PROBLEM: tun/tap crashes if open() /dev/net/tun and then poll() it.

[Eugene Kapun]

This small program causes kernel oops:

#include <fcntl.h>
#include <poll.h>

int main() {
    int fd;
    struct pollfd pfd;
    fd = open("/dev/net/tun", O_RDWR);
    pfd.fd = fd;
    pfd.events = POLLIN | POLLOUT;
    poll(&pfd, 1, 0);
}

Software versions:
Gnu C                  4.4.0
Gnu make               3.81
binutils               2.19.51.20090622
util-linux             2.15.1-rc1
mount                  support
module-init-tools      3.8
e2fsprogs              1.41.5
reiserfsprogs          3.6.21
pcmciautils            014
Linux C Library        2.9
Dynamic linker (ldd)   2.9
Procps                 3.2.8
Net-tools              1.60
Kbd                    1.15
Sh-utils               7.4
wireless-tools         29
Modules Loaded         tun ppp_async option usb_storage binfmt_misc
ppdev kqemu sbp2 lp parport snd_hda_codec_realtek joydev snd_hda_intel
snd_hda_codec snd_pcm_oss snd_mixer_oss snd_pcm arc4 snd_seq_dummy
snd_seq_oss ecb snd_seq_midi snd_rawmidi ath5k snd_seq_midi_event
snd_seq snd_timer snd_seq_device pcmcia mac80211 nsc_ircc ath psmouse
snd sdhci_pci sdhci acer_wmi yenta_socket rsrc_nonstatic uvcvideo
tifm_7xx1 tifm_core serio_raw soundcore iTCO_wdt iTCO_vendor_support
irda led_class pcmcia_core videodev v4l1_compat v4l2_compat_ioctl32
cfg80211 pcspkr snd_page_alloc crc_ccitt ohci1394 ieee1394 tg3 usbhid
fbcon tileblit font bitblit softcursor i915 drm i2c_algo_bit video
output intel_agp

