proc: hold cred_guard_mutex in check_mem_permission()

Avoid a potential race when task exec's and we get a new ->mm but check against
the old credentials in ptrace_may_access().

Holding of the mutex is implemented by factoring out the body of the code into a
helper function __check_mem_permission().  Performing this factorization now
simplifies upcoming changes and minimizes churn in the diff's.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/base.c

@@ -191,10 +191,7 @@ static int proc_root_link(struct inode *inode, struct path *path)
 	return result;
 }
 
-/*
- * Return zero if current may access user memory in @task, -error if not.
- */
-static int check_mem_permission(struct task_struct *task)
+static int __check_mem_permission(struct task_struct *task)
 {
 	/*
 	 * A task can always look at itself, in case it chooses
@@ -222,6 +219,27 @@ static int check_mem_permission(struct task_struct *task)
 	return -EPERM;
 }
 
+/*
+ * Return zero if current may access user memory in @task, -error if not.
+ */
+static int check_mem_permission(struct task_struct *task)
+{
+	int err;
+
+	/*
+	 * Avoid racing if task exec's as we might get a new mm but validate
+	 * against old credentials.
+	 */
+	err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (err)
+		return err;
+
+	err = __check_mem_permission(task);
+	mutex_unlock(&task->signal->cred_guard_mutex);
+
+	return err;
+}
+
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
 	struct mm_struct *mm;