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From: Cong Wang <xiyou.wangcong@gmail.com>
Date: Thu, 23 Mar 2017 12:00:01 -0700
Message-ID: <CAM_iQpWG78yjRAQ673NyiVBJZRfH7WMjbteRw7EfTkfH2tU2-w@mail.gmail.com>
Subject: Re: net/sched: GPF in qdisc_hash_add
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>,
        David Miller <davem@davemloft.net>, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        syzkaller <syzkaller@googlegroups.com>
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On Thu, Mar 23, 2017 at 9:06 AM, Dmitry Vyukov <dvyukov@google.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);