Re: [PATCH] mm/slab: always use cache from obj

From: Vlastimil Babka <vbabka@suse.cz>
To: lijiazi <jqqlijiazi@gmail.com>
Cc: Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	"Jiazi.Li" <jiazi.li@transsion.com>,
	linux-mm@kvack.org, Kees Cook <keescook@chromium.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH] mm/slab: always use cache from obj
Date: Wed, 15 Feb 2023 10:41:32 +0100	[thread overview]
Message-ID: <884f051f-d1ed-756f-aef3-6ed3005de090@suse.cz> (raw)
In-Reply-To: <20230215053748.GA11780@Jiazi.Li>

On 2/15/23 06:49, lijiazi wrote:
> On Tue, Feb 14, 2023 at 11:33:21AM +0100, Vlastimil Babka wrote:
>> On 2/14/23 11:19, Jiazi.Li wrote:
>> > If free obj to a wrong cache, in addition random, different offset
>> > and object_size will also cause problems:
>> > 1. The offset of a cache with a ctor is not zero, free an object from
>> > this cache to cache with offset zero, will write next freepointer to
>> > wrong location, resulting in confusion of freelist.
>> 
>> Kernels hardened against freelist corruption will enable
>> CONFIG_SLAB_FREELIST_HARDENED, so that's already covered, no?
>> 
> Yes, HARDENED already covered.
>> > 2. If wrong cache want init on free, and cache->object_size is large
>> > than obj size, which may lead to overwrite issue.
>> 
>> In general, being defensive against usage errors is part of either hardening
>> or debugging, which is what the existing code takes into account.
>> 
> My consideration is for the wrong cache problem on version without
> HARDENED or debugging, it is likely to cause kernel panic, and such
> problem is difficult to analyze on non-debug version.
> When reproducing this problem on debug version, it will not cause kernel
> panic, but only print the WARN log, then use correct cache to free obj.
> Because we want to reproduce kernel panic problem, so may ignore WARN
> log and think that can not reproduce problem on debug version.

If you need the panic in order to e.g. capture a crash dump, you could
enable slab debugging and boot with panic_on_warn to make the WARN result in
panic.

> Thanks for your reply, I will enable CONFIG_SLAB_FREELIST_HARDENED on
> non-debug version later.
>> > Compared with adding a lot of if-else, it may be better to use obj's
>> > cache directly.
>> > 
>> > Signed-off-by: Jiazi.Li <jiazi.li@transsion.com>
>> > ---
>> >  mm/slab.h | 4 ----
>> >  1 file changed, 4 deletions(-)
>> > 
>> > diff --git a/mm/slab.h b/mm/slab.h
>> > index 63fb4c00d529..ed39b2e4f27b 100644
>> > --- a/mm/slab.h
>> > +++ b/mm/slab.h
>> > @@ -670,10 +670,6 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
>> >  {
>> >  	struct kmem_cache *cachep;
>> >  
>> > -	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
>> > -	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
>> > -		return s;
>> > -
>> >  	cachep = virt_to_cache(x);
>> >  	if (WARN(cachep && cachep != s,
>> >  		  "%s: Wrong slab cache. %s but object is from %s\n",
>> 