netdev - Re: RCU callback crashes

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Date:   Wed, 20 Dec 2017 20:59:22 +0100
From:   Jiri Pirko <jiri@...nulli.us>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Jakub Kicinski <kubakici@...pl>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: RCU callback crashes

Wed, Dec 20, 2017 at 07:17:50PM CET, xiyou.wangcong@...il.com wrote:
>On Tue, Dec 19, 2017 at 10:34 PM, Jakub Kicinski <kubakici@...pl> wrote:
>> Ah, no object debug but KASAN on produces this:
>>
>
>
>I bet it is an ingress qdisc which is being freed?
>
>
>
>> [   39.268209] BUG: KASAN: use-after-free in cpu_needs_another_gp+0x246/0x2b0
>> [   39.275965] Read of size 8 at addr ffff8803aa64f138 by task swapper/13/0
>> [   39.283524]
>> [   39.285256] CPU: 13 PID: 0 Comm: swapper/13 Not tainted 4.15.0-rc3-perf-00955-g1d0b01347dd5-dirty #8
>> [   39.295535] Hardware name: Dell Inc. PowerEdge R730/072T6D, BIOS 2.3.4 11/08/2016
>> [   39.303969] Call Trace:
>> [   39.306769]  <IRQ>
>> [   39.309088]  dump_stack+0xa6/0x118
>> [   39.312957]  ? _atomic_dec_and_lock+0xe8/0xe8
>> [   39.317895]  ? cpu_needs_another_gp+0x246/0x2b0
>> [   39.323030]  print_address_description+0x6a/0x270
>> [   39.328380]  ? cpu_needs_another_gp+0x246/0x2b0
>> [   39.333510]  kasan_report+0x23f/0x350
>> [   39.337672]  cpu_needs_another_gp+0x246/0x2b0
>> ...
>> [   39.383026]  rcu_process_callbacks+0x1a0/0x620
>> ...
>
>
>This is confusing.
>
>I guess it is q->miniqp which is freed in qdisc_graft() without properly
>waiting for rcu readers?

miniqp is inside qdisc private data:
struct ingress_sched_data {
        struct tcf_block *block;
        struct tcf_block_ext_info block_info;
        struct mini_Qdisc_pair miniqp;
};

That is freed along with the qdisc itself in:
qdisc_destroy->qdisc_free

Before miniq, tp was checked in the rcu reader path. In case it was not
null, q was processed. In slow patch, tp is freed after rcu grace period:
tcf_proto_destroy->kfree_rcu

I assumed that since q is processed in rcu reader, it is also freed after
a grace period, but now looking at the code I don't see it happening
like that.

So I think that change to miniq made the existing race window
a bit wider and easier to hit.

I believe that calling kfree_rcu by call_rcu should resolve this.