ftrace: Do not hold references of ftrace_graph_{notrace_}hash out of graph_lock

The hashs ftrace_graph_hash and ftrace_graph_notrace_hash are modified
within the graph_lock being held. Holding a pointer to them and passing them
along can lead to a use of a stale pointer (fgd->hash). Move assigning the
pointer and its use to within the holding of the lock. Note, it's an
rcu_sched protected data, and other instances of referencing them are done
with preemption disabled. But the file manipuation code must be protected by
the lock.

The fgd->hash pointer is set to NULL when the lock is being released.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 0233c8cb45f4c04649a994ebcaa67fde8e2ed767..b3a4896ef78a28a9ac6c596f80457db42220355c 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4616,6 +4616,13 @@ static void *g_start(struct seq_file *m, loff_t *pos)
 
        mutex_lock(&graph_lock);
 
+       if (fgd->type == GRAPH_FILTER_FUNCTION)
+               fgd->hash = rcu_dereference_protected(ftrace_graph_hash,
+                                       lockdep_is_held(&graph_lock));
+       else
+               fgd->hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
+                                       lockdep_is_held(&graph_lock));
+
        /* Nothing, tell g_show to print all functions are enabled */
        if (ftrace_hash_empty(fgd->hash) && !*pos)
                return FTRACE_GRAPH_EMPTY;
@@ -4695,6 +4702,14 @@ __ftrace_graph_open(struct inode *inode, struct file *file,
 
 out:
        fgd->new_hash = new_hash;
+
+       /*
+        * All uses of fgd->hash must be taken with the graph_lock
+        * held. The graph_lock is going to be released, so force
+        * fgd->hash to be reinitialized when it is taken again.
+        */
+       fgd->hash = NULL;
+
        return ret;
 }
 
@@ -4713,7 +4728,8 @@ ftrace_graph_open(struct inode *inode, struct file *file)
 
        mutex_lock(&graph_lock);
 
-       fgd->hash = ftrace_graph_hash;
+       fgd->hash = rcu_dereference_protected(ftrace_graph_hash,
+                                       lockdep_is_held(&graph_lock));
        fgd->type = GRAPH_FILTER_FUNCTION;
        fgd->seq_ops = &ftrace_graph_seq_ops;
 
@@ -4740,7 +4756,8 @@ ftrace_graph_notrace_open(struct inode *inode, struct file *file)
 
        mutex_lock(&graph_lock);
 
-       fgd->hash = ftrace_graph_notrace_hash;
+       fgd->hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
+                                       lockdep_is_held(&graph_lock));
        fgd->type = GRAPH_FILTER_NOTRACE;
        fgd->seq_ops = &ftrace_graph_seq_ops;
 
@@ -4859,17 +4876,18 @@ ftrace_graph_write(struct file *file, const char __user *ubuf,
                ret = ftrace_graph_set_hash(fgd->new_hash,
                                            parser.buffer);
 
-               old_hash = fgd->hash;
                new_hash = __ftrace_hash_move(fgd->new_hash);
                if (!new_hash)
                        ret = -ENOMEM;
 
                if (fgd->type == GRAPH_FILTER_FUNCTION) {
+                       old_hash = rcu_dereference_protected(ftrace_graph_hash,
+                                       lockdep_is_held(&graph_lock));
                        rcu_assign_pointer(ftrace_graph_hash, new_hash);
-                       fgd->hash = ftrace_graph_hash;
                } else {
+                       old_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
+                                       lockdep_is_held(&graph_lock));
                        rcu_assign_pointer(ftrace_graph_notrace_hash, new_hash);
-                       fgd->hash = ftrace_graph_notrace_hash;
                }
 
                mutex_unlock(&graph_lock);