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

[Andy Lutomirski]

On Thu, Jan 5, 2017 at 11:03 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> On Thu, Jan 5, 2017 at 10:58 AM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> wrote:
>> On 1/5/2017 9:54 AM, Thomas Garnier wrote:
>>
>>>
>>> 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.
>>
>>
>> don't we write to the GDT as part of the TLS segment stuff for glibc ?
>>
>
> Not sure which glibc feature it is.
>
> In this design, you can write to the GDT per-cpu variable that will
> remain read-write. You just need to make the remapping writeable when
> we load task registers (ltr) then the processor use the current GDT
> address. At least that the case I know, I might find more through
> testing.

Hmm.  I bet that if we preset the accessed bits in all the segments
then we don't need it to be writable in general.  But your point about
set_thread_area (TLS) is well taken.  However, I strongly suspect that
we could make set_thread_area unconditionally set the accessed bit and
no one would ever notice.