Re: [PATCH] x86: fix kaslr and memmap collision

[Dave Jiang]

On 01/03/2017 11:24 AM, Dan Williams wrote:
> On Tue, Jan 3, 2017 at 12:31 AM, Baoquan He <bhe@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>>>
>>>> * Dave Jiang <dave.jiang@xxxxxxxxx> 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.