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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a critical 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. During the `ls -l` command or similar operations that trigger a `d_path()` call, the `seq_file` interface incorrectly handles the memory allocation for the path string. Specifically, the `seq_buf_alloc()` function may fail to allocate a sufficiently large buffer due to an integer overflow in the size calculation. This results in a buffer that is too small, leading to an out-of-bounds write. If this write corrupts adjacent kernel memory, it can cause a use-after-free condition where a previously freed memory object is referenced again. This memory corruption can be exploited by a local, unprivileged user to achieve arbitrary code execution with kernel privileges, effectively compromising the entire system.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch expected 2021-07-22
What Undercode Say:
Check kernel version
uname -r
Simulate triggering the vulnerability by listing a deep directory
ls -la /path/to/very/deep/directory/structure
Example code snippet showing vulnerable d_path usage
char d_path(const struct path path, char buf, int buflen) {
// ... vulnerable logic ...
}
How Exploit:
An attacker crafts a deep directory structure with path names longer than 8MB. By then performing a simple `ls -l` or other system call that resolves the full path, they trigger the integer overflow and subsequent memory corruption. This can lead to a use-after-free condition, allowing for privilege escalation to root.
Protection from this CVE:
Update the Linux kernel to version 5.13.4 or later. Apply security patches from your distribution vendor. Restrict user access to creating deep directory structures where possible.
Impact:
Full system compromise via local privilege escalation. An unprivileged user can gain root privileges, leading to a complete breach of the operating system’s security boundaries.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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