Re: [PATCH v4 54/57] x86/mm: convert arch_within_stack_frames() to use the new unwinder

[Kees Cook]

On Mon, Aug 22, 2016 at 4:33 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Mon, Aug 22, 2016 at 03:27:19PM -0500, Josh Poimboeuf wrote:
>> On Fri, Aug 19, 2016 at 04:55:22PM -0500, Josh Poimboeuf wrote:
>> > On Fri, Aug 19, 2016 at 11:27:18AM -0700, Kees Cook wrote:
>> > > On Thu, Aug 18, 2016 at 6:06 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > > > Convert arch_within_stack_frames() to use the new unwinder.
>> > > >
>> > > > This also changes some existing behavior:
>> > > >
>> > > > - Skip checking of pt_regs frames.
>> > > > - Warn if it can't reach the grandparent's stack frame.
>> > > > - Warn if it doesn't unwind to the end of the stack.
>> > > >
>> > > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
>> > >
>> > > All the stuff touching usercopy looks good to me. One question,
>> > > though, in looking through the unwinder. It seems like it's much more
>> > > complex than just the frame-hopping that the old
>> > > arch_within_stack_frames() did, but I'm curious to hear what you think
>> > > about its performance. We'll be calling this with every usercopy that
>> > > touches the stack, so I'd like to be able to estimate the performance
>> > > impact of this replacement...
>> >
>> > Yeah, good point.  I'll take some measurements from before and after and
>> > get back to you.
>>
>> I took some before/after measurements by enclosing the affected
>> functions with ktime calls to get the total time spent in each function,
>> and did a "find /usr >/dev/null" to trigger a bunch of user copies.
>>
>>       copy_to/from_user       check_object_size       arch_within_stack_frames
>> before: 13ms                  6.8ms                   0.61ms
>> after:        17ms                    11ms                    4.6ms
>>
>> The unwinder port made arch_within_stack_frames() *much* (8x) slower
>> than its current simple implementation, and added about 30% (4ms) to the
>> total copy_to/from_user() run time.
>>
>> Note that hardened usercopy itself is already quite slow: it made user
>> copies about 52% slower.  With the unwinder port, that worsened to ~65%.
>
> FWIW, I think I messed up my math summary here.  Hardened usercopy was
> roughly 110% slower than normal usercopy (i.e., it took more than twice
> as long) with 52% of the usercopy time being consumed by
> check_object_size().

And this is comparing usercopy to hardened usercopy, which isn't
expected to be super fast, it's just a cheap expense in comparison to
the rest of the work being done for a given syscall.

> With the unwinder, that worsened to 180% slower -- with 65% of the
> usercopy time being consumed by check_object_size().

That's quite a bit more than just a simple frame walk. You mentioned a
few benefits to using the unwinder, but I'm trying to make sure the
cases it covers can actually happen during a usercopy?

-Kees

-- 
Kees Cook
Nexus Security