Revert "ovl: modify ovl_permission() to do checks on two inodes"

Revert "ovl: modify ovl_permission() to do checks on two inodes"

This reverts commit b24be4acd17a8963a29b2a92e1d80b9ddf759c95 which is commit
c0ca3d70e8d3cf81e2255a217f7ca402f5ed0862 upstream.

Commit b24be4acd17a ("ovl: modify ovl_permission() to do checks on two
inodes") (stable kernel  id) breaks r/w access in overlayfs when setting
ACL to files, in 4.4 stable kernel. There is an available reproducer in
[1].

To reproduce the issue :
$./make-overlay.sh
$./test.sh
st_mode is 100644
open failed: -1
cat: /tmp/overlay/animal: Permission denied <---- Breaks access
-rw-r--r-- 1 jo jo 0 Oct 11 09:57 /tmp/overlay/animal

There are two options to fix this; (a) backport commit ce31513a9114
("ovl: copyattr after setting POSIX ACL") to 4.4 or (b) revert offending
commit b24be4acd17a ("ovl: modify ovl_permission() to do checks on two
inodes"). Following option (a) entails high risk of regression since
commit ce31513a9114 ("ovl: copyattr after setting POSIX ACL") has many
dependencies on other commits that need to be backported too (~18
commits).

This patch proceeds with reverting commit b24be4acd17a ("ovl: modify
ovl_permission() to do checks on two inodes").  The reverted commit is
associated with CVE-2018-16597, however the test-script provided in [3]
shows that 4.4 kernel is  NOT affected by this cve and therefore it's
safe to revert it.

The offending commit was introduced upstream in v4.8-rc1. At this point
had nothing to do with any CVE.  It was related with CVE-2018-16597 as
it was the fix for bug [2]. Later on it was backported to stable 4.4.

The test-script [3] tests whether 4.4 kernel is affected by
CVE-2018-16597. It tests the reproducer found in [2] plus a few more
cases. The correct output of the script is failure with "Permission
denied" when a normal user tries to overwrite root owned files.  For
more details please refer to [4].

[1] https://gist.github.com/thomas-holmes/711bcdb28e2b8e6d1c39c1d99d292af7
[2] https://bugzilla.suse.com/show_bug.cgi?id=1106512#c0
[3] https://launchpadlibrarian.net/459694705/test_overlay_permission.sh
[4] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1851243

Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>