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

[Andy Lutomirski]

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.