero pid in event, so think about it this way:
> If a listener is started in (or moved into) its own pid ns, it will
> get zero pid in all
> events (other than those generated by itself and its own children).
> 
> With the proposed change, the same applies also if the listener is started
> without CAP_SYS_ADMIN.
> 
> As a matter of fact, we do not need the flag at all, we can determine whether
> or not to report pid according to capabilities of the event reader at
> event read time.
> And we can check for one of:
> - CAP_SYS_ADMIN
> - CAP_SYS_PACCT
> - CAP_SYS_PTRACE
> 
> Do you prefer this flag-less approach?

Well, I don't have strong opinion what we should do internally either. The
flag seems OK to me. The biggest question is whether we should expose the
FAN_REPORT_SELF_PID flag to userspace or not. If we would not require
explicit flag for unpriv users, I see little reason to expose that flag at
all.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR