Re: [PATCH v4 05/11] mm, kfence: insert KFENCE hooks for SLUB

[Alexander Potapenko <glider@google.com>]

On Fri, Oct 2, 2020 at 9:07 AM Jann Horn <jannh@google.com> wrote:
>
> On Tue, Sep 29, 2020 at 3:38 PM Marco Elver <elver@google.com> wrote:
> > Inserts KFENCE hooks into the SLUB allocator.
> [...]
> > diff --git a/mm/slub.c b/mm/slub.c
> [...]
> > @@ -3290,8 +3314,14 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> >         c = this_cpu_ptr(s->cpu_slab);
> >
> >         for (i = 0; i < size; i++) {
> > -               void *object = c->freelist;
> > +               void *object = kfence_alloc(s, s->object_size, flags);
>
> kfence_alloc() will invoke ->ctor() callbacks if the current slab has
> them. Is it fine to invoke such callbacks from here, where we're in
> the middle of a section that disables interrupts to protect against
> concurrent freelist changes? If someone decides to be extra smart and
> uses a kmem_cache with a ->ctor that can allocate memory from the same
> kmem_cache, or something along those lines, this could lead to
> corruption of the SLUB freelist. But I'm not sure whether that can
> happen in practice.

From cache_init_objs_debug() in mm/slab.c:

                /*
                 * Constructors are not allowed to allocate memory from the same
                 * cache which they are a constructor for.  Otherwise, deadlock.
                 * They must also be threaded.
                 */

So, no, it is not allowed to allocate from the same cache in the constructor.


> Still, it might be nicer if you could code this to behave like a
> fastpath miss: Update c->tid, turn interrupts back on (___slab_alloc()
> will also do that if it has to call into the page allocator), then let
> kfence do the actual allocation in a more normal context, then turn
> interrupts back off and go on. If that's not too complicated?
>
> Maybe Christoph Lameter has opinions on whether this is necessary...
> it admittedly is fairly theoretical.
>
> > +               if (unlikely(object)) {
> > +                       p[i] = object;
> > +                       continue;
> > +               }
> > +
> > +               object = c->freelist;
> >                 if (unlikely(!object)) {
> >                         /*
> >                          * We may have removed an object from c->freelist using



--
Alexander Potapenko
Software Engineer

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