linux-kernel - Re: [PATCH v4 2/2] slab: Introduce kmalloc

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Message-ID: <202510072114.52B93ED736@keescook>
Date: Tue, 7 Oct 2025 21:20:17 -0700
From: Kees Cook <kees@...nel.org>
To: Marco Elver <elver@...gle.com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
	Matthew Wilcox <willy@...radead.org>,
	Vlastimil Babka <vbabka@...e.cz>, 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>,
	"Gustavo A . R . Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, Jann Horn <jannh@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org,
	Miguel Ojeda <ojeda@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Jonathan Corbet <corbet@....net>, Jakub Kicinski <kuba@...nel.org>,
	Yafang Shao <laoar.shao@...il.com>,
	Tony Ambardar <tony.ambardar@...il.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Jan Hendrik Farr <kernel@...rr.cc>,
	Alexander Potapenko <glider@...gle.com>,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	linux-doc@...r.kernel.org, llvm@...ts.linux.dev,
	Matteo Rizzo <matteorizzo@...gle.com>
Subject: Re: [PATCH v4 2/2] slab: Introduce kmalloc_obj() and family

On Tue, Oct 07, 2025 at 08:18:28PM +0200, Marco Elver wrote:
> On Tue, 7 Oct 2025 at 19:47, Christoph Lameter (Ampere) <cl@...two.org> wrote:
> >
> > On Tue, 7 Oct 2025, Kees Cook wrote:
> >
> > > But all of that is orthogonal to just _having_ the type info available.
> >
> > iOS did go the path of creating basically one slab cache for each
> > "type" of kmalloc for security reasons.
> >
> > See https://security.apple.com/blog/towards-the-next-generation-of-xnu-memory-safety/
> 
> We can get something similar to that with:
> https://lore.kernel.org/all/20250825154505.1558444-1-elver@google.com/
> Pending compiler support which is going to become available in a few
> months (probably).
> That version used the existing RANDOM_KMALLOC_CACHES choice of 16 slab
> caches, but there's no fundamental limitation to go higher.

Right -- having compiler support for dealing with types at compile time
means we can create the slab caches statically (instead of any particular
fixed number, even the 16 from RANDOM_KMALLOC_CACHES). Another compiler
feature that might help here is getting a unique u32 for arbitrary type
info, which is also how KCFI works:
https://lore.kernel.org/linux-hardening/20250926030252.2387681-1-kees@kernel.org/

My main issue is that I prefer explicitly exposing the type instead of
having the compiler have to guess. We want it for more than just slab
isolation (e.g. examining alignment).

> Note, this mitigation is likely not as strong as we'd like to without
> SLAB_VIRTUAL (or so I'm told): https://lwn.net/Articles/944647/

True, but both "halves" are needed -- SLAB_VIRTUAL isn't as robust
without the type separation either.

-Kees

-- 
Kees Cook