Subject: Re: [kernel-hardening] [RFC][PATCH 6/7] mm: Add Kconfig option for
 slab sanitization
To: Dave Hansen <dave.hansen@intel.com>, Christoph Lameter <cl@linux.com>
References: <1450755641-7856-1-git-send-email-laura@labbott.name>
 <1450755641-7856-7-git-send-email-laura@labbott.name>
 <567964F3.2020402@intel.com>
 <alpine.DEB.2.20.1512221023550.2748@east.gentwo.org>
 <567986E7.50107@intel.com>
 <alpine.DEB.2.20.1512221124230.14335@east.gentwo.org>
 <56798851.60906@intel.com>
 <alpine.DEB.2.20.1512221207230.14406@east.gentwo.org>
 <5679943C.1050604@intel.com>
Cc: kernel-hardening@lists.openwall.com, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>
From: Laura Abbott <laura@labbott.name>
Message-ID: <5679A0CB.3060707@labbott.name>
Date: Tue, 22 Dec 2015 11:13:15 -0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:38.0)
 Gecko/20100101 Thunderbird/38.4.0
MIME-Version: 1.0
In-Reply-To: <5679943C.1050604@intel.com>
Content-Type: text/plain; charset=windows-1252; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1600
Lines: 40

On 12/22/15 10:19 AM, Dave Hansen wrote:
> On 12/22/2015 10:08 AM, Christoph Lameter wrote:
>> On Tue, 22 Dec 2015, Dave Hansen wrote:
>>>> Why would you use zeros? The point is just to clear the information right?
>>>> The regular poisoning does that.
>>>
>>> It then allows you to avoid the zeroing at allocation time.
>>
>> Well much of the code is expecting a zeroed object from the allocator and
>> its zeroed at that time. Zeroing makes the object cache hot which is an
>> important performance aspect.
>
> Yes, modifying this behavior has a performance impact.  It absolutely
> needs to be evaluated, and I wouldn't want to speculate too much on how
> good or bad any of the choices are.
>
> Just to reiterate, I think we have 3 real choices here:
>
> 1. Zero at alloc, only when __GFP_ZERO
>     (behavior today)
> 2. Poison at free, also Zero at alloc (when __GFP_ZERO)
>     (this patch's proposed behavior, also what current poisoning does,
>      doubles writes)
> 3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO)
>     (what I'm suggesting, possibly less perf impact vs. #2)
>
>

poisoning with non-zero memory makes it easier to determine that the error
came from accessing the sanitized memory vs. some other case. I don't think
the feature would be as strong if the memory was only zeroed vs. some other
data value.

Thanks,
Laura
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/