CVE-2020-16120

Unprivileged overlay + shiftfs read access

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.

Category

5.1
CVSS
Severity: Medium
CVSS 3.1 •
CVSS 2.0 •
EPSS 0.06%
Vendor Advisory ubuntu.com Vendor Advisory ubuntu.com Vendor Advisory ubuntu.com Vendor Advisory launchpad.net Vendor Advisory launchpad.net
Affected: Linux kernel Linux kernel
Published at:
Updated at:

References

Link Tags
https://ubuntu.com/USN-4576-1 third party advisory vendor advisory
https://ubuntu.com/USN-4577-1 third party advisory vendor advisory
https://ubuntu.com/USN-4578-1 third party advisory vendor advisory
https://www.openwall.com/lists/oss-security/2020/10/14/2 third party advisory mailing list
https://launchpad.net/bugs/1894980 patch vendor advisory third party advisory issue tracking
https://launchpad.net/bugs/1900141 vendor advisory third party advisory issue tracking
https://git.kernel.org/linus/56230d956739b9cb1cbde439d76227d77979a04d third party advisory patch
https://git.kernel.org/linus/48bd024b8a40d73ad6b086de2615738da0c7004f third party advisory patch
https://git.kernel.org/linus/05acefb4872dae89e772729efb194af754c877e8 third party advisory patch
https://git.kernel.org/linus/d1d04ef8572bc8c22265057bd3d5a79f223f8f52 third party advisory patch
https://git.kernel.org/linus/b6650dab404c701d7fe08a108b746542a934da84 third party advisory patch

Frequently Asked Questions

What is the severity of CVE-2020-16120?
CVE-2020-16120 has been scored as a medium severity vulnerability.
How to fix CVE-2020-16120?
To fix CVE-2020-16120, make sure you are using an up-to-date version of the affected component(s) by checking the vendor release notes. As for now, there are no other specific guidelines available.
Is CVE-2020-16120 being actively exploited in the wild?
As for now, there are no information to confirm that CVE-2020-16120 is being actively exploited. According to its EPSS score, there is a ~0% probability that this vulnerability will be exploited by malicious actors in the next 30 days.
What software or system is affected by CVE-2020-16120?
CVE-2020-16120 affects Linux kernel Linux kernel.
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