[Intel-wired-lan] [next-queue PATCH v3 2/4] net/sched: Fix accessing invalid dev_queue

[Eric Dumazet]

On Tue, 2017-10-03 at 16:44 -0700, Vinicius Costa Gomes wrote:
> From: Jesus Sanchez-Palencia <jesus.sanchez-palencia at intel.com>
> 
> In qdisc_alloc() the dev_queue pointer was used without any checks being
> performed. If qdisc_create() gets a null dev_queue pointer, it just
> passes it along to qdisc_alloc(), leading to a crash. That happens if a
> root qdisc implements select_queue() and returns a null dev_queue
> pointer for an "invalid handle", for example.
> 
> One way to reproduce that is:
> 
> 1) Setup mqprio
> $ tc qdisc replace dev enp3s0 parent root mqprio num_tc 3 \
>      	   map 2 2 1 0 2 2 2 2 2 2 2 2 2 2 2 2 queues 1 at 0 1 at 1 2 at 2 hw 0
> 
> 2) Replace the first inner qdisc
> $ tc qdisc replace dev enp3s0 parent 8001:1 pfifo_fast
> 
> This will lead to the following crash:

When was this bug added ?

If this is a consequence of your prior patch (1/4), then this must come
before it.

No need to add a stack trace for a not existing bug.
Instead, explain in the changelog that it is a prep work.

We try to not break the tree on purpose, so that future bisection will
not hit a point where the kernel crashes.