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Date: Wed, 2 Nov 2016 16:52:09 -0600
From: Seth Arnold <seth.arnold@...onical.com>
To: oss-security@...ts.openwall.com
Subject: Re: Stack guard canary massaging

[keeping only oss-security]

On Mon, Oct 31, 2016 at 11:48:45AM +0100, Florian Weimer wrote:
> This is an elaborate way of setting ret.bytes[0] = '\0'.
> 
> The intent (determined from an old commit message) is to make it harder to
> obtain the canary value through a read buffer overflow of a NUL-terminated
> string: The read overflow will stop at the NUL byte and not include the
> random canary value, reducing the risk of inappropriate disclosure.

StackGuard used a fixed canary value: CR LF 0x00 0xFF. This was based on
the observation that most unsafe stack buffer manipulations were from
string operations, and most string-handling functions would trip up on at
least one of these values, making it difficult to write the canary with
the functions that were used.

ftp://gcc.gnu.org/pub/gcc/summit/2003/Stackguard.pdf

I suspect the leading 0x00 here is for much the same reason, to trip up
string writing operations more than string reading.

Thanks

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