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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 vulnerability exists within the `seq_file` interface, which is used for producing files in the `/proc` and `/sys` virtual filesystems. The issue arises when a large number of virtual memory areas (VMAs) are created by a process, causing the `mmap` lock to be held for an extended duration during a `lseek()` operation on a `/proc/self/maps` file. If a separate thread concurrently calls fork(), it can trigger a race condition. The `fork()` operation attempts to acquire the same `mmap` lock, which is already held, and in the process, it frees a specific `seq_file` buffer. However, the original thread holding the lock is unaware of this deallocation. When the original thread’s `lseek()` operation completes and it attempts to use the now-freed buffer, it leads to a use-after-free condition. This memory corruption can be exploited by a local, unprivileged user to achieve arbitrary code execution with kernel privileges, ultimately leading to a full system compromise.
Platform: Linux Kernel
Version: All versions before 5.13.4
Vulnerability: Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: Patch available (2021-07-20)
What Undercode Say:
`cat /proc/self/maps`
`lseek(fd, 0, SEEK_SET);`
` Exploit code would create many VMAs and race fork() with lseek()`
How Exploit:
A local attacker creates a program that maximizes the number of virtual memory areas (VMAs) it uses. The program then uses two threads: one to perform a large `lseek()` operation on /proc/self/maps, and a second to repeatedly call fork(). This race condition leads to the use-after-free, allowing kernel memory corruption and privilege escalation.
Protection from this CVE
Update the Linux kernel to version 5.13.4 or later. For older long-term support kernels, apply the relevant security patches provided by your distribution vendor that address CVE-2021-33909. Restrict local user access if immediate patching is not feasible.
Impact:
Successful exploitation allows an unprivileged local attacker to achieve kernel-level code execution, leading to a complete compromise of the system, bypassing all security restrictions.
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Sources:
Reported By: nvd.nist.gov
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