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

[Matthew Bobrowski <repnop@google.com>]

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.

> >
> >         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.

[0]
https://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git/tree/fs/notify/fanotify/fanotify_user.c?h=fsnotify&id=7cea2a3c505e87a9d6afc78be4a7f7be636a73a7#n427  

/M