iommufd: Move vevent memory allocation outside spinlock

Overview

In the Linux kernel, the following vulnerability has been resolved:

iommufd: Move vevent memory allocation outside spinlock

The veventq memory allocation happens inside the spinlock. Given its depth is decided by the user space, this leaves a vulnerability, where userspace can allocate large queues to exhaust atomic memory reserves.

Move the allocation outside the spinlock and use GFP_NOWAIT, which can fail fast under memory pressure without dipping into the GFP_ATOMIC reserves or direct-reclaiming from the threaded IRQ handler. On allocation failure, queue the lost_events_header (so userspace learns of the drop) and return -ENOMEM so the caller learns of the kernel-side memory pressure.

This is intentionally distinct from the queue-overflow path, which also queues the lost_events_header but returns 0: a full queue is an expected userspace-pacing condition rather than a kernel error.

A subsequent change will cap the upper bound of the veventq_depth.

Affected

• Linux >= e36ba5ab808ef6237c3148d469c8238674230e2b < 779480ea79551c31964e74b9aef0e730faa3aa11
• Linux >= e36ba5ab808ef6237c3148d469c8238674230e2b < 6c5fc40200cd0a87d66a368eee00df4d1cca946e
• Linux >= e36ba5ab808ef6237c3148d469c8238674230e2b < 47443565d10c51366c9382dbc8597cd6c460b8a2
• Linux 6.15

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.