Re: [PATCH v2 0/9] slab: Introduce dedicated bucket allocator

[Kees Cook <keescook@chromium.org>]

On Mon, Mar 25, 2024 at 10:03:23AM +0100, Vlastimil Babka wrote:
> On 3/5/24 11:10 AM, Kees Cook wrote:
> > Hi,
> > 
> > Repeating the commit logs for patch 4 here:
> > 
> >     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.
> > 
> >     While CONFIG_RANDOM_KMALLOC_CACHES provides a probabilistic defense
> >     against these kinds of "type confusion" attacks, including for fixed
> >     same-size heap objects, we can create a complementary deterministic
> >     defense for dynamically sized allocations.
> > 
> >     In order to isolate user-controllable sized allocations from system
> >     allocations, introduce kmem_buckets_create(), which behaves like
> >     kmem_cache_create(). (The next patch will introduce kmem_buckets_alloc(),
> >     which behaves like kmem_cache_alloc().)
> > 
> >     Allows for confining allocations to a dedicated set of sized caches
> >     (which have the same layout as the kmalloc caches).
> > 
> >     This can also be used in the future once codetag allocation annotations
> >     exist to implement per-caller allocation cache isolation[0] even for
> >     dynamic allocations.
> > 
> >     Link: https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook [0]
> > 
> > After the implemetation are 2 example patches of how this could be used
> > for some repeat "offenders" that get used in exploits. There are more to
> > be isolated beyond just these. Repeating the commit log for patch 8 here:
> > 
> >     The msg subsystem is a common target for exploiting[1][2][3][4][5][6]
> >     use-after-free type confusion flaws in the kernel for both read and
> >     write primitives. Avoid having a user-controlled size cache share the
> >     global kmalloc allocator by using a separate set of kmalloc buckets.
> > 
> >     Link: https://blog.hacktivesecurity.com/index.php/2022/06/13/linux-kernel-exploit-development-1day-case-study/ [1]
> >     Link: https://hardenedvault.net/blog/2022-11-13-msg_msg-recon-mitigation-ved/ [2]
> >     Link: https://www.willsroot.io/2021/08/corctf-2021-fire-of-salvation-writeup.html [3]
> >     Link: https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html [4]
> >     Link: https://google.github.io/security-research/pocs/linux/cve-2021-22555/writeup.html [5]
> >     Link: https://zplin.me/papers/ELOISE.pdf [6]
> 
> Hi Kees,
> 
> after reading [1] I think the points should be addressed, mainly about the
> feasibility of converting users manually.

Sure, I can do that.

Adding Julien to this thread... Julien can you please respond to LKML
patches in email? It's much easier to keep things in a single thread. :)

] This is playing wack-a-mole

Kind of, but not really. These patches provide a mechanism for having
dedicated dynamically-sized slab caches (to match kmem_cache_create(),
which only works for fixed-size allocations). This is needed to expand
the codetag work into doing per-call-site allocations, as I detailed
here[1].

Also, adding uses manually isn't very difficult, as can be seen in the
examples I included. In fact, my examples between v1 and v2 collapsed
from 3 to 2, because covering memdup_user() actually covered 2 known
allocation paths (attrs and vma names), and given its usage pattern,
will cover more in the future without changes.

] something like AUTOSLAB would be better

Yes, that's the goal of [1]. This is a prerequisite for that, as
mentioned in the cover letter.

] The slabs needs to be pinned

Yes, and this is a general problem[2] with all kmalloc allocations, though.
This isn't unique to to this patch series. SLAB_VIRTUAL solves it, and
is under development.

] Lacks guard pages

Yes, and again, this is a general problem with all kmalloc allocations.
Solving it, like SLAB_VIRTUAL, would be a complementary hardening
improvement to the allocator generally.

] PAX_USERCOPY has been marking these sites since 2012

Either it's whack-a-mole or it's not. :) PAX_USERCOPY shows that it _is_
possible to mark all sites. Regardless, like AUTOSLAB, PAX_USERCOPY isn't
upstream, and its current implementation is an unpublished modification
to a GPL project. I look forward to someone proposing it for inclusion
in Linux, but for now we can work with the patches where an effort _has_
been made to upstream them for the benefit of the entire ecosystem.

] What about CONFIG_KMALLOC_SPLIT_VARSIZE

This proposed improvement is hampered by not having dedicated
_dynamically_ sized kmem caches, which this series provides. And with
codetag-split allocations[1], the goals of CONFIG_KMALLOC_SPLIT_VARSIZE
are more fully realized, providing much more complete coverage.

] I have no idea how the community around the Linux kernel works with
] their email-based workflows

Step 1: reply to the proposal in email instead of (or perhaps in
addition to) making blog posts. :)

> On a related technical note I
> worry what will become of /proc/slabinfo when we convert non-trivial amounts
> of users.

It gets longer. :) And potentially makes the codetag /proc file
redundant. All that said, there are very few APIs in the kernel where
userspace can control both the size and contents of an allocation.

> Also would interested to hear Jann Horn et al.'s opinion, and whether the
> SLAB_VIRTUAL effort will continue?

SLAB_VIRTUAL is needed to address the reclamation UAF gap, and is
still being developed. I don't intend to let it fall off the radar.
(Which is why I included Jann and Matteo in CC originally.)

In the meantime, adding this series as-is kills two long-standing
exploitation methodologies, and paves the way to providing very
fine-grained caches using codetags (which I imagine would be entirely
optional and trivial to control with a boot param).

-Kees

[1] https://lore.kernel.org/lkml/202402211449.401382D2AF@keescook/
[2] https://googleprojectzero.blogspot.com/2021/10/how-simple-linux-kernel-memory.html

-- 
Kees Cook