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

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(), se->vlag is scaled, which overflows and results in

another huge (positive or negative) number.

9. The adjusted lag is subtracted from se->vruntime, which increases or

decreases se->vruntime by a huge number.

10. pick_eevdf() calls entity_eligible()/vruntime_eligible(), which

incorrectly returns false because the vruntime is so far from the

other vruntimes on the queue, causing the

(vruntime - cfs_rq->min_vruntime) * load calulation to overflow.

11. Nothing appears to be eligible, so pick_eevdf() returns NULL.

12. pick_next_entity() tries to dereference the return value of

pick_eevdf() and crashes.

Dumping the cfs_rq states from the core dumps with drgn showed tell-tale

huge vruntime ranges and bogus vlag values, and I also traced se->slice

being set to U64_MAX on live systems (which was usually "benign" since

the rest of the runqueue needed to be in a particular state to crash).

Fix it in dequeue_entities() by always setting slice from the first

non-empty cfs_rq.