Re: [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem collapse error path

[Jan Kara]

On Tue 08-11-16 11:12:45, Johannes Weiner wrote:
> On Tue, Nov 08, 2016 at 10:53:52AM +0100, Jan Kara wrote:
> > On Mon 07-11-16 14:07:36, Johannes Weiner wrote:
> > > The radix tree counts valid entries in each tree node. Entries stored
> > > in the tree cannot be removed by simpling storing NULL in the slot or
> > > the internal counters will be off and the node never gets freed again.
> > > 
> > > When collapsing a shmem page fails, restore the holes that were filled
> > > with radix_tree_insert() with a proper radix tree deletion.
> > > 
> > > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> > > Reported-by: Jan Kara <jack@xxxxxxx>
> > > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > ---
> > >  mm/khugepaged.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 728d7790dc2d..eac6f0580e26 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1520,7 +1520,8 @@ static void collapse_shmem(struct mm_struct *mm,
> > >  				if (!nr_none)
> > >  					break;
> > >  				/* Put holes back where they were */
> > > -				radix_tree_replace_slot(slot, NULL);
> > > +				radix_tree_delete(&mapping->page_tree,
> > > +						  iter.index);
> > 
> > Hum, but this is inside radix_tree_for_each_slot() iteration. And
> > radix_tree_delete() may end up freeing nodes resulting in invalidating
> > current slot pointer and the iteration code will do use-after-free.
> 
> Good point, we need to do another tree lookup after the deletion.
> 
> But there are other instances in the code, where we drop the lock
> temporarily and somebody else could delete the node from under us.
> 
> In the main collapse path, I *think* this is prevented by the fact
> that when we drop the tree lock we still hold the page lock of the
> regular page that's in the tree while we isolate and unmap it, thus
> pin the node. Even so, it would seem a little hairy to rely on that.

Yeah, I think that is mostly right but I'm not sure whether shrinking of
radix tree into direct pointer cannot bite us here as well. Generally that
relies on internal implementatation of the radix tree and its iterator
so what you did makes sense to me.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR