CVE-2019-15794 : Exploit Details and Defense Strategies

Overlayfs and shiftfs in the Linux kernel, specifically in the Ubuntu 5.0 and 5.3 kernel series, introduce a reference counting error that can lead to a refcount underflow when used with aufs.

Understanding CVE-2019-15794

This CVE involves a vulnerability in overlayfs and shiftfs components in the Linux kernel, affecting Ubuntu versions.

What is CVE-2019-15794?

CVE-2019-15794 is a reference counting error in overlayfs/shiftfs error path when utilized alongside aufs in the Linux kernel.

The Impact of CVE-2019-15794

The vulnerability has a CVSS base score of 7.1, indicating a high severity issue with a low attack complexity and privileges required. It can result in high integrity impact and availability impact.

Technical Details of CVE-2019-15794

The vulnerability stems from modifications in the mmap handlers of overlayfs and shiftfs components in the Linux kernel.

Vulnerability Description

Overlayfs and shiftfs replace vma->vm_file in their mmap handlers, leading to a situation where the original value is not restored on error, potentially causing a reference counting error.

Affected Systems and Versions

Affected versions include Ubuntu 5.0 kernel and 5.3 kernel series.

Exploitation Mechanism

The issue arises when an error occurs, and the original value of vm_file is not properly restored, leading to a file reference being put in place.

Mitigation and Prevention

Steps to address and prevent exploitation of CVE-2019-15794:

Immediate Steps to Take

Apply relevant patches provided by Ubuntu to address the reference counting error.
Monitor official Ubuntu security channels for updates and advisories.

Long-Term Security Practices

Regularly update the Linux kernel to the latest stable version to mitigate known vulnerabilities.

Patching and Updates

Stay informed about security patches released by Ubuntu for the Linux kernel and apply them promptly.