Overview
CVE-2025-21879 is a significant vulnerability found in the Linux kernel, specifically in the btrfs file system. This vulnerability arises from a use-after-free condition on an inode when scanning root during em shrinking. The impact of this vulnerability is critical as it potentially allows malicious actors to compromise the system or cause data leakage. It matters because the Linux kernel is widely used in various products and systems, from servers and desktops to embedded systems, making a large number of devices potentially vulnerable to this exploit.
Vulnerability Summary
CVE ID: CVE-2025-21879
Severity: High (7.8 CVSS Score)
Attack Vector: Local
Privileges Required: Low
User Interaction: None
Impact: Compromise of system integrity, potential data leakage
Affected Products
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Product | Affected Versions
Linux kernel | Versions using btrfs filesystem
How the Exploit Works
The exploit takes advantage of a use-after-free condition in the Linux kernel’s btrfs file system. Specifically, the btrfs_scan_root() function is accessing the inode’s root (and fs_info) in a call to btrfs_fs_closing() after the inode has been scheduled for a delayed iput. If the cleaner kthread completes the iput before the inode is dereferenced in the call to btrfs_fs_closing(), it can result in a use-after-free on the inode. This situation can be exploited by an attacker to potentially compromise the system or cause data leakage.
Conceptual Example Code
Below is a conceptual example demonstrating how an attacker might exploit this vulnerability. Here, the attacker triggers the vulnerable condition by causing a delayed iput on the inode and then leveraging the resulting use-after-free condition.
// Create a delayed iput on the inode
trigger_delayed_iput(inode);
// Then, trigger the use-after-free condition
access_inode_after_free(inode);In real-world attacks, the actual code would be much more complex and would involve specific techniques to exploit the use-after-free condition effectively.
Mitigation Guidance
To mitigate this vulnerability, users are advised to apply the vendor patch as soon as it is available. If the patch cannot be applied immediately, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can provide temporary mitigation. However, these measures do not entirely resolve the vulnerability and only limit the potential for exploitation. Therefore, applying the vendor patch remains the most effective way to secure your systems against this vulnerability.