Five privilege escalation flaws fixed in Linux Kernel

Experts found five vulnerabilities in the Linux kernel, tracked as CVE-2021-26708, that could lead to local privilege escalation.

Positive Technologies researcher Alexander Popov found five high severity vulnerabilities in the Linux kernel that could lead to local privilege escalation.

The Linux kernel vulnerabilities are race conditions that reside in AF_VSOCK implementation, they were implicitly introduced in November 2019 in the commits c0cfa2d8a788fcf4 and 6a2c0962105ae8ce that added VSOCK multi-transport support.

A race condition is the condition of an electronics, software, or other system where the system’s substantive behavior is dependent on the sequence or timing of other uncontrollable events. It becomes a bug when one or more of the possible behaviors is undesirable.

The race conditions stems in wrong locking in net/vmw_vsock/af_vsock.c.

“CONFIG_VSOCKETS and CONFIG_VIRTIO_VSOCKETS are shipped as kernel modules in all major GNU/Linux distributions. The vulnerable modules are automatically loaded when you create a socket for AF_VSOCK. That is available for unprivileged users and user namespaces are not needed for that. These vulnerabilities are race conditions caused by wrong locking in net/vmw_vsock/af_vsock.c.” wrote Popov. “The race conditions were implicitly introduced in November 2019 in the commits c0cfa2d8a788fcf4 and 6a2c0962105ae8ce that added VSOCK multi-transport support. These commits were merged in the Linux kernel v5.5-rc1.”

The issues, collectively tracked as CVE-2021-26708, were introduced in kernel version 5.5 in November 2019, they received a CVSS score of 7.0,

The expert successfully developed a PoC exploit for local privilege escalation on Fedora 33 Server, it could allow bypassing x86_64 platform protections such as SMEP and SMAP.

The patch has been merged into mainline kernel version 5.11-rc7 and backported into affected stable trees.

Popov discovered other Linux kernel flaws in the past, including CVE-2019-18683 and CVE-2017-2636 vulnerabilities.

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Pierluigi Paganini

(SecurityAffairs – hacking, Linux)

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