Re: [kernel-hardening] [PATCH v2 00/11] MIPS relocatable kernel & KASLR

Kees Cook
2016-04-04T23:56:58Z

On Mon, Apr 4, 2016 at 4:37 PM, Ralf Baechle <ralf@linux-mips.org> wrote:
> On Mon, Apr 04, 2016 at 12:46:29PM -0700, Kees Cook wrote:
>
>> This is great! Thanks for working on this! :)
>>
>> Without actually reading the code yet, I wonder if the x86 and MIPS
>> relocs tool could be merged at all? Sounds like it might be more
>> difficult though -- the relocation output is different and its storage
>> location is different...
>>
>> > Restrictions:
>> > * The new kernel is not allowed to overlap the old kernel, such that
>> > the original kernel can still be booted if relocation fails.
>>
>> This sounds like physical-only relocation then? Is the virtual offset
>> randomized as well (like arm64) or just physical (like x86 currently
>> -- though there is a series to fix this).
>
> On MIPS we normally place the kernel in KSEG0 or XKPHYS which address
> segments which are not mapped through the TLB so the difference is
> kinda moot.

Ah-ha, excellent. Does this mean that MIPS is effectively doing memory
segmentation between userspace and kernel space (or some version of
x86's SMEP/SMAP or ARM's PXN/PAN)? I don't know much about the MIPS
architecture yet.

What do I need to fill in on these tables for MIPS?

http://kernsec.org/wiki/index.php/Exploit_Methods/Userspace_execution
http://kernsec.org/wiki/index.php/Exploit_Methods/Userspace_data_usage

>
>> > * Relocation is supported only by multiples of 64k bytes. This
>> > eliminates the need to handle R_MIPS_LO16 relocations as the bottom
>> > 16bits will remain the same at the relocated address.
>>
>> IIUC, that's actually better than x86, which needs to be 2MB aligned.
>
> On MIPS a key concern was maintaining a reasonable size for the final
> kernel image. The R_MIPS_LO16 relocatio records make a significant
> portion of the relocations in a relocatable .o file, so we wanted to
> get rid of them. This results in a relocation granularity of 64kB.
> If we were truely, truely stingy we could come up with a relocation format
> to save a few more bits but I doubt that'd make any sense.
>
>> > * In 64 bit kernels, relocation is supported only within the same 4Gb
>> > memory segment as the kernel link address (CONFIG_PHYSICAL_START).
>> > This eliminates the need to handle R_MIPS_HIGHEST and R_MIPS_HIGHER
>> > relocations as the top 32bits will remain the same at the relocated
>> > address.
>>
>> Interesting. Could the relocation code be updated in the future to
>> bump the high addresses too?
>
> It could but yet again, the idea was to keep the size of the final
> generated file under control. The R_MIPS_HIGHER and R_MIPS_HIGHEST
> relocations can be discarded if we constrain the addresses to be in
> a single 4GB segment. Removing this constraint would make a kernel
> image much bigger so I suggested to add this restriction at least for
> this initial version.

Awesome, thanks for the details.

-Kees

--
Kees Cook
Chrome OS & Brillo Security