Re: [PATCH RESEND] mm: mmu_notifier: re-fix freed page still mappedin secondary MMU

[Andrew Morton]

On Mon, 06 May 2013 13:50:18 +0800 Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> wrote:

> The commit 751efd8610d3 (mmu_notifier_unregister NULL Pointer deref
> and multiple ->release()) breaks the fix:
>     3ad3d901bbcfb15a5e4690e55350db0899095a68
>     (mm: mmu_notifier: fix freed page still mapped in secondary MMU)
> 
> Since hlist_for_each_entry_rcu() is changed now, we can not revert that patch
> directly, so this patch reverts the commit and simply fix the bug spotted
> by that patch
> 
> This bug spotted by commit 751efd8610d3 is:
> ======
> 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.
> ======
> 
> This can be fixed by using hlist_del_init_rcu instead of hlist_del_rcu.
> 
> The another issue spotted in the commit is
> "multiple ->release() callouts", we needn't care it too much because
> it is really rare (e.g, can not happen on kvm since mmu-notify is unregistered
> after exit_mmap()) and the later call of multiple ->release should be
> fast since all the pages have already been released by the first call.
> Anyway, this issue should be fixed in a separate patch.

The 751efd8610d3 changelog failed to describe how these duplicated
->release calls can occur.  Races with concurrent notifier
registrations, I assume?

> -stable suggestions:
> Any version has commit 751efd8610d3 need to be backported. I find the oldest
> version has this commit is 3.0-stable.
> 
> ...
>
> Andrew, this patch has been tested by Robin and the test shows that the bug
> of "NULL Pointer deref" bas been fixed. However, we have the argument that
> whether the fix of "multiple ->release" should be merged into this patch.
> (This patch just do fix the bug of "NULL Pointer deref")
> 
> Your thought?

Insufficient information :(
 

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