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

[H. Peter Anvin]

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


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