Merge tag 'fsnotify_for_v5.13-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs

Pull fsnotify fixes from Jan Kara:
 "A fix for permission checking with fanotify unpriviledged groups.

  Also there's a small update in MAINTAINERS file for fanotify"

* tag 'fsnotify_for_v5.13-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs:
  fanotify: fix permission model of unprivileged group
  MAINTAINERS: Add Matthew Bobrowski as a reviewer

MAINTAINERS

@@ -6945,6 +6945,7 @@ F:	net/core/failover.c
 FANOTIFY
 M:	Jan Kara <jack@suse.cz>
 R:	Amir Goldstein <amir73il@gmail.com>
+R:	Matthew Bobrowski <repnop@google.com>
 L:	linux-fsdevel@vger.kernel.org
 S:	Maintained
 F:	fs/notify/fanotify/

fs/notify/fanotify/fanotify_user.c

@@ -424,11 +424,18 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	 * 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;
@@ -1040,6 +1047,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	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);
@@ -1053,6 +1061,13 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, 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
@@ -1105,7 +1120,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 		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();
@@ -1305,11 +1320,13 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 	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;
 
@@ -1460,6 +1477,7 @@ static int __init fanotify_user_setup(void)
 	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

@@ -144,7 +144,7 @@ void fanotify_show_fdinfo(struct seq_file *m, struct file *f)
 	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

@@ -51,6 +51,10 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */
 #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)