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

[Andy Lutomirski]

On Thu, Jan 5, 2017 at 9:54 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> 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.

If you want a small adventure, I think a significant KVM-related
performance improvement is available.  Specifically, on VMX exits, the
GDT limit is hardwired to 0xffff (IIRC -- I could be remembering the
actual vaue wrong).  KVM does LGDT to fix it.

If we actually made the GDT have limit 0xffff (presumably by mapping
the zero page a few times to pad it out without wasting memory), then
we would avoid the LGDT.  LGDT is incredibly slow, so this would be a
big win. Want to see if you can make this work with your patch set?