Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random

Jann Horn
2016-10-31T21:22:56Z

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On Mon, Oct 31, 2016 at 10:10:41PM +0100, Florian Weimer wrote:
> * Daniel Micay:
>
> >> It makes a lot of sense on x86_64 where it means the canary is
> >> still 56 bits. Also, you want -fstack-check for protecting again
> >> stack overflows rather than stack *buffer* overflow. SSP won't
> >> really help you in that regard. Sadly, while -fstack-check now
> >> works well in GCC 6 with little performance cost, it's not really a
>
> I think GCC still does not treat the return address push on
> architectures which have such a CALL instruction as an implicit stack
> probe.
>
> >> complete feature (and Clang impls it as a no-op!).
>
> How many guard pages at the end of the stack does the kernel
> guarantee? I saw some -fstack-check-generated code which seemed to
> jump over a single guard page.

Until recently: Zero, no guard pages below stacks, stack overflow
goes straight into some other allocation.
Now: One guard page, thanks to a lot of work by Andy Lutomirski.
(I think that change is in the current 4.9-rc3 kernel, but not in
any stable kernel yet.)

> The other thing I've seen which could impact the effectiveness of
> -fstack-check: mmap *without* MAP_FIXED and a hint within stack
> allocation can create a mapping inside the stack. That's rather
> surprising, and I'm not sure if the net result is that there actually
> is a guard page in all cases.
>
> > Note: talking about userspace after the entropy bit. The kernel doesn't
> > really -fstack-check, at least in even slightly sane code...
>
> There used to be lots of discussions about kernel stack sizes ...

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