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The CVE-2021-33909 vulnerability, also known as “Sequoia,” is a use-after-free flaw in the Linux kernel’s sequence file system. The issue resides in the `fs/seq_file.c` component. A critical problem exists in the `seq_read_iter` function. When a user-space application interacts with the proc filesystem or sysfs using specific methods like sendfile(), it triggers an integer overflow during the calculation of the buffer size for the `seq_file` interface. This overflow leads to an undersized buffer allocation. Subsequently, when the kernel attempts to copy data into this insufficient buffer, it causes a heap-based buffer overflow. This overflow corrupts adjacent kernel memory objects, potentially freeing a `seq_operations` structure while a file descriptor still holds a reference to it. This creates a use-after-free condition. An attacker can exploit this by manipulating the heap layout to overwrite the freed `seq_operations` structure with a custom payload. When the vulnerable kernel code dereferences the function pointers within this corrupted structure, it leads to arbitrary code execution with kernel privileges, achieving full system compromise.
Platform: Linux Kernel
Version: 3.16 to 5.13.x
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
grep -r "seq_read_iter" /usr/src/linux-kernel-version/ cat /proc/self/maps sendfile -d @/proc/self/mem
//Conceptual Code Snippet showing the flaw
size_t size = req->count + req->buf->size; // Integer overflow possible
char buf = kmalloc(size, GFP_KERNEL); // Buffer too small
if (copy_from_user(buf, req->buf->data, req->count)) { ... } // Heap overflow
How Exploit:
Exploitation involves triggering the integer overflow via a large `count` value when reading from a sequence file. This leads to a heap buffer overflow, which is then used to create a use-after-free condition on a `seq_operations` struct. By spraying the kernel heap with a fake structure containing a malicious function pointer, an attacker gains control when the kernel calls `single_start` or similar functions.
Protection from this CVE:
Apply kernel patch. Update to kernel version 5.13.4 or later. Restrict user namespaces. Use grsecurity/PaX. Implement SELinux in enforcing mode.
Impact:
Full system compromise. Privilege escalation from unprivileged user to root. Bypass of all security mechanisms (SELinux, AppArmor). Remote code execution if combined with another vulnerability.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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