[RFC][PATCH 2/2] fanotify: support limited functionality for unprivileged users

[Amir Goldstein <amir73il@gmail.com>]

Add limited support for unprivileged fanotify event listener.
An unprivileged event listener does not get an open file descriptor in
the event nor the process pid of another process.  An unprivileged event
listener cannot request permission events, cannot set mount/filesystem
marks and cannot request unlimited queue/marks.

This enables the limited functionality similar to inotify when watching a
set of files and directories for OPEN/ACCESS/MODIFY/CLOSE events, without
requiring SYS_CAP_ADMIN privileges.

The FAN_REPORT_DFID_NAME init flag, provide a method for an unprivileged
event listener watching a set of directories (with FAN_EVENT_ON_CHILD)
to monitor all changes inside those directories.

This typically requires that the listener keeps a map of watched directory
fid to dirfd (O_PATH), where fid is obtained with name_to_handle_at()
before starting to watch for changes.

When getting an event, the reported fid of the parent should be resolved
to dirfd and fstatsat(2) with dirfd and name should be used to query the
state of the filesystem entry.

Note that even though events do not report the event creator pid,
fanotify does not merge similar events on the same object that were
generated by different processes. This is aligned with exiting behavior
when generating processes are outside of the listener pidns (which
results in reporting 0 pid to listener).

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/notify/fanotify/fanotify_user.c | 49 +++++++++++++++++++++++++++---
 fs/notify/fdinfo.c                 |  3 +-
 include/linux/fanotify.h           | 16 ++++++++++
 3 files changed, 62 insertions(+), 6 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 4ade3f9df337..b70de273eedb 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -397,9 +397,21 @@ 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);
+	/*
+	 * An unprivileged event listener does not get an open file descriptor
+	 * in the event nor another generating process pid. If the event was
+	 * generated by the unprivileged process itself, self pid is reported.
+	 * We may relax this in the future by checking calling process access
+	 * permissions to the object.
+	 */
+	if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) ||
+	    task_tgid(current) == event->pid)
+		metadata.pid = pid_vnr(event->pid);
+	else
+		metadata.pid = 0;
 
-	if (path && path->mnt && path->dentry) {
+	if (!FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
+	    path && path->mnt && path->dentry) {
 		fd = create_fd(group, path, &f);
 		if (fd < 0)
 			return fd;
@@ -995,12 +1007,29 @@ 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);
 
-	if (!capable(CAP_SYS_ADMIN))
-		return -EPERM;
+	if (!capable(CAP_SYS_ADMIN)) {
+		/*
+		 * An unprivileged user can setup an unprivileged listener with
+		 * limited functionality - an unprivileged event listener cannot
+		 * request permission events, cannot set mount/filesystem marks
+		 * and cannot request unlimited queue/marks.
+		 */
+		if ((flags & ~FANOTIFY_UNPRIV_INIT_FLAGS) ||
+		    class != FAN_CLASS_NOTIF)
+			return -EPERM;
+
+		/*
+		 * We set the internal flag FANOTIFY_UNPRIV on the group, so we
+		 * know that we need to limit setting mount/filesystem marks on
+		 * this group and avoid providing pid and open fd in the event.
+		 */
+		internal_flags |= FANOTIFY_UNPRIV;
+	}
 
 #ifdef CONFIG_AUDITSYSCALL
 	if (flags & ~(FANOTIFY_INIT_FLAGS | FAN_ENABLE_AUDIT))
@@ -1051,7 +1080,7 @@ 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->overflow_event = fanotify_alloc_overflow_event();
@@ -1247,6 +1276,15 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 		goto fput_and_out;
 	group = f.file->private_data;
 
+	/*
+	 * An unprivileged event listener is not allowed to watch a mount
+	 * point nor a filesystem.
+	 */
+	ret = -EPERM;
+	if (FAN_GROUP_FLAG(group, FANOTIFY_UNPRIV) &&
+	    mark_type != FAN_MARK_INODE)
+		goto fput_and_out;
+
 	/*
 	 * group->priority == FS_PRIO_0 == FAN_CLASS_NOTIF.  These are not
 	 * allowed to set permissions events.
@@ -1379,6 +1417,7 @@ SYSCALL32_DEFINE6(fanotify_mark,
  */
 static int __init fanotify_user_setup(void)
 {
+	BUILD_BUG_ON(FANOTIFY_INIT_FLAGS & FANOTIFY_INTERNAL_FLAGS);
 	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_INIT_FLAGS) != 10);
 	BUILD_BUG_ON(HWEIGHT32(FANOTIFY_MARK_FLAGS) != 9);
 
diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
index f0d6b54be412..57f0d5d9f934 100644
--- a/fs/notify/fdinfo.c
+++ b/fs/notify/fdinfo.c
@@ -144,7 +144,8 @@ 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.f_flags);
+		   group->fanotify_data.flags & FANOTIFY_INIT_FLAGS,
+		   group->fanotify_data.f_flags);
 
 	show_fdinfo(m, f, fanotify_fdinfo);
 }
diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index 031a97d8369a..a573c1028c14 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -28,6 +28,22 @@ extern struct ctl_table fanotify_table[]; /* for sysctl */
 				 FAN_CLOEXEC | FAN_NONBLOCK | \
 				 FAN_UNLIMITED_QUEUE | FAN_UNLIMITED_MARKS)
 
+/* Internal flags */
+#define FANOTIFY_UNPRIV		0x80000000
+#define FANOTIFY_INTERNAL_FLAGS	(FANOTIFY_UNPRIV)
+
+/*
+ * fanotify_init() flags allowed for unprivileged listener.
+ * FAN_CLASS_NOTIF in this mask is purely semantic because it is zero,
+ * but it is the only class we allow for unprivileged listener.
+ * Since unprivileged listener does not provide file descriptors in events,
+ * reporting file handles makes sense, but it is not a must.
+ * FAN_REPORT_TID does not make sense for unprivileged listener, which uses
+ * event->pid only to filter out events generated by listener process itself.
+ */
+#define FANOTIFY_UNPRIV_INIT_FLAGS	(FAN_CLOEXEC | FAN_NONBLOCK | \
+					 FAN_CLASS_NOTIF | FANOTIFY_FID_BITS)
+
 #define FANOTIFY_MARK_TYPE_BITS	(FAN_MARK_INODE | FAN_MARK_MOUNT | \
 				 FAN_MARK_FILESYSTEM)
 
-- 
2.25.1