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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, which is used for producing files for the `/proc` filesystem. The vulnerability is triggered when a user creates a large directory structure that exceeds the maximum allowed pathname length. When the `show_mountinfo()` function, which uses the `seq_file` interface, attempts to generate output for such a deeply nested path, it allocates a buffer. However, due to an integer overflow in the `seq_buf_alloc()` function when calculating the buffer size, a much smaller buffer than required is allocated. This leads to a heap buffer overflow when writing the full path. An unprivileged local attacker can exploit this by creating a specially crafted directory structure with a pathname longer than 1GB. When a system command like `df` is executed, it triggers the vulnerable code path. The overflow corrupts kernel memory, potentially leading to a use-after-free condition where the program references memory after it has been freed. This can be leveraged to execute arbitrary code with kernel privileges, resulting in a full compromise of the system.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`mkdir -p /tmp/sequoia$(python3 -c “print(‘/x’ 1000000)”)`
`df /tmp/sequoia`
`dmesg | grep -i “segfault\|oops”`
`cat /sys/kernel/debug/tracing/trace | grep show_mountinfo`
`checksec –kernel`
How Exploit:
Local user creates deep directory tree.
Triggers procfs read via system utility.
Overflows kernel heap buffer.
Achieves arbitrary code execution.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Apply vendor-specific security patches.
Restrict user namespace creation.
Impact:
Full system compromise.
Bypasses all security mechanisms.
Root privilege escalation.
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Sources:
Reported By: nvd.nist.gov
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