lockdep: Fix a race between /proc/lock_stat and module unload

The lock_class iteration of /proc/lock_stat is not serialized against
the lockdep_free_key_range() call from module unload.

Therefore it can happen that we find a class of which ->name/->key are
no longer valid.

There is a further bug in zap_class() that left ->name dangling. Cure
this. Use RCU_INIT_POINTER() because NULL.

Since lockdep_free_key_range() is rcu_sched serialized, we can read
both ->name and ->key under rcu_read_lock_sched() (preempt-disable)
and be assured that if we observe a !NULL value it stays safe to use
for as long as we hold that lock.

If we observe both NULL, skip the entry.

Reported-by: Jerome Marchand <jmarchan@redhat.com>
Tested-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150602105013.GS3644@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 19 insertions, 6 deletions

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index a0831e1b99f4..aaeae885d9af 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -3900,7 +3900,8 @@ static void zap_class(struct lock_class *class)
         list_del_rcu(&class->hash_entry);
         list_del_rcu(&class->lock_entry);
 
-        class->key = NULL;
+        RCU_INIT_POINTER(class->key, NULL);
+        RCU_INIT_POINTER(class->name, NULL);
 }
 
 static inline int within(const void *addr, void *start, unsigned long size)
diff --git a/kernel/locking/lockdep_proc.c b/kernel/locking/lockdep_proc.c
index ef43ac4bafb5..d83d798bef95 100644
--- a/kernel/locking/lockdep_proc.c
+++ b/kernel/locking/lockdep_proc.c
@@ -426,10 +426,12 @@ static void seq_lock_time(struct seq_file *m, struct lock_time *lt)
 
 static void seq_stats(struct seq_file *m, struct lock_stat_data *data)
 {
-        char name[39];
-        struct lock_class *class;
+        struct lockdep_subclass_key *ckey;
         struct lock_class_stats *stats;
+        struct lock_class *class;
+        const char *cname;
         int i, namelen;
+        char name[39];
 
         class = data->class;
         stats = &data->stats;
@@ -440,15 +442,25 @@ static void seq_stats(struct seq_file *m, struct lock_stat_data *data)
         if (class->subclass)
                 namelen -= 2;
 
-        if (!class->name) {
+        rcu_read_lock_sched();
+        cname = rcu_dereference_sched(class->name);
+        ckey  = rcu_dereference_sched(class->key);
+
+        if (!cname && !ckey) {
+                rcu_read_unlock_sched();
+                return;
+
+        } else if (!cname) {
                 char str[KSYM_NAME_LEN];
                 const char *key_name;
 
-                key_name = __get_key_name(class->key, str);
+                key_name = __get_key_name(ckey, str);
                 snprintf(name, namelen, "%s", key_name);
         } else {
-                snprintf(name, namelen, "%s", class->name);
+                snprintf(name, namelen, "%s", cname);
         }
+        rcu_read_unlock_sched();
+
         namelen = strlen(name);
         if (class->name_version > 1) {
                 snprintf(name+namelen, 3, "#%d", class->name_version);