CVE-2023-3776 is a security issue that affects the Linux kernel’s networking code, specifically the net/sched: cls_fw (Firewall Classifier) component. This long post explains the bug in plain language, how it can let a local attacker gain root access, and how to fix and protect systems. You’ll find actual code snippets, links to sources, and explanations to help you really understand this vulnerability.

What is CVE-2023-3776?

CVE-2023-3776 is a use-after-free bug found in the Linux kernel, versions before commit 0323bce598eea038714f941ce2b22541c46d488f. It can be exploited by a local user to gain higher privileges (typically root).

Simply put: because of a bug in the way the kernel handles certain network filter setup operations, an attacker can trick the kernel into using memory that’s already been freed—opening the door to code execution as root.

Technical Background

The bug sits in the cls_fw (Firewall Classifier) part of the kernel (under net/sched/cls_fw.c). Classifiers are used to match packets for various network operations.

The problem lies in how reference counters are managed. Normally, reference counters prevent objects from being freed when they're still in use.

tcf_bind_filter(): Adjusts reference counters on the involved object.

If tcf_change_indev() fails, then the reference counts can get out of sync because of how fw_set_parms() and tcf_bind_filter() handle errors.

Here’s a simplified version of the buggy code flow

int fw_set_parms(...) {
    // ...some code...
    if (tcf_bind_filter(...)) // Increment or decrement ref counter
        goto errout;
    if (tcf_change_indev(...)) // Can fail unexpectedly
        goto errout;
    // ...more code...
    return ;
errout:
    // reference counter might be wrong here!
    kfree(some_object); // Freeing still-referenced object
    return -1;
}

If an attacker deliberately makes tcf_change_indev() fail, fw_set_parms() will not properly handle the reference count: in certain cases, setting it to zero and freeing the object while it’s still potentially in use.

Now, if the attacker can make the kernel access that freed object, they control what it points to (use-after-free), potentially gaining control of kernel execution.

Have permissions to manage traffic control: Usually, this requires CAP_NET_ADMIN.

3. Create filters and force failures: The attacker programs the filter, manipulates the inputs to fail at the right moment, and then triggers the kernel to access the freed object.

If successful, the attacker can run arbitrary code as root.

Example Exploit Overview (Pseudo-Code)

Below is a simplified pseudocode example of how an attacker might exploit this. The real-world exploit is complex and architecture-dependent—this is just illustrative.

// 1. Set up a filter on a network interface
system("tc qdisc add dev eth root handle 1: fw");

// 2. Repeatedly try to modify filter with bad device, cause tcf_change_indev() to fail
for (int i = ; i < 10000; i++) {
    system("tc filter add dev eth protocol ip parent 1: prio 1 fw classid 1:1 indev badDevice");
    
    // 'indev badDevice' refers to a non-existent device, so tcf_change_indev() fails
    // This repeatedly triggers the out-of-sync ref counter
}

// 3. Spray memory to occupy recently-freed memory
spray_memory(); // attacker allocates objects to try to control freed pointer

// 4. Trigger kernel code to access now-freed object (use-after-free)
system("tc filter show dev eth parent 1:");

If the stars align, the attacker’s malicious memory allocations could be used by the kernel in place of the freed object.

How is it fixed?

The issue is fixed by commit 0323bce598eea038714f941ce2b22541c46d488f, which adds better logic ensuring the reference count is properly tracked and objects aren’t freed while still in use.

Patch Example

-    if (tcf_bind_filter(...)) // Could cause problem!
-        goto errout;
+    if (tcf_bind_filter(...)) {
+        /* unwind partial initializations, don't free still-referenced object */
+        goto clean_up;
+    }

Mitigation

- Upgrade: If you maintain your Linux boxes, update your kernel to any version past the above commit.

Limit access: Only trusted users should have CAP_NET_ADMIN or the ability to use tc.

- Check your distribution: Many modern Linux distributions have patched this in their stable kernel updates:
 - Red Hat advisory
 - Debian Security Tracker
 - Ubuntu USN

Conclusion

CVE-2023-3776 is a classic *use-after-free* bug hiding in core Linux networking code. While not remotely exploitable, it’s a prime target for attackers with local access. Updating your Linux kernel as soon as practical is the best solution.

References and Further Reading

- Linux Kernel Patch Commit
- CVE Details page
- Red Hat CVE-2023-3776 Summary
- Alexander Stein Disclosure
- Debian Security Tracker

Timeline

Published on: 07/21/2023 21:15:00 UTC
Last modified on: 08/19/2023 18:17:00 UTC