CVE-2026-52985: Linux Kernel Netdevsim Uninitialized Memory Bug

Technical summary

The vulnerability exists in the netdevsim driver's nsim_dev_trap_skb_build function, which constructs synthetic socket buffers (sk_buff) for testing trap handling. The function allocates space for an IP header using skb_put, which does not initialize the allocated memory. This leaves the iphdr structure containing whatever data was previously in kernel memory (an 'uninit-value'). The Kernel Memory Sanitizer (KMSAN) detected this during fuzzing. The fix replaces skb_put with skb_put_zero, ensuring the entire header is zero-initialized before use.

Business impact

This vulnerability has limited direct business impact because netdevsim is a testing and simulation driver, not a production networking component. However, it represents a broader kernel memory safety issue: uninitialized struct fields can leak sensitive kernel memory or cause unpredictable behavior in production. For organizations running custom kernel builds that include netdevsim or using it in development/CI environments, this bug could lead to information disclosure or instability during testing cycles.

Affected systems

The vulnerability affects the Linux kernel across all versions that include the netdevsim module. Netdevsim is commonly used by network driver developers and in kernel test frameworks. Production systems typically do not load netdevsim unless explicitly configured for network simulation testing or CI/CD validation. The vulnerability is specific to code paths that invoke trap simulation via nsim_dev_trap_skb_build.

Exploitability

This vulnerability is not readily exploitable in typical production scenarios. Exploiting it requires either loading the netdevsim kernel module and having the ability to trigger the trap simulation code path, or running code in a development/test environment where netdevsim is active. There is no remote network attack vector. The vulnerability is primarily a local kernel memory safety issue detected during fuzzing, making it a quality and robustness concern rather than an immediate security threat.

Remediation

Apply the Linux kernel patch that replaces skb_put with skb_put_zero in the nsim_dev_trap_skb_build function. This ensures the IP header structure is zero-initialized at allocation time. Verify the patch is included in your kernel version or apply it if running a custom build that includes netdevsim.

Patch guidance

Check your Linux kernel version against the upstream kernel repository to confirm the patch has been integrated. For users of stable/LTS kernel branches (such as linux-6.1.y, linux-6.6.y, etc.), verify that the fix is included in the specific stable release you are running. If running netdevsim in development or test environments, prioritize updating to a kernel version that includes this patch. Verify against the official Linux kernel commit log or your distribution's security advisories for exact version numbers.

Detection guidance

Monitor kernel logs for KMSAN warnings or other memory sanitizer alerts originating from nsim_dev_trap_skb_build or the netdevsim module. In development and CI environments, enable kernel memory safety tools (KMSAN, KASAN) to catch similar issues. Check if netdevsim is loaded on systems where it should not be required using `lsmod | grep netdevsim`. Review kernel configuration to determine whether netdevsim is compiled in or available as a loadable module.

Why prioritize this

Although this is a real kernel memory bug, prioritization should be moderate. The vulnerability is not remotely exploitable and primarily affects development and testing environments. Organizations running only standard production kernels without netdevsim can deprioritize patching. However, teams using netdevsim for driver development or kernel CI should patch promptly to avoid potential information leaks or test instability.

Risk score, explained

The CVSS 3.1 score of 5.5 (MEDIUM) reflects a local privilege requirement, no confidentiality impact, no integrity impact, but high availability impact (a kernel crash or hang is possible). The score appropriately excludes remote attack scenarios and acknowledges the limited scope to systems running netdevsim code paths.

Frequently asked questions