UBUNTU-CVE-2026-99250: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: printk: Postpone kthread startup There have been various bug reports related to SMP systems hanging on boot. The reports vary but seem to have some overlap with regard to missed IPI's during SMP setup. The reports also show that reverting NBCON console usage allows the bugs to disappear. The NBCON kthreads for console printing are started very early during boot using earlyinitcall(). Since these kthreads are asynchronously enabling/disabling interrupts during early boot, they may be interfering with early SMP setup on some hardware platforms. Starting the console printing kthreads later has shown to avoid the hanging on systems that could previously reproduce the issue. Postpone kthread starting until after all arch initcalls have completed.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Configuration
Postpone starting the NBCON console-printing kthreads until after all architecture initialization callbacks have completed to avoid interference with early SMP setup.
Linux kernel NBCON console printing kthreads kthread startup timing = after all arch initcalls have completed
Event History
Frequently Asked Questions
Which systems are most likely to be affected?
The issue is associated with SMP systems that hang during boot, particularly where early NBCON console-printing kernel threads interfere with early SMP setup. The supplied data does not identify specific hardware platforms or configurations beyond that.
What configuration is involved in triggering the problem?
The reported behavior is tied to NBCON console usage, whose console-printing kthreads start very early during boot. Reports indicate that reverting NBCON console usage makes the boot hangs disappear.
What is the mitigation if an updated kernel cannot be installed immediately?
Reverting or disabling NBCON console usage is indicated as a workaround, based on reports that the hangs disappear without it. The kernel fix postpones startup of the console-printing kthreads until after architecture initcalls complete.
How can I tell whether this issue is affecting a system?
Affected systems may hang during SMP boot. A useful indicator is whether the hang stops occurring when NBCON console usage is reverted.