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Subject[PATCH 4.4 24/91] Revert "ovl: modify ovl_permission() to do checks on two inodes"
Date
From: Ioanna Alifieraki <ioanna-maria.alifieraki@canonical.com>

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

--- 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,
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,
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;

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