Re: [PATCH 2/2] fanotify: Add pidfd support to the fanotify API

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Apr 16, 2021 at 10:06 AM Matthew Bobrowski <repnop@google.com> wrote:
>
> On Fri, Apr 16, 2021 at 09:27:03AM +0300, Amir Goldstein wrote:
> > On Fri, Apr 16, 2021 at 2:22 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 is to be
> > > returned instead of a pid for `struct fanotify_event_metadata.pid`.
> > >
> > > FAN_REPORT_PIDFD is mutually exclusive with FAN_REPORT_TID as the
> > > pidfd API is currently restricted to only support pidfd generation for
> > > thread-group leaders. Attempting to set them both when calling
> > > fanotify_init(2) will result in -EINVAL being returned to the
> > > caller. As the pidfd API evolves and support is added for tids, this
> > > is something that could be relaxed in the future.
> > >
> > > If pidfd creation fails, the pid in struct fanotify_event_metadata is
> > > set to FAN_NOPIDFD(-1).
> >
> > Hi Matthew,
> >
> > All in all looks good, just a few small nits.
>
> Thanks for feedback Amir! :)
>
> > > Falling back and providing a pid instead of a
> > > pidfd on pidfd creation failures was considered, although this could
> > > possibly lead to confusion and unpredictability within userspace
> > > applications as distinguishing between whether an actual pidfd or pid
> > > was returned could be difficult, so it's best to be explicit.
> >
> > I don't think this should have been even "considered" so I see little
> > value in this paragraph in commit message.
>
> Fair point. I will discard this sentence for all subsequent iterations
> of this patch series. I guess the idea was that this patch series was
> meant to be labeled as being "RFC", so some extra thoughts had been
> noted.
>
> > > Signed-off-by: Matthew Bobrowski <repnop@google.com>
> > > ---
> > >  fs/notify/fanotify/fanotify_user.c | 33 +++++++++++++++++++++++++++---
> > >  include/linux/fanotify.h           |  2 +-
> > >  include/uapi/linux/fanotify.h      |  2 ++
> > >  3 files changed, 33 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> > > index 9e0c1afac8bd..fd8ae88796a8 100644
> > > --- a/fs/notify/fanotify/fanotify_user.c
> > > +++ b/fs/notify/fanotify/fanotify_user.c
> > > @@ -329,7 +329,7 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> > >         struct fanotify_info *info = fanotify_event_info(event);
> > >         unsigned int fid_mode = FAN_GROUP_FLAG(group, FANOTIFY_FID_BITS);
> > >         struct file *f = NULL;
> > > -       int ret, fd = FAN_NOFD;
> > > +       int ret, pidfd, fd = FAN_NOFD;
> > >         int info_type = 0;
> > >
> > >         pr_debug("%s: group=%p event=%p\n", __func__, group, event);
> > > @@ -340,7 +340,25 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
> > >         metadata.vers = FANOTIFY_METADATA_VERSION;
> > >         metadata.reserved = 0;
> > >         metadata.mask = event->mask & FANOTIFY_OUTGOING_EVENTS;
> > > -       metadata.pid = pid_vnr(event->pid);
> > > +
> > > +       if (FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD) &&
> > > +               pid_has_task(event->pid, PIDTYPE_TGID)) {
> > > +               /*
> > > +                * Given FAN_REPORT_PIDFD is to be mutually exclusive with
> > > +                * FAN_REPORT_TID, panic here if the mutual exclusion is ever
> > > +                * blindly lifted without pidfds for threads actually being
> > > +                * supported.
> > > +                */
> > > +               WARN_ON(FAN_GROUP_FLAG(group, FAN_REPORT_TID));
> >
> > Better WARN_ON_ONCE() the outcome of this error is not terrible.
> > Also in the comment above I would not refer to this warning as "panic".
>
> ACK.
>
> > > +
> > > +               pidfd = pidfd_create(event->pid, 0);
> > > +               if (unlikely(pidfd < 0))
> > > +                       metadata.pid = FAN_NOPIDFD;
> > > +               else
> > > +                       metadata.pid = pidfd;
> > > +       } else {
> > > +               metadata.pid = pid_vnr(event->pid);
> > > +       }
> >
> > You should rebase your work on:
> > git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git fsnotify
> > and resolve conflicts with "unprivileged listener" code.
>
> ACK.
>
> > Need to make sure that pidfd is not reported to an unprivileged
> > listener even if group was initialized by a privileged process.
> > This is a conscious conservative choice that we made for reporting
> > pid info to unprivileged listener that can be revisited in the future.
>
> OK, I see. In that case, I guess I can add the FAN_REPORT_PIDFD check
> above the current conditional [0]:
>
> ...
> if (!capable(CAP_SYS_ADMIN) && task_tgid(current) != event->pid)
>         metadata.pid = 0;
> ...
>
> That way, AFAIK even if it is an unprivileged listener the pid info
> will be overwritten as intended.
>

