sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf() It was possible to have pick_eevdf() return NULL, which then causes a NULL-deref. This turned out to be due to entity_eligible() returning falsely negative because of a s64 multiplcation overflow. Specifically, reweight_eevdf() computes the vlag without considering the limit placed upon vlag as update_entity_lag() does, and then the scaling multiplication (remember that weight is 20bit fixed point) can overflow. This then leads to the new vruntime being weird which then causes the above entity_eligible() to go side-ways and claim nothing is eligible. Thus limit the range of vlag accordingly. All this was quite rare, but fatal when it does happen.

References

• https://git.kernel.org/stable/c/470d347b14b0ecffa9b39cf8f644fa2351db3efb
• https://git.kernel.org/stable/c/06f27e6d7bf0abf54488259ef36bbf0e1fccb35c
• https://git.kernel.org/stable/c/1560d1f6eb6b398bddd80c16676776c0325fe5fe