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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 mounts a filesystem with a very long directory path name that exceeds the internal page size limit (typically 4KB). During the `ls -l` or `stat` operations on such a path, the kernel’s `seq_file` interface mishandles the allocation of a buffer. Specifically, the `seq_buf_alloc()` function fails to allocate memory for the oversized path but the code proceeds without properly checking for this failure. This results in an internal state where a pointer is not initialized (NULL). Later, when the `seq_file` operations try to free this buffer using kvfree(), a use-after-free condition occurs because the code attempts to free an uninitialized or already-freed pointer. This flaw can be exploited by an unprivileged local user to achieve denial of service or potentially execute arbitrary code with kernel privileges.
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:
`ls -la /proc/$(pidof systemd)/root/aaaaaaaa…aaaaaaaa`
`dmesg | grep “seq_file”`
`cat /sys/kernel/debug/tracing/trace_pipe`
`echo 1 > /proc/sys/vm/drop_caches`
`uname -r`
`!/bin/bash
while true; do
find /path/to/long/directory -type f -exec stat {} \; > /dev/null 2>&1
done &`
How Exploit:
Local user crafts deep directory structure.
Attacker runs `ls` or `stat` on the path.
Kernel crashes or executes attacker-controlled code.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Restrict user access to mount namespaces.
Apply distribution-specific security patches.
Impact:
Denial of Service (system crash).
Potential privilege escalation to root.
Information disclosure.
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Sources:
Reported By: nvd.nist.gov
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