Offensive Security

#Linux structure

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CVE-2025-39727

CVE-2025-39727 is a security vulnerability in the Linux kernel, specifically within the memory management (mm) subsystem, affecting the swap handling mechanism. The issue occurs due to a potential buffer overflow in the setup_clusters() function. In the setup_swap_map() function, only bad pages within the range (0, last_page] are checked. However, if maxpages is less than last_page, setup_clusters() may access memory beyond its allocated buffer when processing a badpage with an index greater than or equal to maxpages. This can lead to undefined behavior, including system crashes, memory corruption, or privilege escalation under certain conditions.

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Potential Impact

This vulnerability is serious because it resides in the Linux kernel, which operates at ring 0, giving full control over system resources. Potential consequences include:
1. Local Privilege Escalation (LPE):
• A user with local access could exploit this vulnerability to overwrite kernel memory structures, potentially gaining root privileges.
• Example: Modifying the cred structure of a process to escalate a low-privilege account to full administrative rights.
2. Denial of Service (DoS):
• The overflow may trigger a kernel panic, crashing the system.
• Example: A server handling swap-intensive workloads could become unavailable until rebooted.
3. Memory Corruption / Data Leakage:
• The buffer overflow could corrupt adjacent kernel memory, leading to unpredictable behavior. In some scenarios, it may allow reading sensitive kernel data.

Note: Unlike some other kernel vulnerabilities, remote exploitation is highly unlikely without local code execution, as attackers must influence the swap system or memory directly.

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Technical Details

The vulnerability arises from improper bounds checking in setup_clusters():

// simplified conceptual example
for (i = 0; i < nr_badpages; i++) {
    page = badpages[i];
    // no check if page >= maxpages
    clusters[page / CLUSTER_SIZE]++;
}

If badpages[i] >= maxpages, clusters[] is accessed out-of-bounds, leading to a heap or stack memory corruption.

This memory corruption can have multiple outcomes:
• Overwriting critical kernel structures → privilege escalation.
• Overwriting control data → kernel panic (DoS).
• In rare cases, manipulating data to execute code in kernel context (requires bypassing SMEP/SMAP/KASLR).

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Example Scenario

1. A Linux server (e.g., Ubuntu 24.04) is running a vulnerable kernel.
2. An attacker has local access (low-privilege user or compromised process).
3. The attacker runs a crafted program that injects invalid badpage entries into the swap system, triggering the overflow in setup_clusters().
4. Consequences may include:
• Escalating privileges to root.
• Crashing the system (DoS).
• Potentially corrupting kernel memory or leaking sensitive data.

Reference:
https://nvd.nist.gov/vuln/detail/CVE-2025-39727
https://git.kernel.org/stable/c/152c1339dc13ad46f1b136e8693de15980750835

#Kernel #Linux

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