On Wed, Jul 21, 2021 at 8:21 AM Matthew Bobrowski <repnop@xxxxxxxxxx> 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.
I realize that existing fanotify code appears to be violating that
rule already, and that you're limiting creation of fanotify file
descriptors that can hit this codepath to CAP_SYS_ADMIN, but still, I
think fanotify_read() probably ought to be an ioctl, or something
along those lines, instead of an f_op->read handler if it messes with
the caller's fd table?
