CVE-2020-16120

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.

Bugs

• <https://bugs.launchpad.net/bugs/1894980&gt;
• <https://bugs.launchpad.net/bugs/1900141&gt;

Notes