Situation is a bit more subtle than that.
If you override event->pid with zero and zero is interpreted as pidfd
that would not be consistent with uapi documentation.
You need to make sure that event->pid is FAN_NOPIDFD in case
(!capable(CAP_SYS_ADMIN) &&
 FAN_GROUP_FLAG(group, FAN_REPORT_PIDFD))
Hopefully, you can do that while keeping the special cases to minimum...


> > >
> > >         if (path && path->mnt && path->dentry) {
> > >                 fd = create_fd(group, path, &f);
> > > @@ -941,6 +959,15 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
> > >  #endif
> > >                 return -EINVAL;
> > >
> > > +       /*
> > > +        * A pidfd can only be returned for a thread-group leader; thus
> > > +        * FAN_REPORT_TID and FAN_REPORT_PIDFD need to be mutually
> > > +        * exclusive. Once the pidfd API supports the creation of pidfds on
> > > +        * individual threads, then we can look at removing this conditional.
> > > +        */
> > > +       if ((flags & FAN_REPORT_PIDFD) && (flags & FAN_REPORT_TID))
> > > +               return -EINVAL;
> > > +
> > >         if (event_f_flags & ~FANOTIFY_INIT_ALL_EVENT_F_BITS)
> > >                 return -EINVAL;
> > >
> > > @@ -1312,7 +1339,7 @@ SYSCALL32_DEFINE6(fanotify_mark,
> > >   */
> > >  static int __init fanotify_user_setup(void)
> > >  {
> > > -       BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
> > > +       BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 11);
> > >         BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
> > >
> > >         fanotify_mark_cache = KMEM_CACHE(fsnotify_mark,
> > > diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
> > > index 3e9c56ee651f..894740a6f4e0 100644
> > > --- a/include/linux/fanotify.h
> > > +++ b/include/linux/fanotify.h
> > > @@ -21,7 +21,7 @@
> > >  #define FANOTIFY_FID_BITS      (FAN_REPORT_FID | FAN_REPORT_DFID_NAME)
> > >
> > >  #define FANOTIFY_INIT_FLAGS    (FANOTIFY_CLASS_BITS | FANOTIFY_FID_BITS | \
> > > -                                FAN_REPORT_TID | \
> > > +                                FAN_REPORT_TID | FAN_REPORT_PIDFD | \
> > >                                  FAN_CLOEXEC | FAN_NONBLOCK | \
> > >                                  FAN_UNLIMITED_QUEUE | FAN_UNLIMITED_MARKS)
> > >
> >
> > FAN_REPORT_PIDFD should be added to FANOTIFY_ADMIN_INIT_FLAGS
> > from fsnotify branch.
>
> ACK.
>
> Before sending any other version of this patch series through I will
> see what Jan and Christian have to say.
>

That makes sense.

Thanks,
Amir.