Re: [PATCH 3/7] vfs: Add a mount-notification facility

Jann Horn
2019-05-28T20:06:53Z

On Tue, May 28, 2019 at 6:05 PM David Howells <dhowells@redhat.com> wrote:
> Add a mount notification facility whereby notifications about changes in
> mount topology and configuration can be received. Note that this only
> covers vfsmount topology changes and not superblock events. A separate
> facility will be added for that.
[...]
> @@ -172,4 +167,18 @@ static inline void notify_mount(struct mount *changed,
> u32 info_flags)
> {
> atomic_inc(&changed->mnt_notify_counter);
> +
> +#ifdef CONFIG_MOUNT_NOTIFICATIONS
> + {
> + struct mount_notification n = {
> + .watch.type = WATCH_TYPE_MOUNT_NOTIFY,
> + .watch.subtype = subtype,
> + .watch.info = info_flags | sizeof(n),
> + .triggered_on = changed->mnt_id,
> + .changed_mount = aux ? aux->mnt_id : 0,
> + };
> +
> + post_mount_notification(changed, &n);
> + }
> +#endif
> }
[...]
> +void post_mount_notification(struct mount *changed,
> + struct mount_notification *notify)
> +{
> + const struct cred *cred = current_cred();

This current_cred() looks bogus to me. Can't mount topology changes
come from all sorts of places? For example, umount_mnt() from
umount_tree() from dissolve_on_fput() from __fput(), which could
happen pretty much anywhere depending on where the last reference gets
dropped?

> + struct path cursor;
> + struct mount *mnt;
> + unsigned seq;
> +
> + seq = 0;
> + rcu_read_lock();
> +restart:
> + cursor.mnt = &changed->mnt;
> + cursor.dentry = changed->mnt.mnt_root;
> + mnt = real_mount(cursor.mnt);
> + notify->watch.info &= ~WATCH_INFO_IN_SUBTREE;
> +
> + read_seqbegin_or_lock(&rename_lock, &seq);
> + for (;;) {
> + if (mnt->mnt_watchers &&
> + !hlist_empty(&mnt->mnt_watchers->watchers)) {
> + if (cursor.dentry->d_flags & DCACHE_MOUNT_WATCH)
> + post_watch_notification(mnt->mnt_watchers,
> + &notify->watch, cred,
> + (unsigned long)cursor.dentry);
> + } else {
> + cursor.dentry = mnt->mnt.mnt_root;
> + }
> + notify->watch.info |= WATCH_INFO_IN_SUBTREE;
> +
> + if (cursor.dentry == cursor.mnt->mnt_root ||
> + IS_ROOT(cursor.dentry)) {
> + struct mount *parent = READ_ONCE(mnt->mnt_parent);
> +
> + /* Escaped? */
> + if (cursor.dentry != cursor.mnt->mnt_root)
> + break;
> +
> + /* Global root? */
> + if (mnt != parent) {
> + cursor.dentry = READ_ONCE(mnt->mnt_mountpoint);
> + mnt = parent;
> + cursor.mnt = &mnt->mnt;
> + continue;
> + }
> + break;

(nit: this would look clearer if you inverted the condition and wrote
it as "if (mnt == parent) break;", then you also wouldn't need that
"continue" or the braces)

> + }
> +
> + cursor.dentry = cursor.dentry->d_parent;
> + }
> +
> + if (need_seqretry(&rename_lock, seq)) {
> + seq = 1;
> + goto restart;
> + }
> +
> + done_seqretry(&rename_lock, seq);
> + rcu_read_unlock();
> +}
[...]
> +SYSCALL_DEFINE5(mount_notify,
> + int, dfd,
> + const char __user *, filename,
> + unsigned int, at_flags,
> + int, watch_fd,
> + int, watch_id)
> +{
> + struct watch_queue *wqueue;
> + struct watch_list *wlist = NULL;
> + struct watch *watch;
> + struct mount *m;
> + struct path path;
> + int ret;
> +
> + if (watch_id < -1 || watch_id > 0xff)
> + return -EINVAL;
> +
> + ret = user_path_at(dfd, filename, at_flags, &path);

The third argument of user_path_at() contains kernel-private lookup
flags, I'm pretty sure userspace isn't supposed to be able to control
these directly.

> + if (ret)
> + return ret;
> +
> + wqueue = get_watch_queue(watch_fd);
> + if (IS_ERR(wqueue))
> + goto err_path;
> +
> + m = real_mount(path.mnt);
> +
> + if (watch_id >= 0) {
> + if (!m->mnt_watchers) {
> + wlist = kzalloc(sizeof(*wlist), GFP_KERNEL);
> + if (!wlist)
> + goto err_wqueue;
> + INIT_HLIST_HEAD(&wlist->watchers);
> + spin_lock_init(&wlist->lock);
> + wlist->release_watch = release_mount_watch;
> + }
> +
> + watch = kzalloc(sizeof(*watch), GFP_KERNEL);
> + if (!watch)
> + goto err_wlist;
> +
> + init_watch(watch, wqueue);
> + watch->id = (unsigned long)path.dentry;
> + watch->private = path.mnt;
> + watch->info_id = (u32)watch_id << 24;
> +
> + down_write(&m->mnt.mnt_sb->s_umount);
> + if (!m->mnt_watchers) {
> + m->mnt_watchers = wlist;
> + wlist = NULL;
> + }
> +
> + ret = add_watch_to_object(watch, m->mnt_watchers);
> + if (ret == 0) {
> + spin_lock(&path.dentry->d_lock);
> + path.dentry->d_flags |= DCACHE_MOUNT_WATCH;
> + spin_unlock(&path.dentry->d_lock);
> + path_get(&path);

So... the watches on a mountpoint create references back to the
mountpoint? Is your plan that umount_tree() breaks the loop by getting
rid of the watches?

If so: Is there anything that prevents installing new watches after
umount_tree()? Because I don't see anything.

It might make sense to redesign this stuff so that watches don't hold
references on the object being watched.

> + }
> + up_write(&m->mnt.mnt_sb->s_umount);
> + if (ret < 0)
> + kfree(watch);
> + } else if (m->mnt_watchers) {
> + down_write(&m->mnt.mnt_sb->s_umount);
> + ret = remove_watch_from_object(m->mnt_watchers, wqueue,
> + (unsigned long)path.dentry,
> + false);
> + up_write(&m->mnt.mnt_sb->s_umount);
> + } else {
> + ret = -EBADSLT;
> + }
> +
> +err_wlist:
> + kfree(wlist);
> +err_wqueue:
> + put_watch_queue(wqueue);
> +err_path:
> + path_put(&path);
> + return ret;
> +}