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

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Feb 24, 2021 at 12:52 PM Jan Kara <jack@suse.cz> wrote:
>
> 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.
>

IMO the only listeners that actually care about event->pid are permission
event listeners. I think that FAN_CLASS_NOTIF listeners do not care
about it. The only thing that *I* ever used event->pid for is to ignore events
from self pid.

My question is, do you mind if we start with this code:

@@ -419,6 +419,14 @@ static ssize_t copy_event_to_user(struct
fsnotify_group *group,
        metadata.reserved = 0;
        metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS;
        metadata.pid = pid_vnr(event->pid);
+
+        /*
+         * For an unprivileged listener, event->pid can be used to identify the
+         * events generated by the listener process itself, without disclosing
+         * the pids of other processes.
+         */
+        if (!capable(CAP_SYS_ADMIN) &&
+            task_tgid(current) != event->pid)
+                metadata.pid = 0;

No need for any visible or invisible user flags.
If users ask for event->pid of other processes later (I don't think they will)
and we decide that it is safe to disclose them, we will require another flag
and then the test will become:

+        if (!capable(CAP_SYS_ADMIN) ||
+            FAN_GROUP_FLAG(group, FAN_REPORT_PID))

and *if* that ever happens, we will document the FAN_REPORT_PID
flag and say that it is enabled by default for CAP_SYS_ADMIN instead of
requiring and documenting FAN_REPORT_SELF_PID now.

The way I see it, the only disadvantage with this negated approach is
that CAP_SYS_ADMIN listeners cannot turn off event->pid reporting,
but why would anybody need to do that?

Thanks,
Amir.