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Date:   Thu, 27 Sep 2018 10:06:44 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        syzbot+37b8770e6d5a8220a039@...kaller.appspotmail.com,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: KASAN: use-after-free Read in tcf_block_find

On Wed, Sep 26, 2018 at 11:55 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> >> Hello,
>> >>
>> >> syzbot found the following crash on:
>> >>
>> >> HEAD commit:    4b1bd6976945 net: phy: marvell: Fix build.
>> >> git tree:       net-next
>> >> console output: https://syzkaller.appspot.com/x/log.txt?x=16f763fa400000
>> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=443816db871edd66
>> >> dashboard link: https://syzkaller.appspot.com/bug?extid=37b8770e6d5a8220a039
>> >> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> >> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17a5614e400000
>> >> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=141a532a400000
>> >>
>> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> >> Reported-by: syzbot+37b8770e6d5a8220a039@...kaller.appspotmail.com
>> >>
>> >> IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
>> >> 8021q: adding VLAN 0 to HW filter on device team0
>> >> ==================================================================
>> >> BUG: KASAN: use-after-free in tcf_block_find+0x9d1/0xb90
>> >> net/sched/cls_api.c:646
>> >> Read of size 4 at addr ffff8801cc126978 by task syz-executor002/5646
>> >>
>> >> CPU: 1 PID: 5646 Comm: syz-executor002 Not tainted 4.19.0-rc5+ #232
>> >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
>> >> Google 01/01/2011
>> >> Call Trace:
>> >>   __dump_stack lib/dump_stack.c:77 [inline]
>> >>   dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
>> >>   print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
>> >>   kasan_report_error mm/kasan/report.c:354 [inline]
>> >>   kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
>> >>   __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
>> >>   tcf_block_find+0x9d1/0xb90 net/sched/cls_api.c:646
>> >
>> > Hmm. looks like missing a rcu_dereference() here:
>> >
>> >                 if (!*parent) {
>> >                         *q = dev->qdisc;
>> >                         *parent = (*q)->handle;
>> >
>>
>> We hold RTNL here.
>>
>> Have we already done the changes allowing dev->qdisc being changed
>> without RTNL being required ?
>
> That transition is not completed yet, but holding RTNL doesn't help
> any more after we now free qdisc in rcu callback.
>
> More interestingly, rcu read lock is already held in this context,
> so it seems we still have to use rcu_deference() to read dev->qdisc
> and of course mark it with __rcu too.

On hardware level rcu_deference() only affects Alpha. On software
level, it can affect all archs due to e.g. a re-read of the variable.
Do we see the potential for compilation that would lead to the UAF
here? A missed rcu_deference() would not be the first thing that I
would suspect for UAF on x86. It's more likely some bolder bug.

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