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

[Andy Lutomirski]

On Fri, Jan 6, 2017 at 10:02 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> On Thu, Jan 5, 2017 at 6:34 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Thu, Jan 5, 2017 at 3:05 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>>
>>>> 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.
>>>
>>> I'm not sure that this is architecturally safe.
>>>
>>
>> Hmm.  Last time I looked, I couldn't find *anything* in the SDM
>> explaining what happened if a GDT access resulted in a page fault.  I
>> did discover that Xen intentionally (!) lazily populates and maps LDT
>> pages.  An attempt to access a not-present page results in #PF with
>> the error cod e indicating kernel access even if the access came from
>> user mode.
>>
>> SDM volume 3 7.2.2 says "Pages corresponding to the previous taskâs
>> TSS, the current taskâs TSS, and the descriptor table entries for
>> each all should be marked as read/write."  But I don't see how a CPU
>> implementation could possibly care what the page table for the TSS
>> descriptor table entries says after LTR is done because the CPU isn't
>> even supposed to *read* that memory.
>>
>> OTOH a valid implementation could easily require that the page table
>> says that the page is writable merely to load a segment, especially in
>> weird cases (IRET?).  That being said, this is all quite easy to test.
>>
>> Also, Thomas, why are you creating a new memory region?  I don't see
>> any benefit to randomizing the GDT address.  How about just putting it
>> in the fixmap?  This  would be NR_CPUS * 4 pages if do my limit=0xffff
>> idea.  I'm not sure if the fixmap code knows how to handle this much
>> space.
>
> When I looked at the fixmap, you had to define the space you need
> ahead of time and I am not sure there was enough space as you said.

Can you try it and see if anything goes wrong?  Even if something does
go wrong, I think we should fix *that* rather than making the memory
layout more complicated.