CVE-2020-16120 Unprivileged overlay + shiftfs read access

2021-02-1019:45:26

CWE-266

canonical

www.cve.org

overlayfs

permission checking

user namespace.

CVSS3

5.1

Attack Vector

LOCAL

Attack Complexity

HIGH

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

NONE

Availability Impact

NONE

CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

AI Score

6.1

Confidence

High

EPSS

0.001

Percentile

17.2%

JSON

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.

CNA Affected

[
  {
    "product": "Linux kernel",
    "vendor": "Linux kernel",
    "versions": [
      {
        "lessThan": "5.11.0",
        "status": "affected",
        "version": "5.11-stable",
        "versionType": "custom"
      }
    ]
  }
]