Affected: Linux

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

sched/eevdf: Fix se->slice being set to U64_MAX and resulting crash

There is a code path in dequeue_entities() that can set the slice of a sched_entity to U64_MAX, which sometimes results in a crash.

The offending case is when dequeue_entities() is called to dequeue a delayed group entity, and then the entity's parent's dequeue is delayed. In that case:

1. In the if (entity_is_task(se)) else block at the beginning of dequeue_entities(), slice is set to cfs_rq_min_slice(group_cfs_rq(se)). If the entity was delayed, then it has no queued tasks, so cfs_rq_min_slice() returns U64_MAX. 2. The first for_each_sched_entity() loop dequeues the entity. 3. If the entity was its parent's only child, then the next iteration tries to dequeue the parent. 4. If the parent's dequeue needs to be delayed, then it breaks from the first for_each_sched_entity() loop _without updating slice_. 5. The second for_each_sched_entity() loop sets the parent's ->slice to the saved slice, which is still U64_MAX.

This throws off subsequent calculations with potentially catastrophic results. A manifestation we saw in production was:

6. In update_entity_lag(), se->slice is used to calculate limit, which ends up as a huge negative number. 7. limit is used in se->vlag = clamp(vlag, -limit, limit). Because limit is negative, vlag > limit, so se->vlag is set to the same huge negative number. 8. In place_entity(),…