CVE-2017-1000405 Information

Description

The Linux Kernel versions 2.6.38 through 4.14 have a problematic use of pmd_mkdirty() in the touch_pmd() function inside the THP implementation. touch_pmd() can be reached by get_user_pages(). In such case the pmd will become dirty. This scenario breaks the new can_follow_write_pmd()’s logic - pmd can become dirty without going through a COW cycle. This bug is not as severe as the original \Dirty cow\ because an ext4 file (or any other regular file) cannot be mapped using THP. Nevertheless it does allow us to overwrite read-only huge pages. For example the zero huge page and sealed shmem files can be overwritten (since their mapping can be populated using THP). Note that after the first write page-fault to the zero page it will be replaced with a new fresh (and zeroed) thp.

CVSS Vector

CVSS:3.0/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H

Reference

http://www.securityfocus.com/bid/102032 http://www.securitytracker.com/id/1040020 https://access.redhat.com/errata/RHSA-2018:0180 https://medium.com/bindecy/huge-dirty-cow-cve-2017-1000405-110eca132de0 https://source.android.com/security/bulletin/pixel/2018-02-01 https://www.exploit-db.com/exploits/43199/

Attack Complexity

HIGH

Privileges Required

LOW

User Interaction Required

LOW

Scope

NONE

Confidentiality Impact

UNCHANGED

Integrity Impact

HIGH

Availability Impact

HIGH

Base Score

HIGH

Base Severity

7.0