/etc/pam.d/chsh is in passwd 1:4.5-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# The PAM configuration file for the Shadow `chsh' service
#
# This will not allow a user to change their shell unless
# their current one is listed in /etc/shells. This keeps
# accounts with special shells from changing them.
auth required pam_shells.so
# This allows root to change user shell without being
# prompted for a password
auth sufficient pam_rootok.so
# The standard Unix authentication modules, used with
# NIS (man nsswitch) as well as normal /etc/passwd and
# /etc/shadow entries.
@include common-auth
@include common-account
@include common-session
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