The Linux kernel is the backbone of millions of computers, servers, and smart devices worldwide. Discoveries of vulnerabilities in its core can have wide-reaching effects. Today we're examining CVE-2024-50087, a recent bug in the Btrfs filesystem that was patched in June 2024. This article will break down the issue, how it arose, and what makes it important — in plain, accessible language.

Background: Btrfs and the Linux Kernel

Btrfs (short for B-tree File System) is a popular modern filesystem for Linux, built for advanced features like snapshotting, checksums, and high fault tolerance.

The kernel is the heart of the Linux operating system, and filesystems like Btrfs are "in-kernel" — so bugs here have critical implications for security and stability.

What Happened?

A kernel function called read_alloc_one_name() is used internally by Btrfs. It helps prepare the encrypted names of files or folders, handling memory (RAM) allocation using the common kmalloc() function.

The relevant structure is

struct fscrypt_str {
    unsigned char *name;
    unsigned int len;
};

If kmalloc() fails (for example, if the system is low on memory), read_alloc_one_name() *did not* initialize the name pointer in this structure. Importantly, later code always tries to free fscrypt_str.name (using kfree) — whether initialization succeeded or not.

Here's a stylized version of what the old code did

int read_alloc_one_name(struct fscrypt_str *str, unsigned int len) {
    str->name = kmalloc(len, GFP_KERNEL);
    if (!str->name)
        return -ENOMEM; // Quit on allocation failure WITHOUT resetting str->name
    str->len = len;
    return ;
}

// ...Elsewhere:
struct fscrypt_str fname;
int err = read_alloc_one_name(&fname, some_length);
...
kfree(fname.name); // Dangerous if fname.name was never set!

If kmalloc failed, fname.name still had *whatever garbage* value was on the stack — or maybe what was in RAM from a previous function. Freeing that is undefined behavior and could crash the kernel or, in rare cases, even be exploited further.

How Did the Bug Appear?

The bug traces back to a large refactoring in Linux kernel commit e43eec81c516 ("btrfs: use struct qstr instead of name and namelen pairs"). That update switched how file/directory name memory was tracked, but left a subtle error for uninitialized pointers.

Can Attackers Use This?

Short answer: In most real-world setups, exploitation is *difficult* but not impossible.

Here's why

- The bug occurs only if kmalloc() fails — usually only when the system is very low on memory or under resource pressure.
- To exploit it, an attacker would already need to run code on the machine as a normal user and trigger these edge cases, hoping that a freed pointer points to something helpful (or can be influenced).

Privilege escalation by manipulating freed pointers, *if* other conditions align

In practice, it's more likely this would trigger an unexpected kernel panic than lead directly to attacker control. Still, any kernel bug like this is serious.

The Fix: Always Initialize the Pointer

The solution is simple but vital: always set fscrypt_str.name to NULL if allocation fails.

Here is the *fixed* code pattern

int read_alloc_one_name(struct fscrypt_str *str, unsigned int len) {
    str->name = kmalloc(len, GFP_KERNEL);
    if (!str->name) {
        str->name = NULL; // Explicit: refuse to leave garbage in the struct!
        return -ENOMEM;
    }
    str->len = len;
    return ;
}

// Now, free is always safe:
kfree(fname.name); // kfree(NULL) is safe

This patch ensures that on a failed allocation, fname.name is guaranteed to be NULL. Calling kfree(NULL) is allowed and safe in the Linux kernel, so there’s no undefined behavior.

References and Further Reading

- Upstream fix commit: btrfs: fix uninitialized pointer free on read_alloc_one_name() error (kernel.org)
- Original bug-introducing commit: e43eec81c516
- Linux Kernel Source for reference

What Should You Do?

- Users: Apply any forthcoming kernel security updates from your Linux distribution. Most users will receive this as part of routine security patches.

Summary

CVE-2024-50087 is a "memory safety" bug in the Linux kernel's Btrfs filesystem. If the system runs out of memory during some operations, Btrfs could try to free a pointer that was never set — leading to unpredictable (and potentially unsafe) behavior. The fix ensures that pointers are always set to a safe value, closing off this subtle but important bug.

Want to learn more about Btrfs or kernel security?

See the Btrfs Wiki and the Linux Security Mailing List for up-to-date info.


*This article was created exclusively for educational purposes, focusing on CVE-2024-50087 and Linux kernel users interested in security and reliability. For real-world deployments, always follow your operating system's security advisories.*

Timeline

Published on: 10/29/2024 01:15:05 UTC
Last modified on: 10/30/2024 14:40:16 UTC