[for-next][PATCH 08/12] user_events: Prevent dyn_event delete racing with ioctl add/delete

Steven Rostedt
2022-03-12T23:26:03Z

From: Beau Belgrave <beaub@linux.microsoft.com>

Find user_events always while under the event_mutex and before leaving
the lock, add a ref count to the user_event. This ensures that all paths
under the event_mutex that check the ref counts will be synchronized.

The ioctl add/delete paths are protected by the reg_mutex. However,
dyn_event is only protected by the event_mutex. The dyn_event delete
path cannot acquire reg_mutex, since that could cause a deadlock between
the ioctl delete case acquiring event_mutex after acquiring the reg_mutex.

Link: https://lkml.kernel.org/r/20220310001141.1660-1-beaub@linux.microsoft.com

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
kernel/trace/trace_events_user.c | 46 +++++++++++++++++++++++++++-----
1 file changed, 40 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
index 9a6191a6a786..4febc1d6ae72 100644
--- a/kernel/trace/trace_events_user.c
+++ b/kernel/trace/trace_events_user.c
@@ -135,6 +135,8 @@ static struct list_head *user_event_get_fields(struct trace_event_call *call)
* NOTE: Offsets are from the user data perspective, they are not from the
* trace_entry/buffer perspective. We automatically add the common properties
* sizes to the offset for the user.
+ *
+ * Upon success user_event has its ref count increased by 1.
*/
static int user_event_parse_cmd(char *raw_command, struct user_event **newuser)
{
@@ -593,8 +595,10 @@ static struct user_event *find_user_event(char *name, u32 *outkey)
*outkey = key;

hash_for_each_possible(register_table, user, node, key)
- if (!strcmp(EVENT_NAME(user), name))
+ if (!strcmp(EVENT_NAME(user), name)) {
+ atomic_inc(&user->refcnt);
return user;
+ }

return NULL;
}
@@ -883,7 +887,12 @@ static int user_event_create(const char *raw_command)
return -ENOMEM;

mutex_lock(&reg_mutex);
+
ret = user_event_parse_cmd(name, &user);
+
+ if (!ret)
+ atomic_dec(&user->refcnt);
+
mutex_unlock(&reg_mutex);

if (ret)
@@ -1050,6 +1059,7 @@ static int user_event_trace_register(struct user_event *user)
/*
* Parses the event name, arguments and flags then registers if successful.
* The name buffer lifetime is owned by this method for success cases only.
+ * Upon success the returned user_event has its ref count increased by 1.
*/
static int user_event_parse(char *name, char *args, char *flags,
struct user_event **newuser)
@@ -1057,7 +1067,12 @@ static int user_event_parse(char *name, char *args, char *flags,
int ret;
int index;
u32 key;
- struct user_event *user = find_user_event(name, &key);
+ struct user_event *user;
+
+ /* Prevent dyn_event from racing */
+ mutex_lock(&event_mutex);
+ user = find_user_event(name, &key);
+ mutex_unlock(&event_mutex);

if (user) {
*newuser = user;
@@ -1121,6 +1136,10 @@ static int user_event_parse(char *name, char *args, char *flags,
goto put_user;

user->index = index;
+
+ /* Ensure we track ref */
+ atomic_inc(&user->refcnt);
+
dyn_event_init(&user->devent, &user_event_dops);
dyn_event_add(&user->devent, &user->call);
set_bit(user->index, page_bitmap);
@@ -1147,12 +1166,21 @@ static int delete_user_event(char *name)
if (!user)
return -ENOENT;

- if (atomic_read(&user->refcnt) != 0)
- return -EBUSY;
+ /* Ensure we are the last ref */
+ if (atomic_read(&user->refcnt) != 1) {
+ ret = -EBUSY;
+ goto put_ref;
+ }

- mutex_lock(&event_mutex);
ret = destroy_user_event(user);
- mutex_unlock(&event_mutex);
+
+ if (ret)
+ goto put_ref;
+
+ return ret;
+put_ref:
+ /* No longer have this ref */
+ atomic_dec(&user->refcnt);

return ret;
}
@@ -1340,6 +1368,9 @@ static long user_events_ioctl_reg(struct file *file, unsigned long uarg)

ret = user_events_ref_add(file, user);

+ /* No longer need parse ref, ref_add either worked or not */
+ atomic_dec(&user->refcnt);
+
/* Positive number is index and valid */
if (ret < 0)
return ret;
@@ -1364,7 +1395,10 @@ static long user_events_ioctl_del(struct file *file, unsigned long uarg)
if (IS_ERR(name))
return PTR_ERR(name);

+ /* event_mutex prevents dyn_event from racing */
+ mutex_lock(&event_mutex);
ret = delete_user_event(name);
+ mutex_unlock(&event_mutex);

kfree(name);

--
2.35.1