kern/45700: /chroot/proc/mounts exposes out-of-chroot pathnames

[Taylor R Campbell <campbell+netbsd%mumble.net@localhost>]

>Number:         45700
>Category:       kern
>Synopsis:       /chroot/proc/mounts exposes out-of-chroot pathnames
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Dec 09 08:25:00 +0000 2011
>Originator:     Taylor R Campbell <campbell+netbsd%mumble.net@localhost>
>Release:        NetBSD 5.99.56
>Organization:
>Environment:
System: NetBSD oberon.local 5.99.56 NetBSD 5.99.56 (RIAMONOHACK) #0: Sun Oct 16 
07:50:03 UTC 2011 
root@oberon.local:/home/riastradh/netbsd/current/obj/sys/arch/i386/compile/RIAMONOHACK
 i386
Architecture: i386
Machine: i386
>Description:

        If I'm chrooted in /chroot, and I mount procfs on /proc (in the
        chroot), then /proc/mounts exposes pathnames from outside the
        chroot.

>How-To-Repeat:

        # chroot /chroot
        # mount -t procfs procfs /proc
        # cat /proc/mounts

>Fix:

        Yes, please!  It's not clear what the right behaviour is, but
        perhaps a hack similar to ptyfs's would be appropriate here.