From 751efd8610d3d7d67b7bdf7f62646edea7365dd7 Mon Sep 17 00:00:00 2001 From: Robin Holt Date: Fri, 22 Feb 2013 16:35:34 -0800 Subject: [PATCH] mmu_notifier_unregister NULL Pointer deref and multiple ->release() callouts 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 Cc: Andrea Arcangeli Cc: Wanpeng Li Cc: Xiao Guangrong Cc: Avi Kivity Cc: Hugh Dickins Cc: Marcelo Tosatti Cc: Sagi Grimberg Cc: Haggai Eran Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/mmu_notifier.c | 82 ++++++++++++++++++++++++----------------------- 1 file changed, 42 insertions(+), 40 deletions(-) diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c index 8a5ac8c686b0..f5c3d968d8c6 100644 --- 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); } @@ -294,31 +296,31 @@ void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm) { 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); -- 2.20.1