linux-kernel - stackprotector: ascii armor the stack canary

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Message-Id: <20170519212636.30440-1-riel@redhat.com>
Date:   Fri, 19 May 2017 17:26:31 -0400
From:   riel@...hat.com
To:     linux-kernel@...r.kernel.org
Cc:     danielmicay@...il.com, 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: stackprotector: ascii armor the stack canary

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/