netdev - Re: net/sched: GPF in qdisc_hash

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Date:   Fri, 24 Mar 2017 10:28:12 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/sched: GPF in qdisc_hash_add

On Thu, Mar 23, 2017 at 12:10 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> On Thu, Mar 23, 2017 at 12:06 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>
>> On Thu, Mar 23, 2017 at 8:00 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> > On Thu, Mar 23, 2017 at 9:06 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> >> kasan: CONFIG_KASAN_INLINE enabled
>> >> kasan: GPF could be caused by NULL-ptr deref or user memory access
>> >> general protection fault: 0000 [#1] SMP KASAN
>> >> Dumping ftrace buffer:
>> >>    (ftrace buffer empty)
>> >> Modules linked in:
>> >> CPU: 2 PID: 12732 Comm: syz-executor6 Not tainted 4.11.0-rc3+ #365
>> >> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> >> task: ffff880062b7a2c0 task.stack: ffff880033480000
>> >> RIP: 0010:qdisc_hash_add.part.19+0xb6/0x3c0 net/sched/sch_api.c:280
>> >> RSP: 0018:ffff880033487820 EFLAGS: 00010202
>> >> RAX: dffffc0000000000 RBX: ffffffff85357e00 RCX: ffffc90002b24000
>> >> RDX: 000000000000007a RSI: ffffffff835a523a RDI: 00000000000003d0
>> >> RBP: ffff8800334878b8 R08: fffffbfff0a6afeb R09: fffffbfff0a6afeb
>> >> R10: 0000000000000001 R11: fffffbfff0a6afea R12: ffffffff85357e48
>> >> R13: 1ffff10006690f06 R14: ffff880033487890 R15: 0000000000000000
>> >> FS:  00007f68665d0700(0000) GS:ffff88006e200000(0000) knlGS:0000000000000000
>> >> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> >> CR2: 00000000004c2d44 CR3: 000000003c6f8000 CR4: 00000000000026e0
>> >> Call Trace:
>> >>  qdisc_hash_add+0x76/0x90 net/sched/sch_api.c:279
>> >>  attach_default_qdiscs net/sched/sch_generic.c:798 [inline]
>> >>  dev_activate+0x6ca/0x920 net/sched/sch_generic.c:829
>> >>  __dev_open+0x25b/0x360 net/core/dev.c:1348
>> >>  __dev_change_flags+0x159/0x3d0 net/core/dev.c:6460
>> >>  dev_change_flags+0x88/0x140 net/core/dev.c:6525
>> >>  dev_ifsioc+0x51f/0x9b0 net/core/dev_ioctl.c:254
>> >>  dev_ioctl+0x1fe/0x1030 net/core/dev_ioctl.c:532
>> >>  sock_do_ioctl+0x94/0xb0 net/socket.c:902
>> >>  sock_ioctl+0x2c2/0x440 net/socket.c:993
>> >>  vfs_ioctl fs/ioctl.c:45 [inline]
>> >>  do_vfs_ioctl+0x1af/0x16d0 fs/ioctl.c:685
>> >>  SYSC_ioctl fs/ioctl.c:700 [inline]
>> >>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
>> >>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>> >
>> > The interesting part is why the NULL dereference is in
>> > qdisc_hash_add(), since we have a check before calling
>> > it:
>> >
>> > #ifdef CONFIG_NET_SCHED
>> >         if (dev->qdisc)
>> >                 qdisc_hash_add(dev->qdisc);
>> > #endif
>> >
>> >
>> > When attach_one_default_qdisc() fails, we should trigger
>> > the NULL pointer dereference bug at:
>> >
>> >     atomic_inc(&dev->qdisc->refcnt);
>>
>> I think qdisc is not NULL, it's something _in_ qdisc that is NULL. The
>> crash happens here:
>>
>>                 struct Qdisc *root = qdisc_dev(q)->qdisc;
>>
>> so it's probably device.
>
>
>
> Looks like this bug came with commit 59cc1f61f09c
> ("net: sched: convert qdisc linked list to hashtable")
>
> I would simply guard qdisc_hash_add()
>
> (Against &noop_qdisc)

Yeah, I missed that dev_init_scheduler() could assign noop_qdisc
to each tx queue. Then the check in attach_default_qdiscs()
is always false? If so we need...

 #ifdef CONFIG_NET_SCHED
-       if (dev->qdisc)
+       if (dev->qdisc != &noop_qdisc)
                qdisc_hash_add(dev->qdisc);
 #endif