Bug 10149

Summary:	stack guard should lead with zero byte to gain protections from str* writes
Product:	glibc	Reporter:	Kees Cook <kees>
Component:	libc	Assignee:	Ulrich Drepper <drepper.fsp>
Status:	RESOLVED FIXED
Severity:	normal	CC:	glibc-bugs
Priority:	P2	Flags:	fweimer: security-
Version:	unspecified
Target Milestone:	---
Host:		Target:
Build:		Last reconfirmed:
Attachments:	keep leading zero

Description Kees Cook 2009-05-12 18:05:34 UTC

When building the stack guard, it has been traditionally important to have the
value start (in memory) with a zero byte to protect the guard value (and the
rest of the stack past it) from being read via strcpy, etc.

This patch reduces the number of random bytes by one, leaving the leading zero byte.

Comment 1 Kees Cook 2009-05-12 18:05:58 UTC

Created attachment 3933 [details]
keep leading zero

Comment 2 Kees Cook 2009-05-14 21:48:40 UTC

I should clarify -- the read-blocking is nice, but the more common reason the
leading zero is important is to avoid the guard being written as part of a
larger overflow being written out by a str* function, if its value were leaked
to an attacker in some other way.

Comment 3 Ulrich Drepper 2011-05-15 15:00:37 UTC

I've applied a cleaner and more efficient patch.