On Mon, Aug 22, 2016 at 05:59:18PM -0700, Kees Cook wrote: > On Mon, Aug 22, 2016 at 4:33 PM, Josh Poimboeuf <jpoim...@redhat.com> 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 <jpoim...@redhat.com> > >> > > 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 <jpoim...@redhat.com> > >> > > > >> > > 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?
Yeah, I really don't know. And given Linus's objections, I think I'll drop it, and the other usercopy patches. Though I think "move arch_within_stack_frames() to usercopy.c" is still a nice cleanup if you want to pick that one up. -- Josh
