fanotify: add watchdog for permission events

JIRA: https://issues.redhat.com/browse/RHEL-44601

```
commit b8cf8fd
Author: Miklos Szeredi <mszeredi@redhat.com>
Date:   Tue Sep 9 16:30:47 2025 +0200

    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>
```

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: Miklos Szeredi

fs/notify/fanotify/fanotify.h

@@ -427,7 +427,9 @@ struct fanotify_perm_event {
 	struct path path;
 	u32 response;			/* userspace answer to the event */
 	unsigned short state;		/* state of the event */
+	unsigned short watchdog_cnt;	/* already scanned by watchdog? */
 	int fd;		/* fd we passed to userspace for this event */
+	pid_t recv_pid;	/* pid of task receiving the event */
 	union {
 		struct fanotify_response_info_header hdr;
 		struct fanotify_response_info_audit_rule audit_rule;

fs/notify/fanotify/fanotify_user.c

@@ -51,6 +51,7 @@
 
 /* configurable via /proc/sys/fs/fanotify/ */
 static int fanotify_max_queued_events __read_mostly;
+static int perm_group_timeout __read_mostly;
 
 #ifdef CONFIG_SYSCTL
 
@@ -86,6 +87,14 @@ static 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)
@@ -96,6 +105,91 @@ 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.
  *
@@ -854,6 +948,7 @@ 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);
 			}
 		}
@@ -913,6 +1008,8 @@ 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.
@@ -1365,6 +1462,10 @@ static int fanotify_add_mark(struct fsnotify_group *group,
 	fsnotify_group_unlock(group);
 
 	fsnotify_put_mark(fsn_mark);
+
+	if (!ret && (mask & FANOTIFY_PERM_EVENTS))
+		fanotify_perm_watchdog_group_add(group);
+
 	return ret;
 }
 
@@ -1514,6 +1615,7 @@ 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;

include/linux/fsnotify_backend.h

@@ -255,6 +255,8 @@ struct fsnotify_group {
 			int f_flags; /* event_f_flags from fanotify_init() */
 			struct ucounts *ucounts;
 			mempool_t error_events_pool;
+			/* chained on perm_group_list */
+			struct list_head perm_grp_list;
 		} fanotify_data;
 #endif /* CONFIG_FANOTIFY */
 	};