Re: [PATCH -mm 7/8] slub: make dead caches discard free slabs immediately

[Vladimir Davydov]

On Mon, Jun 02, 2014 at 11:03:51PM +0900, Joonsoo Kim wrote:
> 2014-06-02 20:47 GMT+09:00 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>:
> > Hi Joonsoo,
> >
> > On Mon, Jun 02, 2014 at 01:24:36PM +0900, Joonsoo Kim wrote:
> >> On Sat, May 31, 2014 at 03:04:58PM +0400, Vladimir Davydov wrote:
> >> > On Fri, May 30, 2014 at 09:57:10AM -0500, Christoph Lameter wrote:
> >> > > On Fri, 30 May 2014, Vladimir Davydov wrote:
> >> > >
> >> > > > (3) is a bit more difficult, because slabs are added to per-cpu partial
> >> > > > lists lock-less. Fortunately, we only have to handle the __slab_free
> >> > > > case, because, as there shouldn't be any allocation requests dispatched
> >> > > > to a dead memcg cache, get_partial_node() should never be called. In
> >> > > > __slab_free we use cmpxchg to modify kmem_cache_cpu->partial (see
> >> > > > put_cpu_partial) so that setting ->partial to a special value, which
> >> > > > will make put_cpu_partial bail out, will do the trick.
> > [...]
> >> I think that we can do (3) easily.
> >> If we check memcg_cache_dead() in the end of put_cpu_partial() rather
> >> than in the begin of put_cpu_partial(), we can avoid the race you
> >> mentioned. If someone do put_cpu_partial() before dead flag is set,
> >> it can be zapped by who set dead flag. And if someone do
> >> put_cpu_partial() after dead flag is set, it can be zapped by who
> >> do put_cpu_partial().
> >
> > After put_cpu_partial() adds a frozen slab to a per cpu partial list,
> > the slab becomes visible to other threads, which means it can be
> > unfrozen and freed. The latter can trigger cache destruction. Hence we
> > shouldn't touch the cache, in particular call memcg_cache_dead() on it,
> > after calling put_cpu_partial(), otherwise we can get use-after-free.
> >
> > However, what you propose makes sense if we disable irqs before adding a
> > slab to a partial list and enable them only after checking if the cache
> > is dead and unfreezing all partials if so, i.e.
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index d96faa2464c3..14b9e9a8677c 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2030,8 +2030,15 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> >         struct page *oldpage;
> >         int pages;
> >         int pobjects;
> > +       unsigned long flags;
> > +       int irq_saved = 0;
> >
> >         do {
> > +               if (irq_saved) {
> > +                       local_irq_restore(flags);
> > +                       irq_saved = 0;
> > +               }
> > +
> >                 pages = 0;
> >                 pobjects = 0;
> >                 oldpage = this_cpu_read(s->cpu_slab->partial);
> > @@ -2062,8 +2069,16 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
> >                 page->pobjects = pobjects;
> >                 page->next = oldpage;
> >
> > +               local_irq_save(flags);
> > +               irq_saved = 1;
> > +
> >         } while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
> >                                                                 != oldpage);
> > +
> > +       if (memcg_cache_dead(s))
> > +               unfreeze_partials(s, this_cpu_ptr(s->cpu_slab));
> > +
> > +       local_irq_restore(flags);
> >  #endif
> >  }
> >
> >
> > That would be safe against possible cache destruction, because to remove
> > a slab from a per cpu partial list we have to run on the cpu it was
> > frozen on. Disabling irqs makes it impossible.
> 
> Hmm... this is also a bit ugly.
> How about following change?
> 
> Thanks.
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2b1ce69..6adab87 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2058,6 +2058,21 @@ static void put_cpu_partial(struct kmem_cache
> *s, struct page *page, int drain)
> 
>         } while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
>                                                                 != oldpage);
> +
> +       if (memcg_cache_dead(s)) {
> +               bool done = false;
> +               unsigned long flags;

Suppose we are preempted here. In the meanwhile all objects are freed to
the cache, all frozen pages are unfrozen and also freed. The cache
destruction is then scheduled (patch 2 of this set). Then when this
thread continues execution it will operate on the cache that was
destroyed - use-after-free.

I admit, this is very unlikely, but can we ignore this possibility?

Thanks.

> +
> +               local_irq_save(flags);
> +               if (this_cpu_read(s->cpu_slab->partial) == page) {
> +                       done = true;
> +                       unfreeze_partials(s, this_cpu_ptr);
> +               }
> +               local_irq_restore(flags);
> +
> +               if (!done)
> +                       flush_all(s);
> +       }
>  #endif
>  }
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