Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

[Thomas Garnier]

On Thu, Jan 5, 2017 at 9:51 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>> Each processor holds a GDT in its per-cpu structure. The sgdt
>> instruction gives the base address of the current GDT. This address can
>> be used to bypass KASLR memory randomization. With another bug, an
>> attacker could target other per-cpu structures or deduce the base of the
>> main memory section (PAGE_OFFSET).
>>
>> In this change, a space is reserved at the end of the memory range
>> available for KASLR memory randomization. The space is big enough to hold
>> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is
>> mapped at specific offset based on the target CPU. Note that if there is
>> not enough space available, the GDTs are not remapped.
>
> Can we remap it read-only?  I.e. use PAGE_KERNEL_RO instead of
> PAGE_KERNEL.  After all, the ability to modify the GDT is instant
> root.

That's my goal too. I started by doing a RO remap and got couple
problems with hibernation. I can try again for the next iteration or
delay it for another patch. I also need to look at KVM GDT usage, I am
not familiar with it yet.

-- 
Thomas