UBUNTU-CVE-2026-99229: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: iouring/wait: don't spin on a stuck overflow list As long as IOCHECKCQOVERFLOWBIT is set, iocqringwaitschedule() returns right away, before it gets to look at signals or arms the timeout, for iocqringwait() to flush the overflow list. If that doesn't empty the list, and the CQ ring doesn't hold the number of events the caller wants, the loop starts over, with the same result. The task spins in the kernel, and can't be killed. The assumption is that this can't happen: if CQEs can't be flushed, the ring is full, and minevents is capped to the size of the ring. But: - With IORINGSETUPCQEMIXED a 32b CQE needs two contiguous entries. It is stuck on the list if a single one is free, and then the ring holds less than cqentries events. A single threaded application that asks for as many events as the ring has entries never gets out of iouringenter(). - Whether the ring is full is up to the CQ head, which the application can write to. While one thread waits for events on an empty ring, another one can move the head back so that the ring looks full and have a CQE overflow. The waiter can't be stopped any more, not even by SIGKILL. Only the application can make room in the CQ ring. Return to it if the list could not be flushed.
Affected Software
Event History
Frequently Asked Questions
Which workloads are exposed to this condition?
Applications that use io_uring completion queues can encounter it when completion queue overflow cannot be flushed. The described cases include rings created with IORING_SETUP_CQE_MIXED and multithreaded use where one thread changes the CQ head while another waits for events.
What does exploitation require?
The condition requires an io_uring completion queue overflow and a state where the overflow list cannot be emptied even though the caller is waiting for more events. With IORING_SETUP_CQE_MIXED, a 32-bit CQE can remain queued when only one contiguous CQ entry is free; alternatively, another thread can move the application-controlled CQ head so the ring appears full.
How might an affected system behave?
A task can spin in the kernel and cannot be killed because the wait path repeatedly returns before checking signals or arming its timeout. In the single-threaded mixed-CQE case, an application requesting as many events as the ring has entries may remain stuck in io_uring_enter().