fanotify: fix permission model of unprivileged group

Reporting event->pid should depend on the privileges of the user that
initialized the group, not the privileges of the user reading the
events.

Use an internal group flag FANOTIFY_UNPRIV to record the fact that the
group was initialized by an unprivileged user.

To be on the safe side, the premissions to setup filesystem and mount
marks now require that both the user that initialized the group and
the user setting up the mark have CAP_SYS_ADMIN.

Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxiA77_P5vtv7e83g0+9d7B5W9ZTE4GfQEYbWmfT1rA=[e-mail]/
Fixes: 7cea2a3c505e ("fanotify: support limited functionality for unprivileged users")
Cc: <[e-mail]> # v5.12+
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Matthew Bobrowski <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Signed-off-by: Amir Goldstein <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>

fs/notify/fanotify/fanotify_user.c

···
 	 * events generated by the listener process itself, without disclosing
 	 * the pids of other processes.
 	 */
-	if (!capable(CAP_SYS_ADMIN) &&
+	if (FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
 	    task_tgid(current) != event->pid)
 		metadata.pid = 0;
 
-	if (path && path->mnt && path->dentry) {
+	/*
+	 * For now, fid mode is required for an unprivileged listener and
+	 * fid mode does not report fd in events.  Keep this check anyway
+	 * for safety in case fid mode requirement is relaxed in the future
+	 * to allow unprivileged listener to get events with no fd and no fid.
+	 */
+	if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
+	    path && path->mnt && path->dentry) {
 		fd = create_fd(group, path, &f);
 		if (fd < 0)
 			return fd;
···
 	int f_flags, fd;
 	unsigned int fid_mode = flags & FANOTIFY_FID_BITS;
 	unsigned int class = flags & FANOTIFY_CLASS_BITS;
+	unsigned int internal_flags = 0;
 
 	pr_debug("%s: flags=%x event_f_flags=%x\n",
 		 __func__, flags, event_f_flags);
···
 		 */
 		if ((flags & FANOTIFY_ADMIN_INIT_FLAGS) || !fid_mode)
 			return -EPERM;
+
+		/*
+		 * Setting the internal flag FANOTIFY_UNPRIV on the group
+		 * prevents setting mount/filesystem marks on this group and
+		 * prevents reporting pid and open fd in events.
+		 */
+		internal_flags |= FANOTIFY_UNPRIV;
 	}
 
 #ifdef CONFIG_AUDITSYSCALL
···
 		goto out_destroy_group;
 	}
 
-	group->fanotify_data.flags = flags;
+	group->fanotify_data.flags = flags | internal_flags;
 	group->memcg = get_mem_cgroup_from_mm(current->mm);
 
 	group->fanotify_data.merge_hash = fanotify_alloc_merge_hash();
···
 	group = f.file->private_data;
 
 	/*
-	 * An unprivileged user is not allowed to watch a mount point nor
-	 * a filesystem.
+	 * An unprivileged user is not allowed to setup mount nor filesystem
+	 * marks.  This also includes setting up such marks by a group that
+	 * was initialized by an unprivileged user.
 	 */
 	ret = -EPERM;
-	if (!capable(CAP_SYS_ADMIN) &&
+	if ((!capable(CAP_SYS_ADMIN) ||
+	     FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV)) &&
 	    mark_type != FAN_MARK_INODE)
 		goto fput_and_out;
 
···
 	max_marks = clamp(max_marks, FANOTIFY_OLD_DEFAULT_MAX_MARKS,
 				     FANOTIFY_DEFAULT_MAX_USER_MARKS);
 
+	BUILD_BUG_ON(FANOTIFY_INIT_FLAGS & FANOTIFY_INTERNAL_GROUP_FLAGS);
 	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
 	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
 

fs/notify/fdinfo.c

···
 	struct fsnotify_group *group = f->private_data;
 
 	seq_printf(m, "fanotify flags:%x event-flags:%x\n",
-		   group->fanotify_data.flags,
+		   group->fanotify_data.flags & FANOTIFY_INIT_FLAGS,
 		   group->fanotify_data.f_flags);
 
 	show_fdinfo(m, f, fanotify_fdinfo);

include/linux/fanotify.h

···
 #define FANOTIFY_INIT_FLAGS	(FANOTIFY_ADMIN_INIT_FLAGS | \
 				 FANOTIFY_USER_INIT_FLAGS)
 
+/* Internal group flags */
+#define FANOTIFY_UNPRIV		0x80000000
+#define FANOTIFY_INTERNAL_GROUP_FLAGS	(FANOTIFY_UNPRIV)
+
 #define FANOTIFY_MARK_TYPE_BITS	(FAN_MARK_INODE | FAN_MARK_MOUNT | \
 				 FAN_MARK_FILESYSTEM)