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

From: Vivek Goyal
Date: Mon Oct 13 2014 - 08:53:56 EST

On Sat, Oct 11, 2014 at 03:34:29AM -0700, H. Peter Anvin wrote:
> 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?

I agree. On x86_64, we should stick to previous design and this new
logic of performing relocations does not sound very clean and makes
things very confusing.

I am wondering that why couldn't we simply adjust page tables in case of
kaslr on x86_64, instead of performing relocations.

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