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Date: Tue, 3 Jan 2017 16:31:37 +0800
From: Baoquan He <bhe@redhat.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>, Dave Jiang <dave.jiang@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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        Kees Cook <keescook@chromium.org>, dyoung@redhat.com
Subject: Re: [PATCH] x86: fix kaslr and memmap collision
Message-ID: <20170103083137.GA15788@x1>
References: <147977413859.13657.2181994710415174471.stgit@djiang5-desk3.ch.intel.com>
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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@kernel.org> wrote:
> >
> > * Dave Jiang <dave.jiang@intel.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.

Thanks
Baoquan