linux-kernel - Re: [PATCH] x86: fix kaslr and memmap collision

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Date:   Tue, 3 Jan 2017 13:15:12 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Baoquan He <bhe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        david <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...1.01.org>,
        Kees Cook <keescook@...omium.org>, dyoung@...hat.com
Subject: Re: [PATCH] x86: fix kaslr and memmap collision



On 01/03/2017 11:24 AM, Dan Williams wrote:
> On Tue, Jan 3, 2017 at 12:31 AM, Baoquan He <bhe@...hat.com> wrote:
>> Hi Dan,
>>
>> On 11/22/16 at 09:26am, Dan Williams wrote:
>>> [ replying for Dave since he's offline today and tomorrow ]
>>>
>>> On Tue, Nov 22, 2016 at 12:47 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>>>
>>>> * Dave Jiang <dave.jiang@...el.com> wrote:
>>>>
>>>>> CONFIG_RANDOMIZE_BASE relocates the kernel to a random base address.
>>>>> However it does not take into account the memmap= parameter passed in from
>>>>> the kernel commandline.
>>>>
>>>> memmap= parameters are often used as a list.
>>>>
>>>>> [...] This results in the kernel sometimes being put in the middle of the user
>>>>> memmap. [...]
>>>>
>>>> What does this mean? If memmap= is used to re-define the memory map then the
>>>> kernel getting in the middle of a RAM area is what we want, isn't it? What we
>>>> don't want is for the kernel to get into reserved areas, right?
>>>
>>> Right, this is about teaching kaslr to not land the kernel in newly
>>> defined reserved regions that were not marked reserved in the initial
>>> e820 map from platform firmware.
>>
>> If only tell kaslr to not land kernel in newly defined reserved regions,
>> memory added by "memmap=nn[KMG]@ss[KMG]" should not be avoided since
>> it's usable memory. Kernel randomized into this region is also what we
>> want. Not sure if I understand it right.
> 
> You're right, this is supposed to be for memmap=nn!ss cases which
> defines reserved persistent memory ranges, not memmap=nn@ss which
> defines usable memory.
> 
> We need to fix mem_avoid_memmap() to only skip memmap= statements that
> specify reserved memory.
> 

I think nn@ss is the only one that we should skip over, otherwise
everything else looks like should be avoided. I'll update.