A vulnerability (CVE-2025-21687) in the Linux kernel has been recently identified and resolved, specifically within the vfio/platform code. This post details the vulnerability, its potential impact and the steps taken to mitigate it. Additionally, we will provide relevant code snippets and links to the initial patches.

Vulnerability Summary

In the Linux kernel, the vfio/platform code handles read and write system calls from user space. It has been discovered that the code fails to properly check the bounds of the count and offset arguments. Specifically, the 40-bit limit on the offset is not enforced, leading to potential out-of-bounds read or write operations.

Exploit Details

An attacker could potentially exploit this vulnerability by sending carefully crafted read or write system calls with large count and offset values. This could lead to either information disclosure or arbitrary code execution within the kernel, depending on the specific device and system configuration.

Patch Overview

To address this vulnerability, bounds checks have been added to the vfio_iommu_type1 module. These checks validate both the count and offset arguments of the read or write system calls, ensuring that they fall within the appropriate limits. With the patch applied, unauthorized out-of-bounds operations should be effectively mitigated.

Code Snippet

In the kernel source file, 'drivers/vfio/platform/vfio_platform_common.c', the following changes have been made:

Old code

ssize_t vfio_platform_rw(struct vfio_platform_device *vdev,
                         char __user *buf,
                         size_t count, loff_t *ppos, bool iswrite)
{
    ...
    BUG_ON(offset & (size - 1));
    ...
}

New code

ssize_t vfio_platform_rw(struct vfio_platform_device *vdev,
                         char __user *buf,
                         size_t count, loff_t *ppos, bool iswrite)
{
    ...
    if ((offset + count) > size || (offset & (size - 1))) {
        pr_warn("%s: requested invalid range @ %zx (%zu)\n",
                __func__, *ppos, count);
        return -EINVAL;
    }
    ...
}

Now, the code checks if (offset + count) > size before proceeding with the read or write operation, effectively mitigating the out-of-bounds access.

Conclusion

While the CVE-2025-21687 vulnerability could have led to serious security concerns, the prompt identification and patching of the issue has minimized the risk. It is essential for users and administrators to keep their Linux kernel up-to-date with the latest security patches to benefit from fixes like these.

Timeline

Published on: 02/10/2025 16:15:38 UTC
Last modified on: 03/24/2025 15:38:54 UTC