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

Alexander Potapenko
2020-10-05T09:31:06Z

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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