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Created attachment 462391 [details]
Proposed patch

pam_ssh_agent_auth sets euid temporarily to the user during the connect to the ssh-agent socket to avoid possible race conditions. However if the connect fails for example if the ssh-agent is not running it will fail to restore the original euid. This causes following modules in the PAM stack to fail.

Testing instructions:

Add 
auth sufficient pam_ssh_agent_auth.so file=~/.ssh/authorized_keys
to the /etc/pam.d/su.

Run 
'ssh-agent -a .ssh-agent-socket'
as user. Then kill the ssh-agent process. Verify that the stale unix socket ~/.ssh-agent-socket is still present. Try to su to root - it asks for password but it fails even if you supply correct root password.

Created attachment 462392 [details]
Better patch adds also missing close(sock) if seteuid fails

An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHBA-2011-0598.html