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Subject: Re: [PATCH v4] mm/vmalloc: randomize vmalloc() allocations
To:     Uladzislau Rezki <urezki@gmail.com>,
        Kees Cook <keescook@chromium.org>
Cc:     linux-hardening@vger.kernel.org, akpm@linux-foundation.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Jann Horn <jannh@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>
References: <20210309135757.5406-1-toiwoton@gmail.com>
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 <20210315174742.GA2038@pc638.lan>
From:   Topi Miettinen <toiwoton@gmail.com>
Message-ID: <85515ea8-744e-acec-76ba-034b38d0f9fa@gmail.com>
Date:   Tue, 16 Mar 2021 10:01:46 +0200
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On 15.3.2021 19.47, Uladzislau Rezki wrote:
> On Mon, Mar 15, 2021 at 09:16:26AM -0700, Kees Cook wrote:
>> On Mon, Mar 15, 2021 at 01:24:10PM +0100, Uladzislau Rezki wrote:
>>> On Mon, Mar 15, 2021 at 11:04:42AM +0200, Topi Miettinen wrote:
>>>> What's the problem with that? It seems to me that nothing relies on specific
>>>> addresses of the chunks, so it should be possible to randomize these too.
>>>> Also the alignment is honored.
>>>>
>>> My concern are:
>>>
>>> - it is not a vmalloc allocator;
>>> - per-cpu allocator allocates chunks, thus it might be it happens only once. It does not allocate it often;
>>
>> That's actually the reason to randomize it: if it always ends up in the
>> same place at every boot, it becomes a stable target for attackers.
>>
> Probably we can randomize a base address only once when pcpu-allocator
> allocates a fist chunk during the boot.
> 
>>> - changing it will likely introduce issues you are not aware of;
>>> - it is not supposed to be interacting with vmalloc allocator. Read the
>>>    comment under pcpu_get_vm_areas();
>>>
>>> Therefore i propose just not touch it.
>>
>> How about splitting it from this patch instead? Then it can get separate
>> testing, etc.
>>
> It should be split as well as tested.

Would you prefer another kernel option `randomize_percpu_allocator=1`, 
or would it be OK to make it a flag in `randomize_vmalloc`, like 
`randomize_vmalloc=3`? Maybe the latter would not be compatible with 
static branches.

-Topi

> 
> --
> Vlad Rezki
>