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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` and `/sys` virtual filesystems. The vulnerability is triggered when a user copies a large directory structure to a specific location. The `seq_file` interface mishandles the buffer allocation for directory entries during a `readdir` operation. Specifically, the `show_mountinfo` function in `fs/proc_namespace.c` does not properly handle a failure of the `seq_buf_alloc()` call. If this allocation fails, the function returns an error but leaves a pointer in the `seq_file` structure pointing to the previously freed buffer (“use-after-free”). A local attacker can exploit this by creating a deep directory hierarchy, forcing the allocation to fail. This corrupts kernel memory, and by carefully controlling the memory layout, the attacker can execute arbitrary code with elevated kernel privileges.
Platform: Linux Kernel
Version: 3.16 to 5.13
Vulnerability : Use-After-Free
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`seq_buf_alloc()` fails, pointer remains.
`show_mountinfo` does not reset.
`m->buf` points to freed memory.
User controls data written there.
Exploit creates deep directories.
`ls` or `readdir` triggers the bug.
How Exploit:
Create deep directory tree.
Force kernel memory pressure.
Trigger `readdir` on `/proc/self/mountinfo`.
Overwrite freed `seq_file` buffer.
Execute arbitrary kernel code.
Gain root privileges.
Protection from this CVE
Update Linux kernel.
Apply vendor patches.
Restrict user namespaces.
Use kernel hardening (SLAB_FREELIST_RANDOM).
Monitor system calls.
Impact:
Local Privilege Escalation.
Kernel Memory Corruption.
Full System Compromise.
Denial of Service.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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