CVE-2024-31076

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

genirq/cpuhotplug, x86/vector: Prevent vector leak during CPU offline

The absence of IRQDMOVEPCNTXT prevents immediate effectiveness of interrupt affinity reconfiguration via procfs. Instead, the change is deferred until the next instance of the interrupt being triggered on the original CPU.

When the interrupt next triggers on the original CPU, the new affinity is enforced within _irqmoveirq(). A vector is allocated from the new CPU, but the old vector on the original CPU remains and is not immediately reclaimed. Instead, apicd->movein_progress is flagged, and the reclaiming process is delayed until the next trigger of the interrupt on the new CPU.

Upon the subsequent triggering of the interrupt on the new CPU, irqcompletemove() adds a task to the old CPU's vectorcleanup list if it remains online. Subsequently, the timer on the old CPU iterates over its vectorcleanup list, reclaiming old vectors.

However, a rare scenario arises if the old CPU is outgoing before the interrupt triggers again on the new CPU.

In that case irqforcecompletemove() is not invoked on the outgoing CPU to reclaim the old apicd->prevvector because the interrupt isn't currently affine to the outgoing CPU, and irqneedsfixup() returns false. Even though _vectorschedulecleanup() is later called on the new CPU, it doesn't reclaim apicd->prevvector; instead, it simply resets both apicd->moveinprogress and apicd->prev_vector to 0.

As a result, the vector remains unreclaimed in vector_matrix, leading to a CPU vector leak.

To address this issue, move the invocation of irqforcecompletemove() before the irqneedsfixup() call to reclaim apicd->prevvector, if the interrupt is currently or used to be affine to the outgoing CPU.

Additionally, reclaim the vector in _vectorschedulecleanup() as well, following a warning message, although theoretically it should never see apicd->moveinprogress with apicd->prevcpu pointing to an offline CPU.