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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 maximum allowed length. When the `seq_file` interface’s `single_start()` function is called to output the pathname, it allocates a buffer of size `PAGE_SIZE 4` (typically 16KB). If the pathname is longer than this buffer, the function `seq_escape()` attempts to escape the string but can write beyond the allocated buffer’s end, causing a heap overflow. This overflow corrupts adjacent kernel memory objects, potentially leading to a use-after-free condition where a freed memory object is later reused. An attacker can exploit this by crafting a specific directory structure, leading to arbitrary code execution with kernel privileges, effectively granting full control over the system.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch available
What Undercode Say:
Check kernel version
uname -r
Simulate long path creation (conceptual)
mkdir -p /mnt/long/path/$(python3 -c "print('a' 5000)")
List /proc entries that use seq_file
ls -la /proc/self/
// Conceptual code snippet showing vulnerable buffer allocation
int single_start(struct seq_file m, loff_t pos) {
char buf = kmalloc(PAGE_SIZE 4, GFP_KERNEL); // 16KB buffer
// ... path is copied and escaped into 'buf', potentially overflowing it
seq_escape(m, buf, "\t\n\");
}
How Exploit:
An attacker creates a deeply nested directory structure with extremely long pathnames. When a utility like `ls` or `stat` traverses this path via the `/proc` filesystem, it triggers the heap buffer overflow in the `seq_file` interface. By carefully crafting the memory layout through this overflow, the attacker can corrupt kernel data structures to achieve a use-after-free state. This allows them to execute arbitrary code in kernel mode, leading to a full system compromise.
Protection from this CVE:
Apply kernel update to version 5.13.4 or later. For systems unable to update immediately, restrict user access to mount namespaces, a capability required to exploit this vulnerability. This can be done by setting `user.max_user_namespaces` to 0 in `/etc/sysctl.conf` or via the command sysctl -w user.max_user_namespaces=0.
Impact:
Full system compromise, privilege escalation to root, and complete bypass of all security mechanisms enforced by the operating system.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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