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

From	Florian Weimer <fw@deneb.enyo.de>
Newsgroups	linux.kernel
Subject	Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random
Date	2016-10-31 22:20 +0100
Message-ID	<sys9Q-7LH-9@gated-at.bofh.it> (permalink)
References	(1 earlier) <synjQ-4H2-31@gated-at.bofh.it> <synDb-4Pf-9@gated-at.bofh.it> <syrGN-7md-15@gated-at.bofh.it> <syrQt-7pC-19@gated-at.bofh.it> <sys09-7Ib-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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* 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.

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 ...

Thread

[PATCH] fork: make whole stack_canary random Jann Horn <jann@thejh.net> - 2016-10-31 15:10 +0100
  Re: [PATCH] fork: make whole stack_canary random Kees Cook <keescook@chromium.org> - 2016-10-31 17:10 +0100
    Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Jann Horn <jann@thejh.net> - 2016-10-31 17:30 +0100
      Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random Florian Weimer <fw@deneb.enyo.de> - 2016-10-31 21:50 +0100
        Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Jann Horn <jann@thejh.net> - 2016-10-31 22:00 +0100
        Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Daniel Micay <danielmicay@gmail.com> - 2016-10-31 22:00 +0100
          Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Daniel Micay <danielmicay@gmail.com> - 2016-10-31 22:10 +0100
            Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random Florian Weimer <fw@deneb.enyo.de> - 2016-10-31 22:20 +0100
              Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Jann Horn <jann@thejh.net> - 2016-10-31 22:30 +0100
                Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random Florian Weimer <fw@deneb.enyo.de> - 2016-10-31 22:30 +0100
                Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Daniel Micay <danielmicay@gmail.com> - 2016-10-31 22:30 +0100
              Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Daniel Micay <danielmicay@gmail.com> - 2016-10-31 22:30 +0100
                Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random Florian Weimer <fw@deneb.enyo.de> - 2016-10-31 22:40 +0100
                Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary  random Daniel Micay <danielmicay@gmail.com> - 2016-10-31 23:10 +0100
                Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random Florian Weimer <fw@deneb.enyo.de> - 2016-10-31 23:20 +0100

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