CVE-2020-16120: Unprivileged overlay + shiftfs read access

Vulnerability summary

CVE-2020-16120 is a medium (CVSS 5.1) Incorrect Privilege Assignment vulnerability in canonical ubuntu_linux. EPSS estimates a 0.41% chance of exploitation in the next 30 days, higher than 35.4% of scored vulnerabilities.

Timeline

1. 10-02-2021
   PublishedVulnerability first disclosed.
2. 16-09-2024
   Last modifiedVulnerability information updated.

Description

Overlayfs did not properly perform permission checking when copying up files in an overlayfs and could be exploited from within a user namespace, if, for example, unprivileged user namespaces were allowed. It was possible to have a file not readable by an unprivileged user to be copied to a mountpoint controlled by the user, like a removable device. This was introduced in kernel version 4.19 by commit d1d04ef ("ovl: stack file ops"). This was fixed in kernel version 5.8 by commits 56230d9 ("ovl: verify permissions in ovl_path_open()"), 48bd024 ("ovl: switch to mounter creds in readdir") and 05acefb ("ovl: check permission to open real file"). Additionally, commits 130fdbc ("ovl: pass correct flags for opening real directory") and 292f902 ("ovl: call secutiry hook in ovl_real_ioctl()") in kernel 5.8 might also be desired or necessary. These additional commits introduced a regression in overlay mounts within user namespaces which prevented access to files with ownership outside of the user namespace. This regression was mitigated by subsequent commit b6650da ("ovl: do not fail because of O_NOATIMEi") in kernel 5.11.

• CWE-266Incorrect Privilege AssignmentA product incorrectly assigns a privilege to a particular actor, creating an unintended sphere of control for that actor.