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@xxxxxxx> wrote:
> >
> > On Fri 30-07-21 08:03:01, Amir Goldstein wrote:
> > > On Thu, Jul 29, 2021 at 6:13 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> > > > On Thu, Jul 29, 2021 at 4:39 PM Jan Kara <jack@xxxxxxx> 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. :)
Christian
