[ 15/53] mmu_notifier_unregister NULL Pointer deref and multiple ->release() callouts

Greg Kroah-Hartman
2013-02-27T00:36:04Z

From: Robin Holt <holt@sgi.com>

commit 751efd8610d3d7d67b7bdf7f62646edea7365dd7 upstream.

There is a race condition between mmu_notifier_unregister() and
__mmu_notifier_release().

Assume two tasks, one calling mmu_notifier_unregister() as a result of a
filp_close() ->flush() callout (task A), and the other calling
mmu_notifier_release() from an mmput() (task B).

A B
t1 srcu_read_lock()
t2 if (!hlist_unhashed())
t3 srcu_read_unlock()
t4 srcu_read_lock()
t5 hlist_del_init_rcu()
t6 synchronize_srcu()
t7 srcu_read_unlock()
t8 hlist_del_rcu() <--- NULL pointer deref.

Additionally, the list traversal in __mmu_notifier_release() is not
protected by the by the mmu_notifier_mm->hlist_lock which can result in
callouts to the ->release() notifier from both mmu_notifier_unregister()
and __mmu_notifier_release().

-stable suggestions:

The stable trees prior to 3.7.y need commits 21a92735f660 and
70400303ce0c cherry-picked in that order prior to cherry-picking this
commit. The 3.7.y tree already has those two commits.

Signed-off-by: Robin Holt <holt@sgi.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Sagi Grimberg <sagig@mellanox.co.il>
Cc: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/mmu_notifier.c | 82 +++++++++++++++++++++++++++---------------------------
1 file changed, 42 insertions(+), 40 deletions(-)

--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -37,49 +37,51 @@ static struct srcu_struct srcu;
void __mmu_notifier_release(struct mm_struct *mm)
{
struct mmu_notifier *mn;
- struct hlist_node *n;
int id;

/*
- * SRCU here will block mmu_notifier_unregister until
- * ->release returns.
+ * srcu_read_lock() here will block synchronize_srcu() in
+ * mmu_notifier_unregister() until all registered
+ * ->release() callouts this function makes have
+ * returned.
*/
id = srcu_read_lock(&srcu);
- hlist_for_each_entry_rcu(mn, n, &mm->mmu_notifier_mm->list, hlist)
- /*
- * if ->release runs before mmu_notifier_unregister it
- * must be handled as it's the only way for the driver
- * to flush all existing sptes and stop the driver
- * from establishing any more sptes before all the
- * pages in the mm are freed.
- */
- if (mn->ops->release)
- mn->ops->release(mn, mm);
- srcu_read_unlock(&srcu, id);
-
spin_lock(&mm->mmu_notifier_mm->lock);
while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) {
mn = hlist_entry(mm->mmu_notifier_mm->list.first,
struct mmu_notifier,
hlist);
+
/*
- * We arrived before mmu_notifier_unregister so
- * mmu_notifier_unregister will do nothing other than
- * to wait ->release to finish and
- * mmu_notifier_unregister to return.
+ * Unlink. This will prevent mmu_notifier_unregister()
+ * from also making the ->release() callout.
*/
hlist_del_init_rcu(&mn->hlist);
+ spin_unlock(&mm->mmu_notifier_mm->lock);
+
+ /*
+ * Clear sptes. (see 'release' description in mmu_notifier.h)
+ */
+ if (mn->ops->release)
+ mn->ops->release(mn, mm);
+
+ spin_lock(&mm->mmu_notifier_mm->lock);
}
spin_unlock(&mm->mmu_notifier_mm->lock);

/*
- * synchronize_srcu here prevents mmu_notifier_release to
- * return to exit_mmap (which would proceed freeing all pages
- * in the mm) until the ->release method returns, if it was
- * invoked by mmu_notifier_unregister.
- *
- * The mmu_notifier_mm can't go away from under us because one
- * mm_count is hold by exit_mmap.
+ * All callouts to ->release() which we have done are complete.
+ * Allow synchronize_srcu() in mmu_notifier_unregister() to complete
+ */
+ srcu_read_unlock(&srcu, id);
+
+ /*
+ * mmu_notifier_unregister() may have unlinked a notifier and may
+ * still be calling out to it. Additionally, other notifiers
+ * may have been active via vmtruncate() et. al. Block here
+ * to ensure that all notifier callouts for this mm have been
+ * completed and the sptes are really cleaned up before returning
+ * to exit_mmap().
*/
synchronize_srcu(&srcu);
}
@@ -300,31 +302,31 @@ void mmu_notifier_unregister(struct mmu_
{
BUG_ON(atomic_read(&mm->mm_count) <= 0);

+ spin_lock(&mm->mmu_notifier_mm->lock);
if (!hlist_unhashed(&mn->hlist)) {
- /*
- * SRCU here will force exit_mmap to wait ->release to finish
- * before freeing the pages.
- */
int id;

- id = srcu_read_lock(&srcu);
/*
- * exit_mmap will block in mmu_notifier_release to
- * guarantee ->release is called before freeing the
- * pages.
+ * Ensure we synchronize up with __mmu_notifier_release().
*/
+ id = srcu_read_lock(&srcu);
+
+ hlist_del_rcu(&mn->hlist);
+ spin_unlock(&mm->mmu_notifier_mm->lock);
+
if (mn->ops->release)
mn->ops->release(mn, mm);
- srcu_read_unlock(&srcu, id);

- spin_lock(&mm->mmu_notifier_mm->lock);
- hlist_del_rcu(&mn->hlist);
+ /*
+ * Allow __mmu_notifier_release() to complete.
+ */
+ srcu_read_unlock(&srcu, id);
+ } else
spin_unlock(&mm->mmu_notifier_mm->lock);
- }

/*
- * Wait any running method to finish, of course including
- * ->release if it was run by mmu_notifier_relase instead of us.
+ * Wait for any running method to finish, including ->release() if it
+ * was run by __mmu_notifier_release() instead of us.
*/
synchronize_srcu(&srcu);