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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 that exceeds the maximum allowed length. During the `ls -l` command or similar operations that trigger a `d_path()` call, the kernel attempts to handle this path. Due to an integer overflow in the `size_t` variable accounting for the path length, the subsequent memory allocation via `kmalloc` is insufficient. This leads to a heap buffer overflow when the full path is written. The overflow corrupts adjacent kernel memory, which can be leveraged to free an object and then reference it later (use-after-free), allowing for arbitrary code execution with kernel privileges.
Platform: Linux Kernel
Version: 3.16 to 5.13
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… (70,000+ characters)`
`d_path(&path, buf, buflen); // buflen overflows`
`kmalloc(size, GFP_KERNEL); // size is too small`
`memcpy(buf, path, real_size); // Heap buffer overflow`
How Exploit:
Craft a deep directory structure exceeding PAGE_SIZE 4.
Mount it using fuse or a network filesystem.
Trigger `d_path()` via `ls` or `getcwd()`.
Overflow the buffer to corrupt adjacent objects.
Gain control of a freed `seq_operations` structure.
Execute privileged kernel code.
Protection from this CVE
Update kernel to version 5.13.4 or later.
Apply vendor security patches.
Restrict user access to FUSE filesystems.
Use kernel hardening (SLAB_FREELIST_HARDENED).
Impact:
Local Privilege Escalation to root.
Full system compromise.
Bypass of all security mechanisms.
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Sources:
Reported By: nvd.nist.gov
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