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

[Thomas Garnier]

On Tue, Jan 10, 2017 at 2:27 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>
>> Coming back on that after a bit more testing. The LTR instruction
>> check if the busy bit is already set, if already set then it will just
>> issue a #GP given a bad selector:
>>
>> [    0.000000] general protection fault: 0040 [#1] SMP
>> ...
>> [    0.000000] RIP: 0010:native_load_tr_desc+0x9/0x10
>> ...
>> [    0.000000] Call Trace:
>> [    0.000000]  cpu_init+0x2d0/0x3c0
>> [    0.000000]  trap_init+0x2a2/0x312
>> [    0.000000]  start_kernel+0x1fb/0x43b
>> [    0.000000]  ? set_init_arg+0x55/0x55
>> [    0.000000]  ? early_idt_handler_array+0x120/0x120
>> [    0.000000]  x86_64_start_reservations+0x2a/0x2c
>> [    0.000000]  x86_64_start_kernel+0x13d/0x14c
>> [    0.000000]  start_cpu+0x14/0x14
>>
>> I assume that's in this part of the pseudo-code:
>>
>> if(!IsWithinDescriptorTableLimit(Source.Offset) || Source.Type !=
>> TypeGlobal) Exception(GP(SegmentSelector));
>> SegmentDescriptor = ReadSegmentDescriptor();
>> if(!IsForAnAvailableTSS(SegmentDescriptor))
>> Exception(GP(SegmentSelector)); <---- That's where I got the GP
>> TSSSegmentDescriptor.Busy = 1;
>> <------------------------------------------------------------------
>> That's the pagefault I get otherwise
>> //Locked read-modify-write operation on the entire descriptor when
>> setting busy flag
>> TaskRegister.SegmentSelector = Source;
>> TaskRegister.SegmentDescriptor.TSSSegmentDescriptor;
>>
>> I assume the best option would be to make the remap read-write for the
>> LTR instruction. What do you think?
>
> So if LTR does not modify the GDT if the busy bit is already set, why don't we set
> the busy bit in the descriptor (via the linear mapping rw alias).
>
> Then the remapped GDT can stay read-only all the time and LTR won't fault.
>
> Am I missing something here?
>

Sorry, I may not have explained myself well.

If you set the busy bit on the GDT TSS entry, you get a #GP on LTR.

When we use the LTR instruction, we need the GDT to be writeable.

I think I can handle it by switching to the remap GDT after
load_TR_desc. I will try this approach.

> Thanks,
>
>         Ingo



-- 
Thomas