CVE-2020-16120 Unprivileged overlay + shiftfs read access

2020-10-1300:00:00

CWE-266

canonical

www.cve.org

5.1 Medium

CVSS3

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

6.1 Medium

AI Score

Confidence

High

0.0005 Low

EPSS

Percentile

17.3%

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"
      }
    ]
  }
]