fanotify: add watchdog for permission events

This is to make it easier to debug issues with AV software, which time and
again deadlocks with no indication of where the issue comes from, and the
kernel being blamed for the deadlock.  Then we need to analyze dumps to
prove that the kernel is not in fact at fault.

The deadlock comes from recursion: handling the event triggers another
permission event, in some roundabout way, obviously, otherwise it would
have been found in testing.

With this patch a warning is printed when permission event is received by
userspace but not answered for more than the timeout specified in
/proc/sys/fs/fanotify/watchdog_timeout.  The watchdog can be turned off by
setting the timeout to zero (which is the default).

The timeout is very coarse (T <= t < 2T) but I guess it's good enough for
the purpose.

Overhead should be minimal.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Link: https://patch.msgid.link/20250909143053.112171-1-mszeredi@redhat.com
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 102 insertions, 0 deletions

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 561339b4cf75..1dadda82cae5 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -50,6 +50,7 @@
 
 /* configurable via /proc/sys/fs/fanotify/ */
 static int fanotify_max_queued_events __read_mostly;
+static int perm_group_timeout __read_mostly;
 
 #ifdef CONFIG_SYSCTL
 
@@ -85,6 +86,14 @@ static const struct ctl_table fanotify_table[] = {
 		.proc_handler	= proc_dointvec_minmax,
 		.extra1		= SYSCTL_ZERO
 	},
+	{
+		.procname	= "watchdog_timeout",
+		.data		= &perm_group_timeout,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+	},
 };
 
 static void __init fanotify_sysctls_init(void)
@@ -95,6 +104,91 @@ static void __init fanotify_sysctls_init(void)
 #define fanotify_sysctls_init() do { } while (0)
 #endif /* CONFIG_SYSCTL */
 
+static LIST_HEAD(perm_group_list);
+static DEFINE_SPINLOCK(perm_group_lock);
+static void perm_group_watchdog(struct work_struct *work);
+static DECLARE_DELAYED_WORK(perm_group_work, perm_group_watchdog);
+
+static void perm_group_watchdog_schedule(void)
+{
+	schedule_delayed_work(&perm_group_work, secs_to_jiffies(perm_group_timeout));
+}
+
+static void perm_group_watchdog(struct work_struct *work)
+{
+	struct fsnotify_group *group;
+	struct fanotify_perm_event *event;
+	struct task_struct *task;
+	pid_t failed_pid = 0;
+
+	guard(spinlock)(&perm_group_lock);
+	if (list_empty(&perm_group_list))
+		return;
+
+	list_for_each_entry(group, &perm_group_list,
+			    fanotify_data.perm_grp_list) {
+		/*
+		 * Ok to test without lock, racing with an addition is
+		 * fine, will deal with it next round
+		 */
+		if (list_empty(&group->fanotify_data.access_list))
+			continue;
+
+		spin_lock(&group->notification_lock);
+		list_for_each_entry(event, &group->fanotify_data.access_list,
+				    fae.fse.list) {
+			if (likely(event->watchdog_cnt == 0)) {
+				event->watchdog_cnt = 1;
+			} else if (event->watchdog_cnt == 1) {
+				/* Report on event only once */
+				event->watchdog_cnt = 2;
+
+				/* Do not report same pid repeatedly */
+				if (event->recv_pid == failed_pid)
+					continue;
+
+				failed_pid = event->recv_pid;
+				rcu_read_lock();
+				task = find_task_by_pid_ns(event->recv_pid,
+							   &init_pid_ns);
+				pr_warn_ratelimited(
+					"PID %u (%s) failed to respond to fanotify queue for more than %d seconds\n",
+					event->recv_pid,
+					task ? task->comm : NULL,
+					perm_group_timeout);
+				rcu_read_unlock();
+			}
+		}
+		spin_unlock(&group->notification_lock);
+	}
+	perm_group_watchdog_schedule();
+}
+
+static void fanotify_perm_watchdog_group_remove(struct fsnotify_group *group)
+{
+	if (!list_empty(&group->fanotify_data.perm_grp_list)) {
+		/* Perm event watchdog can no longer scan this group. */
+		spin_lock(&perm_group_lock);
+		list_del_init(&group->fanotify_data.perm_grp_list);
+		spin_unlock(&perm_group_lock);
+	}
+}
+
+static void fanotify_perm_watchdog_group_add(struct fsnotify_group *group)
+{
+	if (!perm_group_timeout)
+		return;
+
+	spin_lock(&perm_group_lock);
+	if (list_empty(&group->fanotify_data.perm_grp_list)) {
+		/* Add to perm_group_list for monitoring by watchdog. */
+		if (list_empty(&perm_group_list))
+			perm_group_watchdog_schedule();
+		list_add_tail(&group->fanotify_data.perm_grp_list, &perm_group_list);
+	}
+	spin_unlock(&perm_group_lock);
+}
+
 /*
  * All flags that may be specified in parameter event_f_flags of fanotify_init.
  *
@@ -953,6 +1047,7 @@ static ssize_t fanotify_read(struct file *file, char __user *buf,
 				spin_lock(&group->notification_lock);
 				list_add_tail(&event->fse.list,
 					&group->fanotify_data.access_list);
+				FANOTIFY_PERM(event)->recv_pid = current->pid;
 				spin_unlock(&group->notification_lock);
 			}
 		}
@@ -1012,6 +1107,8 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 	 */
 	fsnotify_group_stop_queueing(group);
 
+	fanotify_perm_watchdog_group_remove(group);
+
 	/*
 	 * Process all permission events on access_list and notification queue
 	 * and simulate reply from userspace.
@@ -1465,6 +1562,10 @@ out:
 	fsnotify_group_unlock(group);
 
 	fsnotify_put_mark(fsn_mark);
+
+	if (!ret && (mask & FANOTIFY_PERM_EVENTS))
+		fanotify_perm_watchdog_group_add(group);
+
 	return ret;
 }
 
@@ -1625,6 +1726,7 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 	group->fanotify_data.f_flags = event_f_flags;
 	init_waitqueue_head(&group->fanotify_data.access_waitq);
 	INIT_LIST_HEAD(&group->fanotify_data.access_list);
+	INIT_LIST_HEAD(&group->fanotify_data.perm_grp_list);
 	switch (class) {
 	case FAN_CLASS_NOTIF:
 		group->priority = FSNOTIFY_PRIO_NORMAL;