linux-kernel - Re: [PATCH v3 2/6] mm/slab: Plumb kmem_buckets into __do_kmalloc

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Date: Fri, 31 May 2024 13:59:14 -0700
From: Kees Cook <kees@...nel.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
	linux-hardening@...r.kernel.org,
	"GONG, Ruiqi" <gongruiqi@...weicloud.com>,
	Xiu Jianfeng <xiujianfeng@...wei.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Jann Horn <jannh@...gle.com>, Matteo Rizzo <matteorizzo@...gle.com>,
	Thomas Graf <tgraf@...g.ch>,
	Herbert Xu <herbert@...dor.apana.org.au>, julien.voisin@...tri.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] mm/slab: Plumb kmem_buckets into
 __do_kmalloc_node()

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