Re: [PATCH v2 01/11] mm: Implement stack frame object validation

Josh Poimboeuf
2016-07-14T19:24:06Z

On Thu, Jul 14, 2016 at 11:10:18AM -0700, Kees Cook wrote:
> On Wed, Jul 13, 2016 at 10:48 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Wed, Jul 13, 2016 at 03:04:26PM -0700, Kees Cook wrote:
> >> On Wed, Jul 13, 2016 at 3:01 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >> > On Wed, Jul 13, 2016 at 2:55 PM, Kees Cook <keescook@chromium.org> wrote:
> >> >> This creates per-architecture function arch_within_stack_frames() that
> >> >> should validate if a given object is contained by a kernel stack frame.
> >> >> Initial implementation is on x86.
> >> >>
> >> >> This is based on code from PaX.
> >> >>
> >> >
> >> > This, along with Josh's livepatch work, are two examples of unwinders
> >> > that matter for correctness instead of just debugging. ISTM this
> >> > should just use Josh's code directly once it's been written.
> >>
> >> Do you have URL for Josh's code? I'd love to see what happening there.
> >
> > The code is actually going to be 100% different next time around, but
> > FWIW, here's the last attempt:
> >
> > https://lkml.kernel.org/r/4d34d452bf8f85c7d6d5f93db1d3eeb4cba335c7.1461875890.git.jpoimboe@redhat.com
> >
> > In the meantime I've realized the need to rewrite the x86 core stack
> > walking code to something much more manageable so we don't need all
> > these unwinders everywhere. I'll probably post the patches in the next
> > week or so. I'll add you to the CC list.
>
> Awesome!
>
> > With the new interface I think you'll be able to do something like:
> >
> > struct unwind_state;
> >
> > unwind_start(&state, current, NULL, NULL);
> > unwind_next_frame(&state);
> > oldframe = unwind_get_stack_pointer(&state);
> >
> > unwind_next_frame(&state);
> > frame = unwind_get_stack_pointer(&state);
> >
> > do {
> > if (obj + len <= frame)
> > return blah;
> > oldframe = frame;
> > frame = unwind_get_stack_pointer(&state);
> >
> > } while (unwind_next_frame(&state);
> >
> > And then at the end there'll be some (still TBD) way to query whether it
> > reached the last syscall pt_regs frame, or if it instead encountered a
> > bogus frame pointer along the way and had to bail early.
>
> Sounds good to me. Will there be any frame size information available?
> Right now, the unwinder from PaX just drops 2 pointers (saved frame,
> saved ip) from the delta of frame address to find the size of the
> actual stack area used by the function. If I could shave things like
> padding and possible stack canaries off the size too, that would be
> great.

For x86, stacks are aligned at long word boundaries, so there's no real
stack padding.

Also the CC_STACKPROTECTOR stack canaries are created by a gcc feature
which only affects certain functions (and thus certain frames) and I
don't know of any reliable way to find them.

So with frame pointers, I think the best you can do is just assume that
the frame data area is always two words smaller than the total frame
size.

> Since I'm aiming the hardened usercopy series for 4.8, I figure I'll
> just leave this unwinder in for now, and once yours lands, I can rip
> it out again.

Sure, sounds fine to me. If your code lands before I post mine, I can
convert it myself.

--
Josh