CVE-2024-56788 is a recently resolved vulnerability in the Linux kernel's oa_tc6 Ethernet driver. The bug is a classic race condition in handling transmit socket buffers (skbs) between two concurrent threads—one from the network stack, one from the SPI (Serial Peripheral Interface) handler. This flaw could lead to dropped Ethernet frames (packet loss) and memory leaks. While this may not immediately sound severe, in embedded devices or network appliances using this driver, it could lead to subtle network issues or system resource exhaustion.
This post dives into the bug, shows the logic behind it, provides code snippets for understanding, discusses exploitation angles, and gives you references for more technical details.
Technical Background
The oa_tc6 driver manages Ethernet communication over SPI and keeps track of outgoing packets through two pointers:
ongoing_tx_skb: Points to the skb buffer currently being sent.
- waiting_tx_skb: Points to the skb waiting in queue if a new one arrives while the previous is still transmitting.
Both are shared across the network stack context (thread that queues packets) and the SPI thread (which physically writes them to the wire).
This dual-pointer design aims for performance, but it’s a pitfall if synchronization isn’t perfect.
Scenario Illustration
Let's detail the flow using comments and simplified pseudo-code.
// Thread 1: Network Stack - enqueue skb, aka oa_tc6_start_xmit
if (waiting_tx_skb == NULL) {
waiting_tx_skb = skb; // Save new skb
}
// Thread 2: SPI Handler - handle SPI writes, aka oa_tc6_spi_thread_handler
if (ongoing_tx_skb == NULL) {
ongoing_tx_skb = waiting_tx_skb;
waiting_tx_skb = NULL;
}
...
// After processing, ongoing_tx_skb = NULL
Both threads do not lock access to waiting_tx_skb and ongoing_tx_skb.
- When both values are NULL, swapping and assigning can leave a new skb stranded. One thread can set waiting_tx_skb as NULL while another is about to assign, leading to lost references.
Sets waiting_tx_skb to NULL.
3. If a new packet arrives *just* as these accesses happen, the wrong pointer gets overwritten, and a skb pointer is lost.
This results in dropped packets (never transmitted) and leaked memory (skb ref lost, not freed).
Here's a step-by-step
// 1. SPI thread checks
if (ongoing_tx_skb == NULL) {
ongoing_tx_skb = waiting_tx_skb; // suppose waiting_tx_skb is NULL
waiting_tx_skb = NULL;
// Now ongoing_tx_skb is NULL!
}
// 2. Network thread runs simultaneously
if (waiting_tx_skb == NULL) {
waiting_tx_skb = skb; // enqueue new skb
}
// 3. But, SPI thread may have already set waiting_tx_skb to NULL,
// so the reference to the new skb is lost once SPI thread runs again!
The fix introduces locking (usually a spinlock or mutex) around these shared pointers. Example
spin_lock(&tx_lock);
if (ongoing_tx_skb == NULL) {
ongoing_tx_skb = waiting_tx_skb;
waiting_tx_skb = NULL;
}
spin_unlock(&tx_lock);
Similarly, in the network stack thread
spin_lock(&tx_lock);
if (waiting_tx_skb == NULL) {
waiting_tx_skb = skb;
}
spin_unlock(&tx_lock);
This ensures both threads can't mess with the pointers at the same time.
Who is affected?
- Devices using the oa_tc6 Ethernet controller over SPI — typically embedded hardware or industrial Linux systems.
The system will leak memory for each lost skb,
- In theory, a remote adversary on the network could flood traffic, causing resource exhaustion, DoS (Denial of Service), or simply degrade network reliability.
If you have a system using this driver,
2. Generate bursts of outgoing traffic or manipulate timings (with crafted network code or custom hardware),
Observe packet loss and increasing kernel memory usage.
While exploiting the race for anything beyond DoS is hard, repeated memory leaks can quickly become a concern for resource-limited systems.
Upstream Patch discussion:
LKML link: "net: ethernet: oa_tc6: fix tx skb race condition between reference pointers"
Linux kernel commit:
kernel.org commit - oa_tc6 fix race condition
CVE database:
Conclusion
CVE-2024-56788 reminds us that concurrency bugs still lurk in network drivers—even in mature projects like Linux. If you're using hardware that depends on oa_tc6, update your kernel or apply the patch ASAP, especially if reliability or uptime matter!
For embedded developers: review your driver code for similar race conditions. Always use proper locking around shared pointers!
Stay sharp, and keep your kernels patched.
*This post is exclusive content. If you found it valuable, share responsibly—but remember: fixing upstream helps everyone!*
Timeline
Published on: 01/11/2025 13:15:29 UTC
Last modified on: 05/04/2025 10:04:45 UTC