CVE-2025-21679: Fixing the Linux Kernel Vulnerability - Btrfs Error Handling in "get_canonical_dev_path"

The Linux kernel, the core component of the Linux operating system, recently resolved a potentially critical vulnerability in its btrfs (B-Tree File System) module. This vulnerability can potentially trigger an invalid memory access, causing system crashes or failures. In this post, we'll dive deep into the issue, the code snippet that caused the vulnerability, and the steps taken to resolve it, along with links to the original references.

Background

The B-Tree File System (btrfs) is a modern, and open-source file system primarily designed for Linux systems. Widely used for various applications, btrfs provides advanced features such as fault tolerance, scalability, and data integrity. However, a vulnerability was discovered in the Linux kernel's btrfs module, specifically in the get_canonical_dev_path() function.

The Vulnerability

The function get_canonical_dev_path() is used to obtain the final device path in btrfs by calling the d_path() function. However, there was a missing error handling mechanism in this function. In cases where d_path() returns an error, the subsequent strscpy() call results in an invalid memory access, which can potentially crash or destabilize the system.

To understand the root cause, let's examine the code snippet where the vulnerability exists

static char *get_canonical_dev_path(const char *dev_path, char **errstr)
{
        struct path path;
        char *real_path;
        int ret;

        ret = kern_path(dev_path, , &path);
        if (ret < ) {
                *errstr = strdup(strerror(-ret));
                return NULL;
        }

        real_path = d_path(&path, canonical_path, PATH_MAX);
        strscpy(canonical_path, real_path, PATH_MAX);
        path_put(&path);

        return canonical_path;
}

In the snippet above, the issue lies with d_path() and strscpy() functions. Specifically, after the call to kern_path(), d_path() is called to retrieve the real_path. However, d_path() can return an error; this is not being handled, and the following strscpy() call will trigger an invalid memory access in such cases.

The Solution

To resolve this vulnerability, the missing error handling for d_path() needs to be added. The following code snippet shows the updated get_canonical_dev_path() function with the proper error handling:

static char *get_canonical_dev_path(const char *dev_path, char **errstr)
{
        struct path path;
        const char *real_path;
        int ret;

        ret = kern_path(dev_path, , &path);
        if (ret < ) {
                *errstr = strdup(strerror(-ret));
                return NULL;
        }

        real_path = d_path(&path, canonical_path, PATH_MAX);
        if (IS_ERR(real_path)) {
                *errstr = strdup(strerror(-PTR_ERR(real_path)));
                ret = PTR_ERR(real_path);
                goto out;
        }
        strscpy(canonical_path, real_path, PATH_MAX);
        path_put(&path);

        return canonical_path;
out:
        path_put(&path);
        return ERR_PTR(ret);
}

In the above code, the error handling is added for d_path() by checking if the returned value (real_path) is an error using the IS_ERR() macro. If an error is returned, it is now handled gracefully rather than causing a memory access violation.

Conclusion

The Linux kernel's btrfs module had a potential vulnerability caused by a missing error handling mechanism for the get_canonical_dev_path() function. By updating the function with proper error handling, the issue has been resolved, ensuring that systems running btrfs on Linux kernel remain stable and secure. For details about the original commit and code, check out this link.

Timeline

Published on: 01/31/2025 12:15:29 UTC