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 <[email protected]> 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 <[email protected]>
> >
> > 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%.
"find /usr" took about 170ms of kernel time and 2.3s total. So the
unwinder port added about 2% on the kernel side and 0.2% total for this
particular test case. Though I'm sure there are more I/O-intensive
workloads out there which would be more adversely affected.
I haven't yet looked to see where the bottlenecks are and if there could
be any obvious performance improvements.
BTW, ignoring the performance issues, using the unwinder here would have
some benefits:
- It protects pt_regs frames from being changed. For example, during a
page fault operation, the saved regs->ip on the stack is protected.
- Unlike the existing code, it could potentially work with
__copy_from_user_inatomic() and copy_from_user_nmi(), which can copy
to/from an irq/exception stack. (I think check_stack_object() would
need to be rewritten a bit so that it doesn't always assume the task
stack.)
- It complains loudly if there's stack corruption or something else goes
wrong with walking the stack instead of just silently failing.
- The same code could also work with DWARF if we ever add a DWARF
unwinder (with a possible tweak to the unwinder API to get the stack
frame header size).
--
Josh
