CVE-2020-16120

Published: 13 October 2020

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.

From the Ubuntu Security Team

Giuseppe Scrivano discovered that the overlay file system in the Linux kernel did not properly perform permission checks in some situations. A local attacker could possibly use this to bypass intended restrictions and gain read access to restricted files.

Notes

this issue most likely only has an impact on Ubuntu systems
as it is dependent on both unprivileged user namespaces being enabled
as well as a non-upstream patch that allows overlayfs mounts in user
namespaces.
the backport of this issue introduced a regression, LP: #1900141

Mitigation

disable unprivileged user namespaces if not needed via
  sudo sysctl kernel.unprivileged_userns_clone=0
do this permanently by adding
  kernel.unprivileged_userns_clone = 0
to /etc/sysctl.d/99-userns.conf

Status

Severity score breakdown

References

• https://www.openwall.com/lists/oss-security/2020/10/13/6
• https://ubuntu.com/security/notices/USN-4578-1
• https://ubuntu.com/security/notices/USN-4577-1
• https://ubuntu.com/security/notices/USN-4576-1
• https://www.cve.org/CVERecord?id=CVE-2020-16120
• NVD
• Launchpad
• Debian

Bugs

• https://bugs.launchpad.net/bugs/1894980
• https://bugs.launchpad.net/bugs/1900141