CVE-2024-42245

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

Revert sched/fair: Make sure to try to detach at least one movable task

This reverts commit b0defa7ae03ecf91b8bfd10ede430cff12fcbd06.

b0defa7ae03ec changed the load balancing logic to ignore env.max_loop if all tasks examined to that point were pinned. The goal of the patch was to make it more likely to be able to detach a task buried in a long list of pinned tasks. However, this has the unfortunate side effect of creating an O(n) iteration in detach_tasks(), as we now must fully iterate every task on a cpu if all or most are pinned. Since this load balance code is done with rq lock held, and often in softirq context, it is very easy to trigger hard lockups. We observed such hard lockups with a user who affined O(10k) threads to a single cpu.

When I discussed this with Vincent he initially suggested that we keep the limit on the number of tasks to detach, but increase the number of tasks we can search. However, after some back and forth on the mailing list, he recommended we instead revert the original patch, as it seems likely no one was actually getting hit by the original issue.

Weakness

The product does not properly acquire or release a lock on a resource, leading to unexpected resource state changes and behaviors.

Affected Software

Extended Description

Locking is a type of synchronization behavior that ensures that multiple independently-operating processes or threads do not interfere with each other when accessing the same resource. All processes/threads are expected to follow the same steps for locking. If these steps are not followed precisely - or if no locking is done at all - then another process/thread could modify the shared resource in a way that is not visible or predictable to the original process. This can lead to data or memory corruption, denial of service, etc.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/25.html
• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/27.html

References

• https://git.kernel.org/stable/c/0fa6dcbfa2e2b97c1e6febbea561badf0931a38b
• https://git.kernel.org/stable/c/1e116c18e32b035a2d1bd460800072c8bf96bc44
• https://git.kernel.org/stable/c/2feab2492deb2f14f9675dd6388e9e2bf669c27a
• https://git.kernel.org/stable/c/d467194018dd536fe6c65a2fd3aedfcdb1424903
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html