[4.4.y PATCH] Revert "ovl: modify ovl_permission() to do checks on two inodes"

[Ioanna Alifieraki]

This reverts commit b24be4acd17a8963a29b2a92e1d80b9ddf759c95.

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@xxxxxxxxxxxxx>
---
 fs/overlayfs/inode.c | 13 -------------
 1 file changed, 13 deletions(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 060482e349ef..013d27dc6f58 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -9,7 +9,6 @@
 
 #include <linux/fs.h>
 #include <linux/slab.h>
-#include <linux/cred.h>
 #include <linux/xattr.h>
 #include "overlayfs.h"
 
@@ -92,7 +91,6 @@ int ovl_permission(struct inode *inode, int mask)
 	struct ovl_entry *oe;
 	struct dentry *alias = NULL;
 	struct inode *realinode;
-	const struct cred *old_cred;
 	struct dentry *realdentry;
 	bool is_upper;
 	int err;
@@ -145,18 +143,7 @@ int ovl_permission(struct inode *inode, int mask)
 			goto out_dput;
 	}
 
-	/*
-	 * Check overlay inode with the creds of task and underlying inode
-	 * with creds of mounter
-	 */
-	err = generic_permission(inode, mask);
-	if (err)
-		goto out_dput;
-
-	old_cred = ovl_override_creds(inode->i_sb);
 	err = __inode_permission(realinode, mask);
-	revert_creds(old_cred);
-
 out_dput:
 	dput(alias);
 	return err;
-- 
2.17.1