system/3128: Session scrollback security exposure

>Number: 3128
>Category: system
>Synopsis: Console logout does not clear scrollback buffer.
>Confidential: no
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>Environment:

	System      : OpenBSD 3.2
	Architecture: OpenBSD.i386
	Machine     : i386

>Description:

	When logging out the console screen is cleared, however the scrollback
	buffer is not. Someone not even logged in can view 7 (?) pages of
	output from the previous session. A security exposure.

>How-To-Repeat:

	Log in, log out, then use shift-scrollup/down to view the scrollback
	buffer output.

>Fix:

	Workaround: Right after logout, use ctrl-alt Fn to switch console, then 
	switch back. This erases scrollback buffer (unfortunately, but
	it's useful here).
	Fix: Clear the scrollback buffer at console logout time.