Re: [PATCH v3 2/6] mm/slab: Plumb kmem_buckets into __do_kmalloc_node()

[Kees Cook <kees@kernel.org>]

On Fri, May 31, 2024 at 12:51:29PM -0400, Kent Overstreet wrote:
> On Fri, May 31, 2024 at 09:48:49AM -0700, Kees Cook wrote:
> > On Fri, May 24, 2024 at 11:01:40AM -0400, Kent Overstreet wrote:
> > > On Wed, Apr 24, 2024 at 02:40:59PM -0700, Kees Cook wrote:
> > > > To be able to choose which buckets to allocate from, make the buckets
> > > > available to the lower level kmalloc interfaces by adding them as the
> > > > first argument. Where the bucket is not available, pass NULL, which means
> > > > "use the default system kmalloc bucket set" (the prior existing behavior),
> > > > as implemented in kmalloc_slab().
> > > 
> > > I thought the plan was to use codetags for this? That would obviate the
> > > need for all this plumbing.
> > > 
> > > Add fields to the alloc tag for:
> > >  - allocation size (or 0 if it's not a compile time constant)
> > >  - union of kmem_cache, kmem_buckets, depending on whether the
> > >    allocation size is constant or not
> > 
> > I want to provide "simple" (low-hanging fruit) coverage that can live
> > separately from the codetags-based coverage. The memory overhead for
> > this patch series is negligible, but I suspect the codetags expansion,
> > while not giant, will be more than some deployments will want. I want
> > to avoid an all-or-nothing solution -- which is why I had intended this
> > to be available "by default".
> 
> technically there's no reason for your thing to depend on
> CONFIG_CODETAGGING at all, that's the infrastructure for finding
> codetags for e.g. /proc/allocinfo. you'd just be using the alloc_hoos()
> macro and struct alloc_tag as a place to stash the kmem_buckets pointer.

It's the overhead of separate kmem_cache and kmem_buckets for every
allocation location that I meant. So I'd like the "simple" version for
gaining coverage over the currently-being-regularly-exploited cases, and
then allow for the "big hammer" solution too.

However, I do think I'll still need the codetag infra because of the
sections, etc. I think we'll need to pre-build the caches, but maybe
that could be avoided by adding some kind of per-site READ_ONCE/lock
thingy to create them on demand. We'll see! :)

-- 
Kees Cook