linux-kernel - Re: stackprotector: ascii armor the stack canary

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Date:   Fri, 19 May 2017 19:57:07 -0400
From:   Daniel Micay <danielmicay@...il.com>
To:     riel@...hat.com, linux-kernel@...r.kernel.org
Cc:     tytso@....edu, keescook@...omium.org, hpa@...or.com,
        luto@...capital.net, mingo@...nel.org, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, catalin.marinas@....com,
        linux-sh@...r.kernel.org, ysato@...rs.sourceforge.jp
Subject: Re: stackprotector: ascii armor the stack canary

On Fri, 2017-05-19 at 17:26 -0400, riel@...hat.com wrote:
> Zero out the first byte of the stack canary value on 64 bit systems,
> in order to prevent unterminated C string overflows from being able
> to successfully overwrite the canary, even if an attacker somehow
> guessed or obtained the canary value.
> 
> Inspired by execshield ascii-armor and PaX/grsecurity.
> 
> Thanks to Daniel Micay for extracting code of similar functionality
> from PaX/grsecurity and making it easy to find in his linux-hardened
> git tree on https://github.com/thestinger/linux-hardened/

To clarify something this part isn't from PaX / grsecurity. I marked the
commits from PaX / grsecurity as such in their commit messages and these
are among the ones that aren't from there.

This is from a set of changes that I did for CopperheadOS and forward
ported to mainline recently to start linux-hardened. It was only arm64
for CopperheadOS. The overlap with PaX is that when adding the leading
zero byte for x86, I needed to first fix get_random_int being used for
the per-task canary value. I didn't know PaX fixed it way back in 2007.

I implemented heap canaries for our userspace malloc implementation and
then later did the same thing for slub in the kernel. I added a leading
zero byte to both of those heap canaries later on and then did the SSP
implementation in Bionic and the kernel's arm64 code. I took the idea
from glibc but limited it to 64-bit where there's entropy to spare. The
glibc leading zero might have come from execshield, but I don't know.