Re: [PATCH RFC 1/2] mm: Extract SLAB_QUARANTINE from KASAN

[Andrey Konovalov <andreyknvl@google.com>]

On Mon, Aug 17, 2020 at 7:32 PM Alexander Popov <alex.popov@linux.com> wrote:
>
> On 15.08.2020 19:52, Kees Cook wrote:
> > On Thu, Aug 13, 2020 at 06:19:21PM +0300, Alexander Popov wrote:
> >> Heap spraying is an exploitation technique that aims to put controlled
> >> bytes at a predetermined memory location on the heap. Heap spraying for
> >> exploiting use-after-free in the Linux kernel relies on the fact that on
> >> kmalloc(), the slab allocator returns the address of the memory that was
> >> recently freed. Allocating a kernel object with the same size and
> >> controlled contents allows overwriting the vulnerable freed object.
> >>
> >> Let's extract slab freelist quarantine from KASAN functionality and
> >> call it CONFIG_SLAB_QUARANTINE. This feature breaks widespread heap
> >> spraying technique used for exploiting use-after-free vulnerabilities
> >> in the kernel code.
> >>
> >> If this feature is enabled, freed allocations are stored in the quarantine
> >> and can't be instantly reallocated and overwritten by the exploit
> >> performing heap spraying.
> >
> > It may be worth clarifying that this is specifically only direct UAF and
> > doesn't help with spray-and-overflow-into-a-neighboring-object attacks
> > (i.e. both tend to use sprays, but the former doesn't depend on a write
> > overflow).
>
> Andrey Konovalov wrote:
> > If quarantine is to be used without the rest of KASAN, I'd prefer for
> > it to be separated from KASAN completely: move to e.g. mm/quarantine.c
> > and don't mention KASAN in function/config names.
>
> Hmm, making quarantine completely separate from KASAN would bring troubles.
>
> Currently, in many special places the allocator calls KASAN handlers:
>   kasan_cache_create()
>   kasan_slab_free()
>   kasan_kmalloc_large()
>   kasan_krealloc()
>   kasan_slab_alloc()
>   kasan_kmalloc()
>   kasan_cache_shrink()
>   kasan_cache_shutdown()
>   and some others.
> These functions do a lot of interesting things and also work with the quarantine
> using these helpers:
>   quarantine_put()
>   quarantine_reduce()
>   quarantine_remove_cache()
>
> Making quarantine completely separate from KASAN would require to move some
> internal logic of these KASAN handlers to allocator code.

It doesn't look like there's quite a lot of KASAN-specific logic there.

All those quarantine_*() calls are either at the beginning or at the
end of some kasan annotations, so it should be quite easy to move
those out. E.g. quarantine_reduce() can be moved together with the
gfpflags_allow_blocking(flags) check and put before kasan_kmalloc()
calls (or maybe also into some other places?), quarantine_put() can be
put after kasan_slab_free(), etc.

> In this patch I used another approach, that doesn't require changing the API
> between allocators and KASAN. I added linux/mm/kasan/slab_quarantine.c with slim
> KASAN handlers that implement the minimal functionality needed for quarantine.
>
> Do you think that it's a bad solution?

This solution doesn't look clean. Here you provide a second KASAN
runtime implementation, parallel to the original one, which only does
quarantine. It seems much cleaner to put quarantine logic into a
separate module, which can be either used independently, or together
with KASAN built on top of it.

Maybe other KASAN contributors have an opinion on this?