CVE-2026-52985 Overview
CVE-2026-52985 is an uninitialized memory use vulnerability in the Linux kernel's netdevsim driver. The flaw resides in the nsim_dev_trap_skb_build function, where a dummy sk_buff is constructed without zero-initializing the embedded struct iphdr. Syzbot detected the issue using KMSAN (Kernel Memory Sanitizer), which reported an uninit-value originating from the same function that performed the allocation.
The fix replaces the skb_put call with skb_put_zero, ensuring the entire IP header is zeroed before use. The vulnerability affects the in-kernel network device simulator used primarily for testing and development of networking subsystems.
Critical Impact
Uninitialized stack or heap memory contents may be propagated through simulated network traps, potentially leaking kernel memory bytes to consumers of the trap data path.
Affected Products
- Linux kernel netdevsim driver (multiple stable branches)
- Distributions shipping affected stable kernels prior to backport of the fix
- Test, CI, and development environments using netdevsim for network simulation
Discovery Timeline
- 2026-06-24 - CVE-2026-52985 published to NVD
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-52985
Vulnerability Analysis
The netdevsim module provides a simulated network device used by kernel developers and continuous integration systems. Within nsim_dev_trap_skb_build, the driver constructs a synthetic socket buffer (sk_buff) to emulate trapped packets traversing the device. The function allocates the buffer and then uses skb_put to reserve space for an struct iphdr without explicitly clearing the reserved bytes.
KMSAN flagged this allocation path because subsequent code reads from the struct iphdr fields before all bytes have been deterministically written. Any padding or unset header fields therefore carry whatever values were previously resident in the underlying memory pages. While netdevsim is a simulator, the trapped skb is observable through standard kernel networking interfaces, so the uninitialized bytes can be propagated outside the function.
The issue is classified as Uninitialized Memory Use, a class of bug tracked under CWE-908. The remediation calls skb_put_zero instead of skb_put, which guarantees the entire IP header region is zero before any field is populated.
Root Cause
The root cause is reliance on skb_put to reserve header space without an accompanying memset of the allocated region. skb_put only advances the buffer tail pointer; it does not initialize the memory. Code that later sets only a subset of struct iphdr fields leaves the remaining bytes undefined.
Attack Vector
Reachability requires execution paths that drive nsim_dev_trap_skb_build, which is exercised through netdevsim configuration and trap-generation interfaces typically available to privileged users. The vulnerability does not provide direct code execution. Its impact is limited to potential disclosure of small kernel memory regions through generated trap packets and to KMSAN-flagged undefined behavior that can destabilize fuzzing and testing pipelines.
No verified public proof-of-concept code is available beyond the Syzbot KMSAN reproducer that prompted the upstream fix. Refer to the kernel commits in the references for the exact source-level change.
Detection Methods for CVE-2026-52985
Indicators of Compromise
- KMSAN reports referencing nsim_dev_trap_skb_build in kernel logs from fuzzing or test infrastructure
- Anomalous trap packet contents generated by netdevsim instances containing non-deterministic byte patterns in IP header padding
- Kernel version strings indicating an unpatched stable branch listed in the upstream commits
Detection Strategies
- Enable KMSAN on test kernels and replay netdevsim trap workloads to surface uninitialized reads originating from the driver
- Compare installed kernel package versions against the fixed commits 175556c049ea, 1b7b6ae0e93b, 35eaa6d8d6c2, 6e2cfd090497, 750d0091bebf, 818f7673ed7f, 978ca6ff789f, and bc6002865e8c
- Audit hosts where netdevsim is loaded; production systems generally have no need for this module
Monitoring Recommendations
- Track loaded kernel modules and alert when netdevsim appears outside of test or CI environments
- Monitor distribution security trackers for backports of the fix into long-term stable kernels
- Integrate KMSAN and syzkaller results into CI gates so regressions in netdevsim skb handling are caught before release
How to Mitigate CVE-2026-52985
Immediate Actions Required
- Apply the upstream Linux kernel patches referenced in the kernel.org commits to all affected stable branches
- Unload the netdevsim module on systems where it is not required by running modprobe -r netdevsim
- Restrict CAP_NET_ADMIN and other privileged capabilities required to configure netdevsim instances
Patch Information
The fix replaces skb_put with skb_put_zero in nsim_dev_trap_skb_build to zero-initialize the embedded struct iphdr. The change is distributed across the following stable kernel commits: 175556c049ea, 1b7b6ae0e93b, 35eaa6d8d6c2, 6e2cfd090497, 750d0091bebf, 818f7673ed7f, 978ca6ff789f, and bc6002865e8c.
Workarounds
- Blacklist the netdevsim module on production hosts by adding blacklist netdevsim to /etc/modprobe.d/
- Limit access to the netdevsim sysfs and debugfs interfaces to trusted administrative accounts only
- Run fuzzing and CI workloads that exercise netdevsim in isolated environments until the patched kernel is deployed
# Configuration example
# Prevent netdevsim from loading on production systems
echo 'blacklist netdevsim' | sudo tee /etc/modprobe.d/blacklist-netdevsim.conf
sudo modprobe -r netdevsim 2>/dev/null || true
# Verify the module is not loaded
lsmod | grep netdevsim || echo 'netdevsim not loaded'
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
