netdev - Re: RCU callback crashes

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Date:   Wed, 20 Dec 2017 21:15:55 +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>,
        john.fastabend@...il.com
Subject: Re: RCU callback crashes

Wed, Dec 20, 2017 at 08:59:22PM CET, jiri@...nulli.us wrote:
>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.

Aha! It was removed by:
commit c5ad119fb6c09b0297446be05bd66602fa564758
Author: John Fastabend <john.fastabend@...il.com>
Date:   Thu Dec 7 09:58:19 2017 -0800

    net: sched: pfifo_fast use skb_array


>
>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.