In the world of Linux kernel development, addressing vulnerabilities is of utmost importance to ensure the robustness and security of millions of systems running on various Linux distributions. One such recent vulnerability, identified by its CVE number CVE-2025-21665, is related to the inadvertent truncation of a 64-bit offset to 32 bits in the filemap subsystem of the Linux kernel. This article will discuss the vulnerability in detail, including its impact, the code snippet containing the error, and the exploit details. Furthermore, we will delve into the solution applied to resolve this issue and provide links to the original references for further study.

Understanding the Vulnerability

The issue lies within the Linux kernel's filemap subsystem, specifically in the function folio_seek_hole_data(). On 32-bit kernels, this function incorrectly truncates a 64-bit value to 32 bits, which could potentially lead to an infinite loop when writing to an XFS filesystem. This poses a significant risk to the system's stability and performance, as well as potentially opening a vector for malicious attacks.

Here's the faulty code snippet that introduced the vulnerability

static loff_t folio_seek_hole_data(struct folio *folio, loff_t start)
{
    if (folio_empty(folio)) {
        unsigned long index = folio->index;
        if (index >= start >> PAGE_SHIFT)
            return index << PAGE_SHIFT;
        return start;
    }
    // ...
    return (u32)-1;   // <-- 64-bit value incorrectly truncated to 32 bits
}

In the original implementation, the function incorrectly returns a value of (u32)-1, which is cast as a 32-bit unsigned integer, causing the truncation of the 64-bit loff_t return type.

Exploit Details

Although no known exploits have been used in the wild at the time of writing, the vulnerability might be abused by an attacker seeking to cause excessive CPU usage, system slowdowns, or even crashes. Additionally, there is a chance that a malicious exploit might leverage this vulnerability to bypass certain security measures or gain unauthorized access to sensitive filesystem data.

Resolution

The vulnerability was addressed by correcting the return statement in the folio_seek_hole_data() function. The fix, submitted by the Linux kernel developers, ensures that a 64-bit value is returned, thus eliminating the potential for an infinite loop and other adverse impacts on the system.

Here's the corrected code snippet with the vulnerability addressed

static loff_t folio_seek_hole_data(struct folio *folio, loff_t start)
{
    if (folio_empty(folio)) {
        unsigned long index = folio->index;
        if (index >= start >> PAGE_SHIFT)
            return index << PAGE_SHIFT;
        return start;
    }
    // ...
    return (u64)-1;   // <-- 64-bit value preserved as-is
}

As seen in the above code, the (u32)-1 return value has been changed to (u64)-1, correctly preserving the 64-bit value and resolving the vulnerability.

Original References

Further information about the vulnerability, its resolution, and the discussions that led to determining the best course of action can be found in the following resources:

1. Linux Kernel Mailing List (LKML) announcement discussing the vulnerability
2. Patch submission addressing the issue on the LKML
3. Documentation on the filemap subsystem of the Linux kernel

Conclusion

CVE-2025-21665 is a critical vulnerability that could have severely impacted systems running on 32-bit Linux kernels. Thanks to the Linux kernel development community's open and collaborative approach, it was quickly identified and addressed, ensuring that the Linux ecosystem remains a reliable and robust choice for users worldwide. As always, staying updated with the latest security patches and kernel versions ensures the best possible protection against such vulnerabilities.

Timeline

Published on: 01/31/2025 12:15:27 UTC
Last modified on: 02/03/2025 19:57:54 UTC