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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 within the `fs/seq_file.c` file. A critical function, seq_buf_putmem(), is used to write data to a sequential file. However, when handling a maliciously crafted `loff_t` position value that is excessively large, the function miscalculates the buffer size. This miscalculation leads to an integer overflow during the memory allocation request in the `seq_buf_alloc()` function. Consequently, a much smaller buffer is allocated than what is required. The `seq_file` operations then write data beyond the bounds of this undersized allocated buffer, causing a heap-based buffer overflow. This overflow corrupts adjacent kernel memory structures, potentially leading to a use-after-free condition where a program continues to use a pointer after the memory it points to has been freed. This memory corruption can be exploited by a local, unprivileged user to achieve arbitrary code execution with elevated kernel privileges, completely compromising the system.
Platform: Linux Kernel
Version: 3.16 to 5.13.x
Vulnerability: Buffer Overflow
Severity: Critical
date: 2021-07-20
Prediction: 2021-07-27
What Undercode Say:
`grep -r “seq_buf_putmem” /usr/src/linux/`
`cat /proc/self/mountinfo`
`dmesg | grep “seq_file”`
How Exploit:
Local Privilege Escalation via crafted `lseek()` call on /proc/self/mountinfo.
Protection from this CVE:
Update kernel to version 5.13.4 or apply security patch for fs/seq_file.c.
Impact:
Arbitrary code execution, full system compromise, kernel panic.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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