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

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Message-ID: <CAPcyv4jTsUD25OGW2iwF1V7i2CTF_-+e4CsTw8NQXXVE8QhsYw@mail.gmail.com>
Date:   Tue, 3 Jan 2017 10:24:46 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Dave Jiang <dave.jiang@...el.com>,
        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 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.