CVE-2026-53274: 
Linux Kernel vulnerability analysis and mitigation

CVE-2026-53274 is a sleep-inside-lock vulnerability in the Linux kernel's SMC (Shared Memory Communications) socket option handler (__smc_setsockopt()) that allows a local unprivileged user to cause a Denial of Service (DoS). The flaw was disclosed on June 25, 2026, and affects Linux kernel versions from 5.18 up to (but not including) the fixed stable releases. Fixed versions include 6.1.176, 6.6.143, 6.12.94, 6.18.36, 7.0.13, and 7.1+. It carries a CVSS v3.1 base score of 5.5 (Medium) (GitHub Advisory, Red Hat).

The root cause is an incorrect synchronization flaw (CWE-821): __smc_setsockopt() calls copy_from_sockptr() while holding lock_sock(sk), creating a window where a blocking user-space memory operation can stall indefinitely inside the critical section. An attacker exploits this by passing a userfaultfd-monitored memory page (or FUSE-backed memory on systems with unprivileged userfaultfd disabled) as the optval argument to setsockopt(), causing the kernel to block during the copy while holding the socket lock. When combined with concurrent asynchronous tear-down operations such as shutdown(), this exhausts kernel worker threads (kworkers) and triggers the hung task watchdog, resulting in a system hang. The fix relocates the copy_from_sockptr() call to outside the lock_sock() critical section (GitHub Advisory, Red Hat Bugzilla).

Successful exploitation results in a local Denial of Service: kernel worker threads are exhausted, the hung task watchdog fires, and the affected system becomes unresponsive or requires a reboot. There is no confidentiality or integrity impact — the vulnerability is limited to availability. Because exploitation requires only low-privileged local access, any multi-tenant Linux system (e.g., shared servers, container hosts) running an affected kernel with SMC support enabled is at risk (GitHub Advisory, Red Hat).

1. Gain local access: Obtain a low-privileged shell on a target Linux system running a vulnerable kernel (5.18 through the unpatched stable branches) with SMC socket support compiled in.
2. Set up userfaultfd or FUSE: Register a userfaultfd handler on a memory page, or mount a FUSE filesystem, so that any read from a specific memory address will block until the handler responds.
3. Craft the setsockopt call: Open an SMC socket (AF_SMC) and call setsockopt() with an optval pointer pointing to the userfaultfd-monitored or FUSE-backed memory page. This causes copy_from_sockptr() inside __smc_setsockopt() to block while lock_sock(sk) is held.
4. Trigger concurrent shutdown: From a second thread or process, call shutdown() on the same socket. This operation attempts to acquire the socket lock, which is already held, causing a kworker thread to block.
5. Exhaust kernel workers: Repeat steps 3–4 to consume all available kernel worker threads (kworkers), triggering the hung task watchdog and causing the system to hang or become unresponsive (GitHub Advisory, Red Hat Bugzilla).

• Logs: Kernel messages such as INFO: task kworker/uX:Y blocked for more than 120 seconds in /var/log/kern.log or dmesg; call traces referencing smc_shutdown and lock_sock_nested.
• Process: Large number of kworker threads in a blocked/uninterruptible (D) state visible via ps aux or top; system load average rising sharply without corresponding CPU usage.
• Network/Socket: Unusual AF_SMC socket activity from unprivileged user accounts, particularly repeated setsockopt() calls combined with shutdown() on the same socket.
• File System: Presence of userfaultfd-related or FUSE mount activity initiated by non-root users on systems where such access is normally restricted.

Apply the upstream kernel patch that moves copy_from_sockptr() outside the lock_sock() critical section in __smc_setsockopt(). Fixed stable kernel versions are 6.1.176, 6.6.143, 6.12.94, 6.18.36, 7.0.13, and 7.1+. As a temporary workaround on systems where immediate patching is not feasible, restrict unprivileged userfaultfd access by setting vm.unprivileged_userfaultfd=0 via sysctl, which raises the bar for exploitation (though FUSE-backed memory may still be usable on some configurations). Disabling SMC socket support (CONFIG_SMC) if not required by the workload also eliminates the attack surface (GitHub Advisory, Red Hat Bugzilla).

Red Hat has acknowledged the vulnerability and assigned it a Moderate severity rating, with tracking via Bugzilla. The Yocto Project security mailing list also flagged the issue for embedded Linux maintainers. No significant broader media coverage or notable researcher commentary beyond standard CVE tracking has been observed (Red Hat, Red Hat Bugzilla).