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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s sequence file system operation functionality. The issue resides in the `fs/seq_file.c` file. When a user mounts a filesystem with a pathname exceeding the maximum allowed size (PAGE_SIZE << MAX_ORDER), the `seq_file` interface fails to properly handle the error condition. Specifically, the `seq_buf_alloc()` function fails and returns NULL, but the calling function, seq_open(), does not adequately check for this error. This results in a struct `seq_file` being allocated but its associated buffer being NULL. Later, when the file is closed via seq_release(), the kernel attempts to free this NULL buffer using kfree(), which is safe. However, if an attacker can trigger a concurrent operation that manipulates the file descriptor, they can create a situation where the `seq_file` structure is freed (use-after-free) but still referenced, leading to a kernel panic or potential code execution.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected by 2021-07-27
What Undercode Say:
`cat /proc/self/mountinfo | grep -i longpath`
`dmesg | grep -i “seq_file”`
`uname -r`
`!/bin/bash`
`while true; do cat /proc/self/mountinfo; done`
How Exploit:
An attacker creates a deep directory structure or a very long pathname that exceeds the system’s maximum allowed size. By repeatedly querying the `/proc` filesystem (e.g., /proc/self/mountinfo) while manipulating the mount namespace, the attacker can trigger the race condition to exploit the use-after-free vulnerability, potentially leading to privilege escalation.
Protection from this CVE:
Update kernel to version 5.13.4 or later. Apply vendor-specific security patches. Restrict user access to privileged operations involving mount namespaces. Use kernel hardening features like SLAB freelist hardening.
Impact:
Local privilege escalation from an unprivileged user to root. Denial-of-service (kernel panic) causing system instability. Complete compromise of the affected system.
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Sources:
Reported By: nvd.nist.gov
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