Patch "mm, mempolicy: task->mempolicy must be NULL before dropping final reference" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

    mm, mempolicy: task->mempolicy must be NULL before dropping final reference

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-mempolicy-task-mempolicy-must-be-null-before-dropping-final-reference.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Rientjes <[email protected]>
Date: Thu, 1 Sep 2016 16:15:07 -0700
Subject: mm, mempolicy: task->mempolicy must be NULL before dropping final reference

From: David Rientjes <[email protected]>

commit c11600e4fed67ae4cd6a8096936afd445410e8ed upstream.

KASAN allocates memory from the page allocator as part of kmem_cache_free(), and that can reference current->mempolicy through any number of allocation functions. It needs to be NULL'd out before the final reference is dropped to prevent a use-after-free bug:

	BUG: KASAN: use-after-free in alloc_pages_current+0x363/0x370 at addr ffff88010b48102c
	CPU: 0 PID: 15425 Comm: trinity-c2 Not tainted 4.8.0-rc2+ #140
	...
	Call Trace:
		dump_stack
		kasan_object_err
		kasan_report_error
		__asan_report_load2_noabort
		alloc_pages_current	<-- use after free
		depot_save_stack
		save_stack
		kasan_slab_free
		kmem_cache_free
		__mpol_put		<-- free
		do_exit

This patch sets current->mempolicy to NULL before dropping the final reference.

---
 include/linux/mempolicy.h |    4 ++++
 kernel/exit.c             |    7 +------
 mm/mempolicy.c            |   17 +++++++++++++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

@@ -195,6 +195,7 @@ static inline bool vma_migratable(struct
 }
 
 extern int mpol_misplaced(struct page *, struct vm_area_struct *, unsigned long);
+extern void mpol_put_task_policy(struct task_struct *);
 
 #else
 

@@ -297,5 +298,8 @@ static inline int mpol_misplaced(struct
 	return -1; /* no node preference */
 }
 
+static inline void mpol_put_task_policy(struct task_struct *task)
+{
+}
 #endif /* CONFIG_NUMA */
 #endif

@@ -768,12 +768,7 @@ void do_exit(long code)
 	TASKS_RCU(preempt_enable());
 	exit_notify(tsk, group_dead);
 	proc_exit_connector(tsk);
-#ifdef CONFIG_NUMA
-	task_lock(tsk);
-	mpol_put(tsk->mempolicy);
-	tsk->mempolicy = NULL;
-	task_unlock(tsk);
-#endif
+	mpol_put_task_policy(tsk);
 #ifdef CONFIG_FUTEX
 	if (unlikely(current->pi_state_cache))
 		kfree(current->pi_state_cache);

@@ -2334,6 +2334,23 @@ out:
 	return ret;
 }
 
+/*
+ * Drop the (possibly final) reference to task->mempolicy.  It needs to be
+ * dropped after task->mempolicy is set to NULL so that any allocation done as
+ * part of its kmem_cache_free(), such as by KASAN, doesn't reference a freed
+ * policy.
+ */
+void mpol_put_task_policy(struct task_struct *task)
+{
+	struct mempolicy *pol;
+
+	task_lock(task);
+	pol = task->mempolicy;
+	task->mempolicy = NULL;
+	task_unlock(task);
+	mpol_put(pol);
+}
+
 static void sp_delete(struct shared_policy *sp, struct sp_node *n)
 {
 	pr_debug("deleting %lx-l%lx\n", n->start, n->end);

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/mm-oom-prevent-premature-oom-killer-invocation-for-high-order-request.patch queue-4.7/mm-mempolicy-task-mempolicy-must-be-null-before-dropping-final-reference.patch