GHSA-j4rf-7357-f4cg: Unpatched extfs vulnerabilities are exploitable through suid-mode Apptainer and Singularity

Impact

There is an ext4 use-after-free flaw described in CVE-2022-1184 that is exploitable through versions of Apptainer < 1.1.0, installations that include apptainer-suid < 1.1.8, and all versions of Singularity in their default configurations on older operating systems where that CVE has not been patched. That includes Red Hat Enterprise Linux 7, Debian 10 buster (unless the linux-5.10 package is installed), Ubuntu 18.04 bionic and Ubuntu 20.04 focal. Use-after-free flaws in the kernel can be used to attack the kernel for denial of service and potentially for privilege escalation.

Background

Historically there have been many CVEs published for extfs and a smaller number for squashfs, including serious use-after-free and buffer overrun vulnerabilities, that are scored as “Moderate” or “Low” impact only because unprivileged users were assumed to not have write access to the raw data. Because of those ratings, vendors treat such CVEs as low urgency and either delay a patch until their next major release or never patch older but still supported operating systems at all. Many Linux distributions automatically mount user-writable USB-drive volumes, but those are considered low risk because they require physical access to the machine. However, since setuid-root installations of Apptainer and Singularity by default allow all users to mount any extfs (specifically, ext3, which is implemented by the ext4 driver) and squashfs filesystem using kernel drivers even though the users have write access to the raw data, the setuid-root installations raise the severity of such unpatched CVEs.

CVE-2022-1184 is currently such an unpatched CVE, at least on the above listed operating systems. The descriptions from the operating system vendors about the CVE (referenced below) are incomplete, saying only that it allows a local attacker with user privilege to cause a denial of service. Normally users would not be able to cause it because they cannot modify the filesystem image, and normally vulnerabilities that involve kernel memory corruption by unprivileged users are considered high severity even when there is not yet a known privilege escalation because someone with sufficient kernel knowledge can usually turn such a corruption into a privilege escalation.

Red Hat did not list RHEL7 as vulnerable, but they also did not list it as unaffected, and testing confirmed that a filesystem image could be corrupted to get past the check inserted into the filesystem driver to fix the vulnerability (patches linked below).

All published squashfs CVEs have been patched in currently supported major operating systems.

Patches

Apptainer 1.1.8 includes a patch that by default disables mounting of extfs filesystem types in setuid-root mode, while continuing to allow mounting of extfs filesystems in non-setuid “rootless” mode using fuse2fs.

Workarounds

These workarounds are possible:

1. Either do not install apptainer-suid (for versions 1.1.0 through 1.1.7) or set allow setuid = no in apptainer.conf (or singularity.conf for singularity versions). This requires having unprivileged user namespaces enabled and except for apptainer 1.1.x versions will disallow mounting of sif files, extfs files, and squashfs files in addition to other, less significant impacts. (Encrypted sif files are also not supported unprivileged in apptainer 1.1.x.)
2. Alternatively, use the limit containers options in apptainer.conf/singularity.conf to limit sif files to trusted users, groups, and/or paths. (The option allow container extfs = no disallows mounting extfs overlay files but does not disallow mounting of extfs overlay partitions inside SIF files, so it does not help work around the problem.)

References

• GHSA-j4rf-7357-f4cg
• https://nvd.nist.gov/vuln/detail/CVE-2022-1184
• torvalds/linux@61a1d87
• torvalds/linux@65f8ea4
• https://access.redhat.com/security/cve/cve-2022-1184
• https://security-tracker.debian.org/tracker/CVE-2022-1184
• https://ubuntu.com/security/CVE-2022-1184