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Date:   Mon, 12 Mar 2018 20:04:15 +0800
From:   Chao Fan <fanc.fnst@cn.fujitsu.com>
To:     Ingo Molnar <mingo@kernel.org>
CC:     Baoquan He <bhe@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-kernel@vger.kernel.org>, <x86@kernel.org>, <hpa@zytor.com>,
        <tglx@linutronix.de>, <mingo@redhat.com>, <keescook@chromium.org>,
        <yasu.isimatu@gmail.com>, <indou.takao@jp.fujitsu.com>,
        <lcapitulino@redhat.com>, <fanc.fnst@cn.fujitsu.com>
Subject: Re: [PATCH v9 0/5] x86/KASLR: Add parameter
 kaslr_boot_mem=nn[KMG]@ss[KMG]
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On Mon, Mar 12, 2018 at 11:57:27AM +0100, Ingo Molnar wrote:
>
>* Baoquan He <bhe@redhat.com> wrote:
>
>> Hi Ingo,
>> 
>> On 03/12/18 at 10:35am, Ingo Molnar wrote:
>> > 
>> > * Chao Fan <fanc.fnst@cn.fujitsu.com> wrote:
>> > 
>> > > Long time no reply, rebase the patchset, change the parameter name
>> > > from 'kaslr_mem' to 'kaslr_boot_mem'. There's no more code change.
>> > > 
>> > > ***Background:
>> > > People reported that kaslr may randomly chooses some positions
>> > > which are located in movable memory regions. This will break memory
>> > > hotplug feature.
>> > 
>> > [...]
>> > 
>> > > ***Solutions:
>> > > Introduce a new kernel parameter 'kaslr_boot_mem=nn@ss' to let users to
>> > > specify the memory regions where kernel can be allowed to randomize
>> > > safely.
>> > 
>> > Manual solutions like that are pretty suboptimal to users, aren't they?
>> > 
>> > In what way does memory hotplug feature 'break'? Does it crash or misbehave? Or 
>> > simply does it not allow the movement of the affected memory region, while still 
>> > allowing the rest to be moved?
>> 
>> AFAIT, if kernel is randomized into the movable memory region, the
>> affected memory region can not be hot added/removed since it has kernel
>> data. Surely, the system can still work, the unaffected part still can
>> be moved. Still it will cause regression on memory hotplug.
>> 
>> Mainly we parse SRAT table to get the ranges of memory provided by
>> hot-added memory devices in initmem_init(), that's very late. During boot,
>> we don't know it. Chao ever posted patches to grab SRAT at decompressing
>> stage, the code is very complicated and not elegant, ACPI maintainer
>> NACKed that.

Thanks for Ingo's suggestion and Baoquan's explaination.

Yes, I did ever try to dig SRAT table in boot period in early RFC PATCH:
https://lkml.org/lkml/2017/9/3/77
But the change is too huge so made this patchset to avoid this bug in a
small change, which will not make the code looks messy.

Thanks,
Chao Fan

>
>So there's apparently a mis-design here:
>
> - KASLR needs to be done very early on during bootup: - it's not realistic to 
>   expect KASLR to be done with a booted up kernel, because pointers to various 
>   KASLR-ed objects are already widely spread out in memory.
>
> - But for some unfathomable reason the memory hotplug attribute of memory
>   regions is not part of the regular memory map but part of late-init ACPI data
>   structures.
>
>The right solution would be _not_ to fudge the KASLR location, but to provide the 
>memory hotplug information to early code, preferably via the primary memory map. 
>KASLR can then make use of it and avoid those regions, just like it avoids other 
>memory regions already.
>
>In addition to that hardware makers (including virtualized hardware) should also 
>fix their systems to provide memory hotplug information to early code.
>
>Thanks,
>
>	Ingo
>
>