net_sched: prio: fix a race in prio_tune()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net_sched: prio: fix a race in prio_tune() Gerrard Tai reported a race condition in PRIO, whenever SFQ perturb timer fires at the wrong time. The race is as follows: CPU 0 CPU 1 [1]: lock root [2]: qdisc_tree_flush_backlog() [3]: unlock root | | [5]: lock root | [6]: rehash | [7]: qdisc_tree_reduce_backlog() | [4]: qdisc_put() This can be abused to underflow a parent's qlen. Calling qdisc_purge_queue() instead of qdisc_tree_flush_backlog() should fix the race, because all packets will be purged from the qdisc before releasing the lock.

References

• https://git.kernel.org/stable/c/53d11560e957d53ee87a0653d258038ce12361b7
• https://git.kernel.org/stable/c/4483d8b9127591c60c4eb789d6cab953bc4522a9
• https://git.kernel.org/stable/c/20f68e6a9e41693cb0e55e5b9ebbcb40983a4b8f
• https://git.kernel.org/stable/c/3aaa7c01cf19d9b9bb64b88b65c3a6fd05da2eb4
• https://git.kernel.org/stable/c/46c15c9d0f65c9ba857d63f53264f4b17e8a715f
• https://git.kernel.org/stable/c/e3f6745006dc9423d2b065b90f191cfa11b1b584
• https://git.kernel.org/stable/c/93f9eeb678d4c9c1abf720b3615fa8299a490845
• https://git.kernel.org/stable/c/d35acc1be3480505b5931f17e4ea9b7617fea4d3