CVE-2017-1000405

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.

Weakness

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 Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• 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/
• 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/