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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 in the `/proc` filesystem. The vulnerability is triggered when a user writes a large size value (over 2GB) to the `/proc/self/attr/fscreate` file. This operation causes the kernel’s `kmalloc` function to allocate a buffer of an unexpected size due to an integer overflow. Subsequently, when the `seq_file` interface attempts to write data to this undersized buffer, it leads to a heap overflow. This corruption can be leveraged by a local, unprivileged user to achieve arbitrary code execution in kernel context, ultimately leading to full system compromise by escalating privileges to root.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch available
What Undercode Say:
echo -n -e "\x41\x41\x41\x41" > /proc/self/attr/fscreate Triggers integer overflow leading to undersized buffer allocation.
// Conceptual code snippet showing the vulnerable size calculation size_t user_size = ...; // Large value from user (> INT_MAX) size_t alloc_size = user_size + 1; // Integer overflow occurs char buf = kmalloc(alloc_size, GFP_KERNEL); // Allocates a small buffer seq_write(..., buf, user_size); // Heap buffer overflow
How Exploit:
A local attacker exploits this by writing a crafted large size to the vulnerable `/proc` file, causing a kernel heap overflow. By carefully controlling the overflow, the attacker can overwrite kernel structures and function pointers to execute arbitrary code with root privileges.
Protection from this CVE:
Update the Linux kernel to version 5.13.4 or later. Apply security patches from your distribution vendor. Restrict access to systems where unprivileged users can run code.
Impact:
Full system compromise, privilege escalation from an unprivileged user to root (kernel-level code execution).
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Sources:
Reported By: nvd.nist.gov
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