[RFC PATCH 09/20] rv: Handle unregistered monitors safely in tracefs

[Gabriele Monaco <gmonaco@redhat.com>]

With the introduction of pluggable BPF monitors, a monitor can be
dynamically unregistered while its tracefs files are still open by a
userspace process. This leaves a dangling pointer in the tracefs file
descriptor's private_data.

Validate that the monitor pointer still exists in the rv_monitors_list
before dereferencing it during read/write operations and return an error
otherwise.

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
---
 kernel/trace/rv/rv.c          | 32 +++++++++++++++++++++++++++++---
 kernel/trace/rv/rv.h          |  1 +
 kernel/trace/rv/rv_reactors.c |  6 ++++++
 3 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
index 33919c5fbd6a..6ddd3b92da1d 100644
--- a/kernel/trace/rv/rv.c
+++ b/kernel/trace/rv/rv.c
@@ -245,13 +245,31 @@ struct rv_monitor *rv_get_monitor_by_name(const char *name)
 /*
  * This section collects the monitor/ files and folders.
  */
+
+bool rv_is_monitor_registered(struct rv_monitor *mon)
+{
+	struct rv_monitor *m;
+
+	list_for_each_entry_rcu(m, &rv_monitors_list, list,
+				lockdep_is_held(&rv_interface_lock)) {
+		if (m == mon)
+			return true;
+	}
+	return false;
+}
+
 static ssize_t monitor_enable_read_data(struct file *filp, char __user *user_buf, size_t count,
 					loff_t *ppos)
 {
 	struct rv_monitor *mon = filp->private_data;
 	const char *buff;
 
-	buff = mon->enabled ? "1\n" : "0\n";
+	scoped_guard(rcu) {
+		if (!rv_is_monitor_registered(mon))
+			return -ENODEV;
+
+		buff = mon->enabled ? "1\n" : "0\n";
+	}
 
 	return simple_read_from_buffer(user_buf, count, ppos, buff, strlen(buff)+1);
 }
@@ -384,6 +402,9 @@ static ssize_t monitor_enable_write_data(struct file *filp, const char __user *u
 
 	guard(mutex)(&rv_interface_lock);
 
+	if (!rv_is_monitor_registered(mon))
+		return -ENODEV;
+
 	if (val)
 		retval = rv_enable_monitor(mon);
 	else
@@ -407,9 +428,14 @@ static ssize_t monitor_desc_read_data(struct file *filp, char __user *user_buf,
 	struct rv_monitor *mon = filp->private_data;
 	char buff[MAX_RV_DESCRIPTION_SIZE + 2];
 
-	memset(buff, 0, sizeof(buff));
+	scoped_guard(rcu) {
+		if (!rv_is_monitor_registered(mon))
+			return -ENODEV;
 
-	snprintf(buff, sizeof(buff), "%.*s\n", MAX_RV_DESCRIPTION_SIZE, mon->description);
+		memset(buff, 0, sizeof(buff));
+		snprintf(buff, sizeof(buff), "%.*s\n", MAX_RV_DESCRIPTION_SIZE,
+			 mon->description);
+	}
 
 	return simple_read_from_buffer(user_buf, count, ppos, buff, strlen(buff) + 1);
 }
diff --git a/kernel/trace/rv/rv.h b/kernel/trace/rv/rv.h
index 4d2c9cb284c9..6650ee9de308 100644
--- a/kernel/trace/rv/rv.h
+++ b/kernel/trace/rv/rv.h
@@ -28,6 +28,7 @@ int rv_enable_monitor(struct rv_monitor *mon);
 bool rv_is_container_monitor(struct rv_monitor *mon);
 bool rv_is_nested_monitor(struct rv_monitor *mon);
 struct rv_monitor *rv_get_monitor_by_name(const char *name);
+bool rv_is_monitor_registered(struct rv_monitor *mon);
 
 #ifdef CONFIG_RV_REACTORS
 int reactor_populate_monitor(struct rv_monitor *mon, struct dentry *root);
diff --git a/kernel/trace/rv/rv_reactors.c b/kernel/trace/rv/rv_reactors.c
index 2f5fc8d18dea..e23e0b5cc0ca 100644
--- a/kernel/trace/rv/rv_reactors.c
+++ b/kernel/trace/rv/rv_reactors.c
@@ -142,6 +142,9 @@ static int monitor_reactor_show(struct seq_file *m, void *p)
 	struct rv_monitor *mon = m->private;
 	struct rv_reactor *reactor = container_of(p, struct rv_reactor, list);
 
+	if (!rv_is_monitor_registered(mon))
+		return -ENODEV;
+
 	if (mon->reactor == reactor)
 		seq_printf(m, "[%s]\n", reactor->name);
 	else
@@ -235,6 +238,9 @@ monitor_reactors_write(struct file *file, const char __user *user_buf,
 
 	guard(mutex)(&rv_interface_lock);
 
+	if (!rv_is_monitor_registered(mon))
+		return -ENODEV;
+
 	list_for_each_entry(reactor, &rv_reactors_list, list) {
 		if (strcmp(ptr, reactor->name) != 0)
 			continue;
-- 
2.55.0