Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI to enable/disable PTI

[Andy Lutomirski]

On Tue, Jan 9, 2018 at 6:54 AM, Willy Tarreau <w@xxxxxx> wrote:
> On Tue, Jan 09, 2018 at 03:51:57PM +0100, Borislav Petkov wrote:
>> On Tue, Jan 09, 2018 at 03:36:53PM +0100, Willy Tarreau wrote:
>> > I see and am not particularly against this, but what use case do you
>> > have in mind precisely ? I doubt it's just saving a few tens of bytes,
>> > so probably you're more concerned about the potential risks this opens ?
>> > But given we only allow this for CAP_SYS_RAWIO and these ones already
>> > have access to /dev/mem and many other things, don't you think there
>> > are much easier ways to dump kernel memory in this case than trying to
>> > inject some meltdown code into the victim process ? Or maybe you have
>> > other cases in mind that I'm not seeing.
>>
>> I'd like this to be config-controllable so that distros can make the
>> decision whether/if they want to support the whole per-mm thing.
>
> OK.
>
>> Also, if CAP_SYS_RAWIO is going to protect, please make the
>> ARCH_GET_NOPTI variant check it too.
>
> Interestingly I removed the check consecutive to the discussions. But
> I think I'll simply remove the whole ARCH_GET_NOPTI as it has no real
> value beyond initial development.
>

I've thought about this a bit more.  Here are my thoughts:

1. I don't like it being per-mm.  I think it should be a per-thread
control so that a program can have a thread with PTI that runs
less-trusted JavaScript and other network threads with PTI off.
Obviously we lose NX protection mm-wide if any threads have PTI off.
I think the way to implement this is:

Have this in struct mm_context:

bool has_non_pti_thread;

To turn PTI off on a thread:

Take pagetable_lock.
if (!has_non_pti_thread) {
  context.has_non_pti_thread = true;
  clear the NX bits;
}
drop pagetable_lock;
set the TI flag;

Fork clears the per-mm flag in the new mm.  Exec clears it, too.  I
think that's all that's needed.  Newly created threads always have PTI
on.

To turn PTI back on, just clear the TI flag.

2.Turning off PTI is, in general, a terrible idea.  It totally breaks
any semblance of a security model on a Meltdown-affected CPU.  So I
think we should require CAP_SYS_RAWIO *and* that the system is booted
with pti=allow_optout or something like that.

--Andy