2 files changed, 52 insertions, 24 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 12327b2bb78..fbb87cf138c 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -653,6 +653,21 @@ config DEBUG_NOTIFIERS
 	  This is a relatively cheap check but if you care about maximum
 	  performance, say N.
 
+config DEBUG_CREDENTIALS
+	bool "Debug credential management"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on some debug checking for credential
+	  management.  The additional code keeps track of the number of
+	  pointers from task_structs to any given cred struct, and checks to
+	  see that this number never exceeds the usage count of the cred
+	  struct.
+
+	  Furthermore, if SELinux is enabled, this also checks that the
+	  security pointer in the cred struct is never seen to be invalid.
+
+	  If unsure, say N.
+
 #
 # Select this config option from the architecture Kconfig, if it
 # it is preferred to always offer frame pointers as a config
diff --git a/lib/is_single_threaded.c b/lib/is_single_threaded.c
index f1ed2fe76c6..bd2bea96336 100644
--- a/lib/is_single_threaded.c
+++ b/lib/is_single_threaded.c
@@ -12,34 +12,47 @@
 
 #include <linux/sched.h>
 
-/**
- * is_single_threaded - Determine if a thread group is single-threaded or not
- * @p: A task in the thread group in question
- *
- * This returns true if the thread group to which a task belongs is single
- * threaded, false if it is not.
+/*
+ * Returns true if the task does not share ->mm with another thread/process.
  */
-bool is_single_threaded(struct task_struct *p)
+bool current_is_single_threaded(void)
 {
-	struct task_struct *g, *t;
-	struct mm_struct *mm = p->mm;
+	struct task_struct *task = current;
+	struct mm_struct *mm = task->mm;
+	struct task_struct *p, *t;
+	bool ret;
 
-	if (atomic_read(&p->signal->count) != 1)
-		goto no;
+	if (atomic_read(&task->signal->live) != 1)
+		return false;
 
-	if (atomic_read(&p->mm->mm_users) != 1) {
-		read_lock(&tasklist_lock);
-		do_each_thread(g, t) {
-			if (t->mm == mm && t != p)
-				goto no_unlock;
-		} while_each_thread(g, t);
-		read_unlock(&tasklist_lock);
-	}
+	if (atomic_read(&mm->mm_users) == 1)
+		return true;
 
-	return true;
+	ret = false;
+	rcu_read_lock();
+	for_each_process(p) {
+		if (unlikely(p->flags & PF_KTHREAD))
+			continue;
+		if (unlikely(p == task->group_leader))
+			continue;
+
+		t = p;
+		do {
+			if (unlikely(t->mm == mm))
+				goto found;
+			if (likely(t->mm))
+				break;
+			/*
+			 * t->mm == NULL. Make sure next_thread/next_task
+			 * will see other CLONE_VM tasks which might be
+			 * forked before exiting.
+			 */
+			smp_rmb();
+		} while_each_thread(p, t);
+	}
+	ret = true;
+found:
+	rcu_read_unlock();
 
-no_unlock:
-	read_unlock(&tasklist_lock);
-no:
-	return false;
+	return ret;
 }