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

[Matthew Bobrowski <repnop@google.com>]

Hey Jann,

On Tue, Jul 27, 2021 at 02:23:38AM +0200, Jann Horn wrote:
> On Wed, Jul 21, 2021 at 8:21 AM Matthew Bobrowski <repnop@google.com> wrote:
> > Introduce a new flag FAN_REPORT_PIDFD for fanotify_init(2) which
> > allows userspace applications to control whether a pidfd info record
> > containing a pidfd is to be returned with each event.
> >
> > If FAN_REPORT_PIDFD is enabled for a notification group, an additional
> > struct fanotify_event_info_pidfd object will be supplied alongside the
> > generic struct fanotify_event_metadata within a single event. This
> > functionality is analogous to that of FAN_REPORT_FID in terms of how
> > the event structure is supplied to the userspace application. Usage of
> > FAN_REPORT_PIDFD with FAN_REPORT_FID/FAN_REPORT_DFID_NAME is
> > permitted, and in this case a struct fanotify_event_info_pidfd object
> > will follow any struct fanotify_event_info_fid object.
> >
> > Currently, the usage of FAN_REPORT_TID is not permitted along with
> > FAN_REPORT_PIDFD as the pidfd API only supports the creation of pidfds
> > for thread-group leaders. Additionally, the FAN_REPORT_PIDFD is
> > limited to privileged processes only i.e. listeners that are running
> > with the CAP_SYS_ADMIN capability. Attempting to supply either of
> > these initialization flags with FAN_REPORT_PIDFD will result with
> > EINVAL being returned to the caller.
> >
> > In the event of a pidfd creation error, there are two types of error
> > values that can be reported back to the listener. There is
> > FAN_NOPIDFD, which will be reported in cases where the process
> > responsible for generating the event has terminated prior to fanotify
> > being able to create pidfd for event->pid via pidfd_create(). The
> > there is FAN_EPIDFD, which will be reported if a more generic pidfd
> > creation error occurred when calling pidfd_create().
> [...]
> > @@ -524,6 +562,34 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> >         }
> >         metadata.fd = fd;
> >
> > +       if (pidfd_mode) {
> > +               /*
> > +                * Complain if the FAN_REPORT_PIDFD and FAN_REPORT_TID mutual
> > +                * exclusion is ever lifted. At the time of incoporating pidfd
> > +                * support within fanotify, the pidfd API only supported the
> > +                * creation of pidfds for thread-group leaders.
> > +                */
> > +               WARN_ON_ONCE(FAN_GROUP_FLAG(group, FAN_REPORT_TID));
> > +
> > +               /*
> > +                * The PIDTYPE_TGID check for an event->pid is performed
> > +                * preemptively in attempt to catch those rare instances where
> > +                * the process responsible for generating the event has
> > +                * terminated prior to calling into pidfd_create() and acquiring
> > +                * a valid pidfd. Report FAN_NOPIDFD to the listener in those
> > +                * cases. All other pidfd creation errors are represented as
> > +                * FAN_EPIDFD.
> > +                */
> > +               if (metadata.pid == 0 ||
> > +                   !pid_has_task(event->pid, PIDTYPE_TGID)) {
> > +                       pidfd = FAN_NOPIDFD;
> > +               } else {
> > +                       pidfd = pidfd_create(event->pid, 0);
> > +                       if (pidfd < 0)
> > +                               pidfd = FAN_EPIDFD;
> > +               }
> > +       }
> > +
> 
> As a general rule, f_op->read callbacks aren't allowed to mess with
> the file descriptor table of the calling process. A process should be
> able to receive a file descriptor from an untrusted source and call
> functions like read() on it without worrying about affecting its own
> file descriptor table state with that.

Interesting, thanks for bringing this up. I never knew about this general
rule. Do you mind elaborating a little on why f_op->read() callbacks aren't
allowed to mess with the fdtable of the calling process? I don't quite
exactly understand why this is considered to be suboptimal.

/M