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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s filesystem layer. The issue resides in the `seq_file` interface’s handling of specially crafted paths. An attacker can exploit this by creating a deep directory structure that exceeds the maximum allowed path length. When a privileged process attempts to traverse this path using the `seq_file` interface, a race condition is triggered during the `d_path()` function call. This race condition leads to an incorrect calculation of the path length, causing a buffer overflow. Subsequently, the kernel frees a memory object but a stale pointer to it remains. This dangling pointer is later used, corrupting kernel memory and allowing for arbitrary code execution with elevated kernel privileges, effectively bypassing all security restrictions on the system.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`cat /proc/self/mountinfo`
`mkdir -p $(python3 -c ‘print(“A”5000 + “/B”)’)`
`timeout 1 ls -l /proc/self/mountinfo`
How Exploit:
Craft a deep directory tree exceeding PAGE_SIZE.
Trigger a race condition via `d_path()`.
Utilize the freed `seq_file` buffer for code execution.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Apply security patches from distribution vendors.
Restrict user access to filesystem mounting.
Impact:
Local Privilege Escalation to root.
Complete system compromise.
Kernel memory corruption.
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Sources:
Reported By: nvd.nist.gov
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