Summary:
Recently, a critical vulnerability was patched in the Linux kernel’s CAKE scheduler module (sch_cake). CVE-2025-21647 highlights an issue where improper bounds checking on per-host bulk flow counters could lead to an *underflow* and *out-of-bounds memory access*. In plain words, certain network traffic conditions could let attackers crash the kernel or, in some cases, affect its integrity. Here, I'll break down the mechanics, risks, and how the latest fix works—with easy examples.

What is sch_cake?

sch_cake (Common Applications Kept Enhanced) is a sophisticated network queueing discipline for Linux created to simplify advanced traffic control. It provides fair bandwidth sharing between flows, hosts, and uses various modes to handle bulk traffic.

What Went Wrong?

Originally, the *per-host* counters for tracking "bulk" flows had a logic bug. Even after a bug was supposedly fixed, advanced fuzzing tools (like syzbot) *still* found ways to trigger an integer underflow. This caused negative numbers in counters—leading the code to read outside its intended memory (out-of-bounds access).

The impact:

Code Deep-Dive: The Problem

Imagine you're decrementing a counter, but you don't check if it's already zero. Oops—it becomes -1 (underflow)! Next, you use this negative index with an array—crash or arbitrary data access.

Here’s a *simplified* pseudo-snippet of the risky pattern

// BAD: No bounds-check!
host->bulk_flow_count[idx]--;

If bulk_flow_count[idx] was already at zero, now it’s -1. The logic error can then lead to

if (host->bulk_flow_count[idx] < )
    do_something_bad();

Or, if used for memory indexing

some_array[host->bulk_flow_count[idx]] = data; // Dangerous!

The Fix: Using Safe Helper Functions

The patch introduces *helper functions* that safeguard all increments and decrements, always making sure counters stay within safe bounds.

Example of new, safe code (hand-simplified)

static void safe_decrement(int *counter, int min) {
    if (*counter > min)
        (*counter)--;
    else
        *counter = min; // Never goes below min
}

static void safe_increment(int *counter, int max) {
    if (*counter < max)
        (*counter)++;
    else
        *counter = max; // Never goes above max
}

Now, every access is wrapped

safe_decrement(&host->bulk_flow_count[idx], ); // Can't underflow now

Result: No more negative counters, no more accidental out-of-bounds access.

Exploit Details (How was it triggered?)

Researchers using syzbot built strange traffic patterns that made the counters go negative. For example:

Rapidly close and open flows, causing counters to be decremented too many times.

- The logic slip allowed the counter to become less than zero, then be used as an array index or for other unintended logic.

This can be automated—here’s a *very reduced* PoC concept (not directly exploitable/pasteable)

# Pseudo-PoC: Using netem and traffic control in a loop
import os
for i in range(100):
    os.system("tc filter add dev eth protocol ip ...") # design flows
    os.system("tc filter del dev eth ...")

*Note:* The actual kernel exploit requires more sophistication and privileged access—this just illustrates the class of bug.

User-Visible Effects

The minor fallout is that packet size allowances may fluctuate by one byte, due to dithered quantum calculations being reused in more places. But this is a purely technical/metrics detail, with no noticeable impact for users.

- CVE Record: CVE-2025-21647

Upstream Patch:

- Linux commit: sch_cake: add bounds checks to host bulk flow fairness counts (replace with the final hash when public)
- syzkaller/syzbot

Detailed Discussion:

- Linux Netdev List: Patch discussion
- General sch_cake info: CAKE home

Mitigation & Upgrade Advice

Are you affected?
- If you use Linux as a router (especially OpenWrt installers) or run many containerized network users, this can affect you. Regular desktop users are less likely to hit the bug.

What to do:

If you maintain your own kernel, backport or cherry-pick the patch above.

- Consider disabling sch_cake temporarily if patching is not possible and you are in a high-risk, multi-tenant networking environment.

Conclusion

CVE-2025-21647 is a perfect example why even subtle integer bugs in complex kernel scheduler code can have severe results: memory safety and system reliability. The fix is comprehensive: it factors all risky counter accesses out into simple, repeatable helper functions, improving both security and future maintainability.

As always, keep your kernel up to date!

Share this post if you run your own routers or are a kernel/NetEm enthusiast.
Questions? Drop me a line on Twitter.

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Timeline

Published on: 01/19/2025 11:15:10 UTC
Last modified on: 11/03/2025 21:19:00 UTC