There is a repeat of bug #982043 on el7. The strange thing is that the fix in that bug was yum-utils-1.1.31-19 and that's precisely what is in el7.

result/root.log:

DEBUG util.py:314:  Executing command: /usr/bin/repoquery -c /tmp/tmpvWkMW6 --installed -a --qf '%{nevra} %{buildtime} %{size} %{pkgid} %{yumdb_info.from_repo}' > /var/lib/mock/rhel7-beta1-i686/result/installed_pkgs with env {'LANG': 'en_US.utf8', 'TERM': 'vt100', 'SHELL': '/bin/bash', 'HOSTNAME': 'mock', 'HOME': '/builddir', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin'}
DEBUG util.py:264:  error: cannot open Packages index using db5 - Permission denied (13)
DEBUG util.py:264:  error: cannot open Packages database in /var/lib/mock/rhel7-beta1-i686/root/var/lib/rpm

# ls -l /var/lib/mock/rhel7-beta1-i686/root/var/lib/rpm/
total 24996
-rw------- 1 root mock  1544192 Jan  8 21:55 Basenames
-rw------- 1 root mock     8192 Jan  8 21:55 Conflictname
-rw------- 1 root mock   233472 Jan  8 21:55 Dirnames
-rw------- 1 root mock     8192 Jan  8 21:55 Group
-rw------- 1 root mock    16384 Jan  8 21:55 Installtid
-rw------- 1 root mock    20480 Jan  8 21:55 Name
-rw------- 1 root mock     8192 Jan  8 21:55 Obsoletename
-rw------- 1 root mock 23400448 Jan  8 21:55 Packages
-rw------- 1 root mock   147456 Jan  8 21:55 Providename
-rw------- 1 root mock   139264 Jan  8 21:55 Requirename
-rw------- 1 root mock    32768 Jan  8 21:55 Sha1header
-rw------- 1 root mock    24576 Jan  8 21:55 Sigmd5
-rw------- 1 root mock     8192 Jan  8 21:54 Triggername

# rpm -q yum-utils
yum-utils-1.1.31-19.el7.noarch

Changing the permissions doesn't help; they go back to 600 when mock does chroot init. 

Workaround by initialising the chroot, changing the rpm db permissions manually to 660 and then running mock with --no-clean runs into the next problem:

DEBUG util.py:314:  Executing command: /usr/bin/repoquery -c /tmp/tmpT8pf5x --installed -a --qf '%{nevra} %{buildtime} %{size} %{pkgid} %{yumdb_info.from_repo}' > /var/lib/mock/rhel7-beta1-i686/result/installed_pkgs with env {'LANG': 'en_US.utf8', 'TERM': 'vt100', 'SHELL': '/bin/bash', 'HOSTNAME': 'mock', 'HOME': '/builddir', 'PATH': '/usr/bin:/bin:/usr/sbin:/sbin'}
DEBUG util.py:264:  Traceback (most recent call last):
DEBUG util.py:264:    File "/usr/bin/repoquery", line 1547, in <module>
DEBUG util.py:264:      main(sys.argv)
DEBUG util.py:264:    File "/usr/bin/repoquery", line 1415, in main
DEBUG util.py:264:      except YumBaseError, e:
DEBUG util.py:264:  NameError: global name 'YumBaseError' is not defined
DEBUG util.py:354:  Child return code was: 1