Messages in this thread | | | Date | Thu, 6 Jul 2017 10:55:39 -0500 (CDT) | From | Christoph Lameter <> | Subject | Re: [PATCH v3] mm: Add SLUB free list pointer obfuscation |
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On Thu, 6 Jul 2017, Kees Cook wrote:
> On Thu, Jul 6, 2017 at 6:43 AM, Christoph Lameter <cl@linux.com> wrote: > > On Wed, 5 Jul 2017, Kees Cook wrote: > > > >> @@ -3536,6 +3565,9 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags) > >> { > >> s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor); > >> s->reserved = 0; > >> +#ifdef CONFIG_SLAB_FREELIST_HARDENED > >> + s->random = get_random_long(); > >> +#endif > >> > >> if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU)) > >> s->reserved = sizeof(struct rcu_head); > >> > > > > So if an attacker knows the internal structure of data then he can simply > > dereference page->kmem_cache->random to decode the freepointer. > > That requires a series of arbitrary reads. This is protecting against > attacks that use an adjacent slab object write overflow to write the > freelist pointer. This internal structure is very reliable, and has > been the basis of freelist attacks against the kernel for a decade.
These reads are not arbitrary. You can usually calculate the page struct address easily from the address and then do a couple of loads to get there.
Ok so you get rid of the old attacks because we did not have that hardening in effect when they designed their approaches?
> It is a probabilistic defense, but then so is the stack protector. > This is a similar defense; while not perfect it makes the class of > attack much more difficult to mount.
Na I am not convinced of the "much more difficult". Maybe they will just have to upgrade their approaches to fetch the proper values to decode.
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