Received-SPF: pass (google.com: domain of linux-kernel+bounces-75727-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Date: Wed, 21 Feb 2024 19:34:44 -0500
From: Kent Overstreet <kent.overstreet@linux.dev>
To: Kees Cook <keescook@chromium.org>
Cc: Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org, 
	mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, roman.gushchin@linux.dev, 
	mgorman@suse.de, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, 
	penguin-kernel@i-love.sakura.ne.jp, corbet@lwn.net, void@manifault.com, peterz@infradead.org, 
	juri.lelli@redhat.com, catalin.marinas@arm.com, will@kernel.org, arnd@arndb.de, 
	tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org, 
	peterx@redhat.com, david@redhat.com, axboe@kernel.dk, mcgrof@kernel.org, 
	masahiroy@kernel.org, nathan@kernel.org, dennis@kernel.org, tj@kernel.org, 
	muchun.song@linux.dev, rppt@kernel.org, paulmck@kernel.org, pasha.tatashin@soleen.com, 
	yosryahmed@google.com, yuzhao@google.com, dhowells@redhat.com, hughd@google.com, 
	andreyknvl@gmail.com, ndesaulniers@google.com, vvvvvv@google.com, 
	gregkh@linuxfoundation.org, ebiggers@google.com, ytcoode@gmail.com, 
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, 
	bsegall@google.com, bristot@redhat.com, vschneid@redhat.com, cl@linux.com, 
	penberg@kernel.org, iamjoonsoo.kim@lge.com, 42.hyeyoo@gmail.com, glider@google.com, 
	elver@google.com, dvyukov@google.com, shakeelb@google.com, 
	songmuchun@bytedance.com, jbaron@akamai.com, rientjes@google.com, minchan@google.com, 
	kaleshsingh@google.com, kernel-team@android.com, linux-doc@vger.kernel.org, 
	linux-kernel@vger.kernel.org, iommu@lists.linux.dev, linux-arch@vger.kernel.org, 
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-modules@vger.kernel.org, 
	kasan-dev@googlegroups.com, cgroups@vger.kernel.org
Subject: Re: [PATCH v4 14/36] lib: add allocation tagging support for memory
 allocation profiling
Message-ID: <vxx2o2wdcqjkxauglu7ul52mygu4tti2i3yc2dvmcbzydvgvu2@knujflwtakni>
References: <20240221194052.927623-1-surenb@google.com>
 <20240221194052.927623-15-surenb@google.com>
 <202402211449.401382D2AF@keescook>
 <4vwiwgsemga7vmahgwsikbsawjq5xfskdsssmjsfe5hn7k2alk@b6ig5v2pxe5i>
 <202402211608.41AD94094@keescook>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <202402211608.41AD94094@keescook>

On Wed, Feb 21, 2024 at 04:25:02PM -0800, Kees Cook wrote:
> On Wed, Feb 21, 2024 at 06:29:17PM -0500, Kent Overstreet wrote:
> > On Wed, Feb 21, 2024 at 03:05:32PM -0800, Kees Cook wrote:
> > > On Wed, Feb 21, 2024 at 11:40:27AM -0800, Suren Baghdasaryan wrote:
> > > > [...]
> > > > +struct alloc_tag {
> > > > +	struct codetag			ct;
> > > > +	struct alloc_tag_counters __percpu	*counters;
> > > > +} __aligned(8);
> > > > [...]
> > > > +#define DEFINE_ALLOC_TAG(_alloc_tag)						\
> > > > +	static DEFINE_PER_CPU(struct alloc_tag_counters, _alloc_tag_cntr);	\
> > > > +	static struct alloc_tag _alloc_tag __used __aligned(8)			\
> > > > +	__section("alloc_tags") = {						\
> > > > +		.ct = CODE_TAG_INIT,						\
> > > > +		.counters = &_alloc_tag_cntr };
> > > > [...]
> > > > +static inline struct alloc_tag *alloc_tag_save(struct alloc_tag *tag)
> > > > +{
> > > > +	swap(current->alloc_tag, tag);
> > > > +	return tag;
> > > > +}
> > > 
> > > Future security hardening improvement idea based on this infrastructure:
> > > it should be possible to implement per-allocation-site kmem caches. For
> > > example, we could create:
> > > 
> > > struct alloc_details {
> > > 	u32 flags;
> > > 	union {
> > > 		u32 size; /* not valid after __init completes */
> > > 		struct kmem_cache *cache;
> > > 	};
> > > };
> > > 
> > > - add struct alloc_details to struct alloc_tag
> > > - move the tags section into .ro_after_init
> > > - extend alloc_hooks() to populate flags and size:
> > > 	.flags = __builtin_constant_p(size) ? KMALLOC_ALLOCATE_FIXED
> > > 					    : KMALLOC_ALLOCATE_BUCKETS;
> > > 	.size = __builtin_constant_p(size) ? size : SIZE_MAX;
> > > - during kernel start or module init, walk the alloc_tag list
> > >   and create either a fixed-size kmem_cache or to allocate a
> > >   full set of kmalloc-buckets, and update the "cache" member.
> > > - adjust kmalloc core routines to use current->alloc_tag->cache instead
> > >   of using the global buckets.
> > > 
> > > This would get us fully separated allocations, producing better than
> > > type-based levels of granularity, exceeding what we have currently with
> > > CONFIG_RANDOM_KMALLOC_CACHES.
> > > 
> > > Does this look possible, or am I misunderstanding something in the
> > > infrastructure being created here?
> > 
> > Definitely possible, but... would we want this?
> 
> Yes, very very much. One of the worst and mostly unaddressed weaknesses
> with the kernel right now is use-after-free based type confusion[0], which
> depends on merged caches (or cache reuse).
> 
> This doesn't solve cross-allocator (kmalloc/page_alloc) type confusion
> (as terrifyingly demonstrated[1] by Jann Horn), but it does help with
> what has been a very common case of "use msg_msg to impersonate your
> target object"[2] exploitation.

We have a ton of code that references PAGE_SIZE and uses the page
allocator completely unnecessarily - that's something worth harping
about at conferences; if we could motivate people to clean that stuff up
it'd have a lot of positive effects.

> > That would produce a _lot_ of kmem caches
> 
> Fewer than you'd expect, but yes, there is some overhead. However,
> out-of-tree forks of Linux have successfully experimented with this
> already and seen good results[3].

So in that case - I don't think there's any need for a separate
alloc_details; we'd just add a kmem_cache * to alloc_tag and then hook
into the codetag init/unload path to create and destroy the kmem caches.

No need to adjust the slab code either; alloc_hooks() itself could
dispatch to kmem_cache_alloc() instead of kmalloc() if this is in use.