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

[Baoquan He]

On 10/11/14 at 08:38pm, Baoquan He wrote:
> On 10/11/14 at 03:34am, 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?

Btw, except of this bug, I didn't find other risk of kaslr currently.
The code flow is straightforward and clear.

> 
> Yes, this shouldn't happen this way on x86_64. With this patch, those
> are fixed as expected. If kernel location is not chosen randomly, we
> should not do the relocations handling. If and only if kaslr is enabled
> and it relocated the kernel randomly as expected, we do the relocations
> handling.
> 
> I think this patch really makes sense and it's simple and won't impact
> i386 and other implementations.
> 
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