linux-hardening - Re: [PATCH RFC v2] Randomized slab caches for kmalloc()

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Message-ID: <19707cc6-fa5e-9835-f709-bc8568e4c9cd@huawei.com>
Date:   Mon, 22 May 2023 15:35:02 +0800
From:   Gong Ruiqi <gongruiqi1@...wei.com>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        <kasan-dev@...glegroups.com>,
        Wang Weiyang <wangweiyang2@...wei.com>,
        Xiu Jianfeng <xiujianfeng@...wei.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        "GONG, Ruiqi" <gongruiqi@...weicloud.com>
Subject: Re: [PATCH RFC v2] Randomized slab caches for kmalloc()



On 2023/05/17 6:35, Hyeonggon Yoo wrote:
> [Resending this email after noticing I did not reply-to-all]
> 
> On Fri, May 12, 2023 at 7:11 PM Gong Ruiqi <gongruiqi1@...wei.com> wrote:
>>

[...]

> 
>>>> +#ifdef CONFIG_RANDOM_KMALLOC_CACHES
>>>> +# define SLAB_RANDOMSLAB       ((slab_flags_t __force)0x01000000U)
>>>> +#else
>>>> +# define SLAB_RANDOMSLAB       0
>>>> +#endif
> 
> There is already the SLAB_KMALLOC flag that indicates if a cache is a
> kmalloc cache. I think that would be enough for preventing merging
> kmalloc caches?

After digging into the code of slab merging (e.g. slab_unmergeable(),
find_mergeable(), SLAB_NEVER_MERGE, SLAB_MERGE_SAME etc), I haven't
found an existing mechanism that prevents normal kmalloc caches with
SLAB_KMALLOC from being merged with other slab caches. Maybe I missed
something?

While SLAB_RANDOMSLAB, unlike SLAB_KMALLOC, is added into
SLAB_NEVER_MERGE, which explicitly indicates the no-merge policy.