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

[Thomas Garnier]

On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 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.

Not sure I fully understood and I don't want to miss an important
point. Do you mean making GDT (remapping and per-cpu) read-only and
switch the writeable flag only when we write to the per-cpu entry?

-- 
Thomas