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

[Thomas Garnier <thgarnie@google.com>]

On Fri, Jan 6, 2017 at 11:35 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Thomas Garnier <thgarnie@google.com> wrote:
>
>> > No, and I had the way this worked on 64-bit wrong.  LTR requires an
>> > available TSS and changes it to busy.  So here are my thoughts on how
>> > this should work:
>> >
>> > Let's get rid of any connection between this code and KASLR.  Every
>> > time KASLR makes something work differently, a kitten turns all
>> > Schrödinger on us.  This is moving the GDT to the fixmap, plain and
>> > simple.  For now, make it one page per CPU and don't worry about the
>> > GDT limit.
>>
>> I am all for this change but that's more significant.
>>
>> Ingo: What do you think about that?
>
> I agree with Andy: as I alluded to earlier as well this should be an unconditional
> change (tested properly, etc.) that robustifies the GDT mapping for everyone. That
> KASLR kernels improve too is a happy side effect!
>
>> > On 32-bit, we're going to have to make the fixmap GDT be read-write because
>> > making it read-only will break double-fault handling.
>> >
>> > On 64-bit, we can use your trick of temporarily mapping the GDT read-write
>> > every time we load TR, which should happen very rarely. Alternatively, we can
>> > reload the *GDT* every time we reload TR, which should be comparably slow.
>> > This is going to regress performance in the extremely rare case where KVM
>> > exits to a process that uses ioperm() (I think), but I doubt anyone cares.  Or
>> > maybe we could arrange to never reload TR when GDT points at the fixmap by
>> > having KVM set the host GDT to the direct version and letting KVM's code to
>> > reload the GDT switch to the fixmap copy.
>
> Please check whether the LTR write generates a page fault to a RO PTE even if the
> busy bit is already set. LTR is pretty slow which suggests that it's microcode,
> and microcode is usually not sloppy about such things: i.e. LTR would only
> generate an unconditional write if there's a compatibility dependency on it. But I
> could easily be wrong ...
>

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?

>> > If we need a quirk to keep the fixmap copy read-write, so be it.
>> >
>> > None of this should depend on KASLR.  IMO it should happen unconditionally.
>>
>> I looked back at the fixmap, and I can see a way it could be done
>> (using NR_CPUS) like the other fixmap ranges. It would limit the
>> number of cpus to 512 (there is 2M memory left on fixmap on the
>> default configuration). That's if we never add any other fixmap on
>> x64. I don't know if it is an acceptable number and if the fixmap
>> region could be increased. (128 if we do your kvm trick, of course).
>>
>> Ingo: What do you think?
>
> I think we should scale the fixmap size flexibly with NR_CPUs on 64-bit, and we
> should limit CPUs on 32-bit to a reasonable value.
>
> I.e. let's just do it, if we run into problems it's all solvable AFAICS.
>
> Thanks,
>
>         Ingo



-- 
Thomas