From 7e348b325bc40eb52aead4d57a1f90d33ea834fc Mon Sep 17 00:00:00 2001 From: Beau Belgrave Date: Wed, 9 Mar 2022 16:11:41 -0800 Subject: [PATCH] user_events: Prevent dyn_event delete racing with ioctl add/delete 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 Signed-off-by: Steven Rostedt (Google) --- 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(®_mutex); + ret = user_event_parse_cmd(name, &user); + + if (!ret) + atomic_dec(&user->refcnt); + mutex_unlock(®_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);