CVE-2017-1000405

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 Metrics

• v3.1•HIGH•Score: 7CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
• v2.0•MEDIUM•Score: 6.9AV:L/AC:M/Au:N/C:C/I:C/A:C

EPSS Trends

Current EPSS score: 7.27%• Percentile: 92%

Techniques & Countermeasures

• CWE-362•Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

  The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Systems

• linux•linux_kernel

  ≥ 3.2.87, < 3.3 | ≥ 3.10.106, < 3.11 | ≥ 3.12.73, < 3.13 | ≥ 3.16.42, < 3.16.52 | ≥ 3.18.55, < 3.18.86 | ≥ 4.1.41, < 4.1.48 | ≥ 4.4.70, < 4.4.104 | ≥ 4.9.7, < 4.9.67 | ≥ 4.10, < 4.14.4

References (6)

• http://www.securityfocus.com/bid/102032
• 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
• http://www.securitytracker.com/id/1040020
• https://www.exploit-db.com/exploits/43199/