linux-kernel - Re: [kernel-hardening] Re: [PATCH] fork: make whole stack

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Date:   Mon, 31 Oct 2016 22:10:41 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Daniel Micay <danielmicay@...il.com>
Cc:     Jann Horn <jann@...jh.net>, Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [PATCH] fork: make whole stack_canary random

* 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 ...