Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

[Kees Cook]

On Tue, Jul 25, 2017 at 10:58 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> [ Adding Kees to CC for the hardened usercopy discussion. ]
>
> Kees, FYI: frame pointers may be disabled by default on x86 relatively
> soon (presumably weeks or months) in favor of the ORC unwinder.  So the
> hardened usercopy stack walk will no longer work as advertised.
>
> Using the ORC unwinder for hardened usercopy would probably be pretty
> bad performance-wise.  I'm not sure what else could be done.  Ingo did
> have a few ideas for sanity checks:
>
> On Tue, Jul 25, 2017 at 11:09:44AM +0200, Ingo Molnar wrote:
>> > > > Well, on x86, hardened usercopy relies on frame pointers, but not the
>> > > > unwinder.  It does the frame pointer walk manually to avoid the full
>> > > > unwinder overhead.  See arch_within_stack_frames().
>>
>> BTW., I think this aspect of the hardened user-copy is crazy stuff - there can be
>> many stack frames, and this adds a serious amount of overhead even with frame
>> pointers...
>>
>> I think the current behavior is fine: if frame pointers are disabled then
>> arch_within_stack_frames() returns NOT_STACK. Maybe it could do a few sanity
>> checks: we do know the kernel stack range and we could check alignment as well.
>
> I believe it checks the kernel stack range already in
> check_stack_object() before deciding whether to call
> arch_within_stack_frames().  It also has an overlapping stack check.

Right, pointers starting in the stack are already checked to not go
beyond the stack.

As far as dropping inter-frame overflow checking, while I'd prefer to
keep it, but its benefit in my mind is already pretty minimal since it
already doesn't protect/exclude the stack canary. And since this is a
check for a linear overflow (i.e. a contiguous access) we're mostly
protected by the existing stack canary for writes. For reads, we do
risk allowing return addresses to get exposed, though without the
frame pointer, we've got even less to expose in the first place.

So, mainly, I'm fine with this. I'm slightly sad, but it's not a huge
loss. The main benefit of usercopy hardening is the slab cache object
size protections...

-Kees

-- 
Kees Cook
Pixel Security