dmesg output:
[14610.772456] BUG: unable to handle kernel NULL pointer dereference at
0000000000000080
[14610.772464] IP: [<ffffffffa0455b63>] tun_chr_poll+0x33/0xf0 [tun]
[14610.772473] PGD 2885f067 PUD 2bd8f067 PMD 0
[14610.772477] Oops: 0000 [#4] SMP
[14610.772480] last sysfs file:
/sys/devices/LNXSYSTM:00/device:00/PNP0A08:00/device:29/PNP0C09:00/PNP0C0A:00/power_supply/BAT0/voltage_now
[14610.772484] CPU 0
[14610.772486] Modules linked in: tun ppp_async option usb_storage
binfmt_misc ppdev kqemu sbp2 lp parport snd_hda_codec_realtek joydev
snd_hda_intel snd_hda_codec snd_pcm_oss snd_mixer_oss snd_pcm arc4
snd_seq_dummy snd_seq_oss ecb snd_seq_midi snd_rawmidi ath5k
snd_seq_midi_event snd_seq snd_timer snd_seq_device pcmcia mac80211
nsc_ircc ath psmouse snd sdhci_pci sdhci acer_wmi yenta_socket
rsrc_nonstatic uvcvideo tifm_7xx1 tifm_core serio_raw soundcore iTCO_wdt
iTCO_vendor_support irda led_class pcmcia_core videodev v4l1_compat
v4l2_compat_ioctl32 cfg80211 pcspkr snd_page_alloc crc_ccitt ohci1394
ieee1394 tg3 usbhid fbcon tileblit font bitblit softcursor i915 drm
i2c_algo_bit video output intel_agp
[14610.772532] Pid: 7896, comm: tun-crash Tainted: G      D   
2.6.31-rc1-git10-generic-vanilla #1 Extensa 5220                  
[14610.772534] RIP: 0010:[<ffffffffa0455b63>]  [<ffffffffa0455b63>]
tun_chr_poll+0x33/0xf0 [tun]
[14610.772539] RSP: 0018:ffff88002bdbdb08  EFLAGS: 00010246
[14610.772541] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
ffff88002bdbdb88
[14610.772544] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
ffff88007add0fe0
[14610.772546] RBP: ffff88002bdbdb28 R08: 00007feea2ddb2e0 R09:
00007feea2def160
[14610.772548] R10: 00007fffc0c0a150 R11: 0000000000000246 R12:
ffff88002bdf8000
[14610.772550] R13: 0000000000000000 R14: ffff88002bdf8000 R15:
0000000000000000
[14610.772553] FS:  00007feea2fdf6f0(0000) GS:ffff88000199d000(0000)
knlGS:0000000000000000
[14610.772555] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[14610.772557] CR2: 0000000000000080 CR3: 000000002bdf7000 CR4:
00000000000006b0
[14610.772560] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[14610.772562] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[14610.772565] Process tun-crash (pid: 7896, threadinfo
ffff88002bdbc000, task ffff880028838000)
[14610.772566] Stack:
[14610.772568]  ffff88002bdbdf04 ffff88002bdbde0c ffff88002bdbde04
ffff88002bdf8000
[14610.772572] <0> ffff88002bdbdf38 ffffffff8111eef5 ffff88002bdbdb58
01ffffff810ccaf2
[14610.772575] <0> 00007fffc0c0a3d0 0000000000000000 ffff88002bdbdb88
00000001810e221d
[14610.772580] Call Trace:
[14610.772587]  [<ffffffff8111eef5>] do_sys_poll+0x235/0x510
[14610.772591]  [<ffffffff8111eaf0>] ? __pollwait+0x0/0xf0
[14610.772596]  [<ffffffff81504b49>] ? _spin_lock+0x9/0x10
[14610.772598]  [<ffffffff81504b49>] ? _spin_lock+0x9/0x10
[14610.772601]  [<ffffffff811219de>] ? __d_lookup+0x12e/0x150
[14610.772605]  [<ffffffffa0455862>] ? tun_chr_open+0x32/0xe0 [tun]
[14610.772609]  [<ffffffffa0455862>] ? tun_chr_open+0x32/0xe0 [tun]
[14610.772612]  [<ffffffff81504dcd>] ? unlock_kernel+0x2d/0x40
[14610.772617]  [<ffffffff812e351b>] ? misc_open+0x1bb/0x200
[14610.772622]  [<ffffffff811118da>] ? chrdev_open+0xfa/0x1b0
[14610.772626]  [<ffffffff810cb849>] ? find_get_page+0x19/0xa0
[14610.772629]  [<ffffffff810cd831>] ? filemap_fault+0xb1/0x450
[14610.772632]  [<ffffffff810ccaf2>] ? unlock_page+0x22/0x30
[14610.772636]  [<ffffffff810e4ce9>] ? __do_fault+0x419/0x4e0
[14610.772639]  [<ffffffff810e8d12>] ? handle_mm_fault+0x1b2/0x950
[14610.772643]  [<ffffffff811271bb>] ? mntput_no_expire+0x2b/0x100
[14610.772647]  [<ffffffff81030389>] ? default_spin_lock_flags+0x9/0x10
[14610.772651]  [<ffffffff81260f9a>] ? __up_read+0x9a/0xc0
[14610.772656]  [<ffffffff81073199>] ? up_read+0x9/0x10
[14610.772659]  [<ffffffff8111f3b7>] sys_poll+0x77/0x110
[14610.772663]  [<ffffffff81011ec2>] system_call_fastpath+0x16/0x1b
[14610.772665] Code: 48 89 1c 24 4c 89 64 24 08 49 89 fc 4c 89 6c 24 10
4c 89 74 24 18 48 89 f3 48 8b bf a0 00 00 00 e8 73 f6 ff ff 48 85 db 49
89 c5 <4c> 8b b0 80 00 00 00 0f 85 98 00 00 00 49 8d 45 18 49 39 45 18
[14610.772694] RIP  [<ffffffffa0455b63>] tun_chr_poll+0x33/0xf0 [tun]
[14610.772698]  RSP <ffff88002bdbdb08>
[14610.772699] CR2: 0000000000000080
[14610.772702] ---[ end trace 9813be85d2509042 ]---
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