Re: [PATCH v3 0/6] slab: Introduce dedicated bucket allocator

[Kees Cook]

On Fri, May 24, 2024 at 10:54:58AM -0400, Kent Overstreet wrote:
> On Wed, Apr 24, 2024 at 02:40:57PM -0700, Kees Cook wrote:
> > Hi,
> > 
> > Series change history:
> > 
> >  v3:
> >   - clarify rationale and purpose in commit log
> >   - rebase to -next (CONFIG_CODE_TAGGING)
> >   - simplify calling styles and split out bucket plumbing more cleanly
> >   - consolidate kmem_buckets_*() family introduction patches
> >  v2: https://lore.kernel.org/lkml/20240305100933.it.923-kees@xxxxxxxxxx/
> >  v1: https://lore.kernel.org/lkml/20240304184252.work.496-kees@xxxxxxxxxx/
> > 
> > For the cover letter, I'm repeating commit log for patch 4 here, which has
> > additional clarifications and rationale since v2:
> > 
> >     Dedicated caches are available for fixed size allocations via
> >     kmem_cache_alloc(), but for dynamically sized allocations there is only
> >     the global kmalloc API's set of buckets available. This means it isn't
> >     possible to separate specific sets of dynamically sized allocations into
> >     a separate collection of caches.
> >     
> >     This leads to a use-after-free exploitation weakness in the Linux
> >     kernel since many heap memory spraying/grooming attacks depend on using
> >     userspace-controllable dynamically sized allocations to collide with
> >     fixed size allocations that end up in same cache.
> 
> This is going to increase internal fragmentation in the slab allocator,
> so we're going to need better, more visible numbers on the amount of
> memory stranded thusly, so users can easily see the effect this has.

Yes, but not significantly. It's less than the 16-buckets randomized
kmalloc implementation. The numbers will be visible in /proc/slabinfo
just like any other.

> Please also document this effect and point users in the documentation
> where to check, so that we devs can get feedback on this.

Okay, sure. In the commit log, or did you have somewhere else in mind?

-- 
Kees Cook