Re: [PATCH 0/6] randomize kernel physical address and virtual address separately

[Baoquan He]

On 01/20/15 at 08:19pm, Andy Lutomirski wrote:
> On 01/20/2015 07:37 PM, Baoquan He wrote:
> > Currently kaslr only randomize physical address of kernel loading, then add the delta
> > to virtual address of kernel text mapping. Because kernel virtual address can only be
> > from __START_KERNEL_map to LOAD_PHYSICAL_ADDR+CONFIG_RANDOMIZE_BASE_MAX_OFFSET, namely
> > [0xffffffff80000000, 0xffffffffc0000000], so physical address can only be randomized
> > in region [LOAD_PHYSICAL_ADDR, CONFIG_RANDOMIZE_BASE_MAX_OFFSET], namely [16M, 1G].
> > 
> > So hpa and Vivek suggested the randomization should be done separately for both physical
> > and virtual address. In this patchset I tried it. And after randomization, relocation
> > handling only depends on virtual address changing, means I only check whether virtual
> > address is randomized to other position, if yes relocation need be handled, if no just
> > skip the relocation handling though physical address is randomized to different place.
> > Now physical address can be randomized from 16M to 4G, virtual address offset can be
> > from 16M to 1G.
> > 
> > Leftover problem:
> >     hpa want to see the physical randomization can cover the whole physical memory. I
> > checked code and found it's hard to do. Because in arch/x86/boot/compressed/head_64.S
> > an identity mapping of 4G is built and then kaslr and decompressing are done. The #PF
> > handler solution which he suggested is only available after jump into decompressed
> > kernel, namely in arch/x86/kernel/head_64.S. I didn't think of a way to do the whole
> > memory covering for physical address randomization, any suggestion or idea?
> > 
> 
> I have no idea what the #PF thing you're referring to is, but I have
> code to implement a #PF handler in boot/compressed if it would be
> helpful.  It's two patches:
> 
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=sync_rand_seed&id=89476ea6a2becbaee4f45c3b6689ff31b6aa959a

It's awesome, I am gonna try it.

Thanks a lot!

Thanks
Baoquan

> 
> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=sync_rand_seed&id=142d86921e6f271261584016fc8cfa5cdbf455ba
> 
> You can't recover from a page fault in my version of this code, but that
> would be straightforward to add.
> 
> --Andy
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