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Tavis noted that it'd be possible with a 32 bit counter for someone to cause the cookie to wrap by creating Authentication requests in a loop. Something important to note here is that wrapping of signed integers is undefined behavior in C, so we definitely want to fix that. All counter integers used in this patch are unsigned. See the comment above `authentication_agent_generate_cookie` for details, but basically we're now using a cookie of the form: ``` <agent serial> - <agent random id> - <session serial> - <session random id> ``` Which has multiple 64 bit counters, plus unpredictable random 128 bit integer ids (effectively UUIDs, but we're not calling them that because we don't need to be globally unique. We further ensure that the cookies are not visible to other processes by changing the setuid helper to accept them over standard input. This means that an attacker would have to guess both ids. In any case, the security hole here is better fixed with the other change to bind user id (uid) of the agent with cookie lookups, making cookie guessing worthless. Nevertheless, I think it's worth doing this change too, for defense in depth. Bug: https://bugs.freedesktop.org/show_bug.cgi?id=90832 CVE: CVE-2015-4625 Reported-by:Tavis Ormandy <taviso@google.com> Reviewed-by:
Miloslav Trmač <mitr@redhat.com> Signed-off-by:
Colin Walters <walters@redhat.com>
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