CVE-2026-52980
In the Linux kernel, the following vulnerability has been resolved:
sched/fair: Clear rel_deadline when initializing forked entities
A yield-triggered crash can happen when a newly forked sched_entity enters the fair class with se->rel_deadline unexpectedly set.
The failing sequence is:
- A task is forked while se->rel_deadline is still set.
- __sched_fork() initializes vruntime, vlag and other sched_entity state, but does not clear rel_deadline.
- On the first enqueue, enqueue_entity() calls place_entity().
- Because se->rel_deadline is set, place_entity() treats se->deadline as a relative deadline and converts it to an absolute deadline by adding the current vruntime.
- However, the forked entity's deadline is not a valid inherited relative deadline for this new scheduling instance, so the conversion produces an abnormally large deadline.
- If the task later calls sched_yield(), yield_task_fair() advances se->vruntime to se->deadline.
- The inflated vruntime is then used by the following enqueue path, where the vruntime-derived key can overflow when multiplied by the entity weight.
- This corrupts cfs_rq->sum_w_vruntime, breaks EEVDF eligibility calculation, and can eventually make all entities appear ineligible. pick_next_entity() may then return NULL unexpectedly, leading to a later NULL dereference.
A captured trace shows the effect clearly. Before yield, the entity's vruntime was around:
9834017729983308
After yield_task_fair() executed:
se->vruntime = se->deadline
the vruntime jumped to:
19668035460670230
and the deadline was later advanced further to:
19668035463470230
This shows that the deadline had already become abnormally large before yield_task_fair() copied it into vruntime.
rel_deadline is only meaningful when se->deadline really carries a relative deadline that still needs to be placed against vruntime. A freshly forked sched_entity should not inherit or retain this state. Clear se->rel_deadline in __sched_fork(), together with the other sched_entity runtime state, so that the first enqueue does not interpret the new entity's deadline as a stale relative deadline.
No affected software listed.
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