Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

["H. Peter Anvin" <hpa@zytor.com>]

On 10/10/2014 08:14 PM, Baoquan He wrote:
> On 10/08/14 at 03:27pm, Vivek Goyal wrote:
>> On Wed, Oct 08, 2014 at 08:09:59AM -0700, H. Peter Anvin wrote:
>
>>> Sorry... this makes no sense.
>>>
>>> For x86-64, there is no direct connection between the physical and
>>> virtual address spaces that the kernel runs in...
>>
>> I am sorry I did not understand this one. I thought that initial
>> relocatable kernel implementaion did not have any direct connection
>> between virtual and physical address. One could load kernel anywhere
>> and kernel virtual address will not change and we will just adjust
>> page tables to map virtual address to right physical address.
>>
>> Now handle_relocation() stuff seems to introduce a close coupling
>> between physical and virtual address. So if kernel shifts by 16MB
>> in physical address space, then it will shift by equal amount
>> in virtual address space. So there seems to be a direct connection
>> between virtual and physical address space in this case.
>
> Yeah, it's exactly as Vivek said.
>
> Before kaslr was introduced, x86_64 kernel can be put anywhere, and
> always _text is 0xffffffff81000000. Meanwhile phys_base contains the
> offset between the compiled addr (namely 0x1000000) and kernel loaded
> addr. After kaslr implementation was added, as long as kernel loaded
> addr is different 0x1000000, it will call handle_relocations(). The
> offset now is added onto each symbols including _text and phys_base
> becomes 0.
>
> It's clearly showing that by checking /proc/kallsyms and value of
> phys_base.
>

This really shouldn't have happened this way on x86-64.  It has to 
happen this way on i386, but I worry that this may be a serious 
misdesign in kaslr on x86-64.  I'm also wondering if there is any other 
fallout of this?

	-hpa