Re: [PATCH v3 5/5] fanotify: add pidfd support to the fanotify API

[Jan Kara <jack@suse.cz>]

On Tue 03-08-21 11:37:53, Christian Brauner wrote:
> On Mon, Aug 02, 2021 at 05:38:20PM +0300, Amir Goldstein wrote:
> > On Mon, Aug 2, 2021 at 3:34 PM Jan Kara <jack@suse.cz> wrote:
> > >
> > > On Fri 30-07-21 08:03:01, Amir Goldstein wrote:
> > > > On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > On Thu, Jul 29, 2021 at 4:39 PM Jan Kara <jack@suse.cz> wrote:
> > > > > > Well, but pidfd also makes sure that /proc/<pid>/ keeps belonging to the
> > > > > > same process while you read various data from it. And you cannot achieve
> > > > > > that with pid+generation thing you've suggested. Plus the additional
> > > > > > concept and its complexity is non-trivial So I tend to agree with
> > > > > > Christian that we really want to return pidfd.
> > > > > >
> > > > > > Given returning pidfd is CAP_SYS_ADMIN priviledged operation I'm undecided
> > > > > > whether it is worth the trouble to come up with some other mechanism how to
> > > > > > return pidfd with the event. We could return some cookie which could be
> > > > > > then (by some ioctl or so) either transformed into real pidfd or released
> > > > > > (so that we can release pid handle in the kernel) but it looks ugly and
> > > > > > complicates things for everybody without bringing significant security
> > > > > > improvement (we already can pass fd with the event). So I'm pondering
> > > > > > whether there's some other way how we could make the interface safer - e.g.
> > > > > > so that the process receiving the event (not the one creating the group)
> > > > > > would also need to opt in for getting fds created in its file table.
> > > > > >
> > > > > > But so far nothing bright has come to my mind. :-|
> > > > > >
> > > > >
> > > > > There is a way, it is not bright, but it is pretty simple -
> > > > > store an optional pid in group->fanotify_data.fd_reader.
> > > > >
> > > > > With flag FAN_REPORT_PIDFD, both pidfd and event->fd reporting
> > > > > will be disabled to any process other than fd_reader.
> > > > > Without FAN_REPORT_PIDFD, event->fd reporting will be disabled
> > > > > if fd_reaader is set to a process other than the reader.
> > > > >
> > > > > A process can call ioctl START_FD_READER to set fd_reader to itself.
> > > > > With FAN_REPORT_PIDFD, if reaader_fd is NULL and the reader
> > > > > process has CAP_SYS_ADMIN, read() sets fd_reader to itself.
> > > > >
> > > > > Permission wise, START_FD_READER is allowed with
> > > > > CAP_SYS_ADMIN or if fd_reader is not owned by another process.
> > > > > We may consider YIELD_FD_READER ioctl if needed.
> > > > >
> > > > > I think that this is a pretty cheap price for implementation
> > > > > and maybe acceptable overhead for complicating the API?
> > > > > Note that without passing fd, there is no need for any ioctl.
> > > > >
> > > > > An added security benefit is that the ioctl adds is a way for the
> > > > > caller of fanotify_init() to make sure that even if the fanotify_fd is
> > > > > leaked, that event->fd will not be leaked, regardless of flag
> > > > > FAN_REPORT_PIDFD.
> > > > >
> > > > > So the START_FD_READER ioctl feature could be implemented
> > > > > and documented first.
> > > > > And then FAN_REPORT_PIDFD could use the feature with a
> > > > > very minor API difference:
> > > > > - Without the flag, other processes can read fds by default and
> > > > >   group initiator can opt-out
> > > > > - With the flag, other processes cannot read fds by default and
> > > > >   need to opt-in
> > > >
> > > > Or maybe something even simpler... fanotify_init() flag
> > > > FAN_PRIVATE (or FAN_PROTECTED) that limits event reading
> > > > to the initiator process (not only fd reading).
> > > >
> > > > FAN_REPORT_PIDFD requires FAN_PRIVATE.
> > > > If we do not know there is a use case for passing fanotify_fd
> > > > that reports pidfds to another process why implement the ioctl.
> > > > We can always implement it later if the need arises.
> > > > If we contemplate this future change, though, maybe the name
> > > > FAN_PROTECTED is better to start with.
> > >
> > > Good ideas. I think we are fine with returning pidfd only to the process
> > > creating the fanotify group. Later we can add an ioctl which would indicate
> > > that the process is also prepared to have fds created in its file table.
> > > But I have still some open questions:
> > > Do we want threads of the same process to still be able to receive fds?
> > 
> > I don't see why not.
> > They will be bloating the same fd table as the thread that called
> > fanotify_init().
> > 
> > > Also pids can be recycled so they are probably not completely reliable
> > > identifiers?
> > 
> > Not sure I follow. The group hold a refcount on struct pid of the process that
> > called fanotify_init() - I think that can used to check if reader process is
> > the same process, but not sure. Maybe there is another way (Christian?).
> 
> If the fanotify group hold's a reference to struct pid it won't get
> recycled. And it can be used to check if the reader thread is the same
> thread with some care. You also have to be specific what exactly you
> want to know.  If you're asking if the reading process is the same as
> the fanotify_init() process you can be asking one of two things.
> 
> You can be asking if the reader is a thread in the same thread-group as
> the thread that called fanotify_init(). In that case you might need to
> do something like
> 
> rcu_read_lock();
> struct task_struct *fanotify_init_task_struct = pid_task(stashed_struct_pid, PIDTYPE_PID);
> if (!fanotify_init_task_struct) {
> 	/* The thread which called fanotify_init() has died already. */ 
> 	return -ESRCH;
> }
> if (same_thread_group(fanotify_init_task_struct, current))
> rcu_read_unlock();
> 
> though thinking about it makes me realise that there's a corner case. If
> the thread that called fanotify_init() is a thread in a non-empty
> thread-group it can already have died and been reaped. This would mean,
> pid_task(..., PIDTYPE_PID) will return NULL but there are still other
> threads alive in the thread-group. Handling that case might be a bit
> complicated.
> 
> If you're asking whether the reading thread is really the same as the
> thread that created the fanotify instance then you might need to do sm
> like
> 
> rcu_read_lock();
> if (pid_task(stashed_struct_pid, PIDTYPE_PID) == current)
> rcu_read_unlock();
> 
> Just for completeness if I remember all of this right: there's a corner
> case because of how de_thread() works.
> During exec the thread that is execing will assume the struct pid of the
> old thread-group leader. (All other threads in the same thread-group
> will get killed.)
> Assume the thread that created the fanotify instance is not the
> thread-group leader in its non-empty thread-group. And further assume it
> exec's. Then it will assume the struct pid of the old thread-group
> leader during de_thread().
> Assume the thread inherits the fanotify fd across the exec. Now, when it
> tries to read a new event after the exec then pid_task() will return
> NULL.
> However, if the thread was already the thread-group leader before the
> exec then pid_task() will return the same task struct as before after
> the exec (because no struct pid swapping needed to take place).
> 
> I hope this causes more clarity then confusion. :)

Thanks for the details Christian! After some more reading and your comments
I think the situation is relatively clear. What we could do is:

On fanotify_init(2) do get_task_pid(current, PIDTYPE_TGID) and stash it. On
read from fanotify fd do
	rcu_read_lock();
	if (rcu_dereference(*task_pid_ptr(current, PIDTYPE_TGID)) !=
								stashed_pid) {
		rcu_read_unlock();
		return -EPERM;
	}
	rcu_read_unlock();

This should avoid all the nasty cornercases. So it can be done but I have
more and more doubts whether it is all worth it.

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