net/sched: taprio: extend minimum interval restriction to entire cycle too

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: taprio: extend minimum interval restriction to entire cycle too It is possible for syzbot to side-step the restriction imposed by the blamed commit in the Fixes: tag, because the taprio UAPI permits a cycle-time different from (and potentially shorter than) the sum of entry intervals. We need one more restriction, which is that the cycle time itself must be larger than N * ETH_ZLEN bit times, where N is the number of schedule entries. This restriction needs to apply regardless of whether the cycle time came from the user or was the implicit, auto-calculated value, so we move the existing "cycle == 0" check outside the "if "(!new->cycle_time)" branch. This way covers both conditions and scenarios. Add a selftest which illustrates the issue triggered by syzbot.

References

• https://git.kernel.org/stable/c/34d83c3e6e97867ae061d14eb52123404aab1cbc
• https://git.kernel.org/stable/c/b939d1e04a90248b4cdf417b0969c270ceb992b2
• https://git.kernel.org/stable/c/91f249b01fe490fce11fbb4307952ca8cce78724
• https://git.kernel.org/stable/c/fb66df20a7201e60f2b13d7f95d031b31a8831d3