BUG: spinlock bad magic in tun_do_read

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    75bc37fefc44 Linux 4.17-rc4
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1162c697800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=31f4b3733894ef79
dashboard link: https://syzkaller.appspot.com/bug?extid=e8b902c3c3fadf0a9dba
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
userspace arch: i386
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=172e4c97800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+e8b902c3c3fadf0a9dba@xxxxxxxxxxxxxxxxxxxxxxxxx

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
IPVS: ftp: loaded support on port[0] = 21
BUG: spinlock bad magic on CPU#0, syz-executor0/4586
 lock: 0xffff8801ae8928c8, .magic: 00000000, .owner: <none>/-1, .owner_cpu:  
0
CPU: 0 PID: 4586 Comm: syz-executor0 Not tainted 4.17.0-rc4+ #62
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+0x1b9/0x294 lib/dump_stack.c:113
 spin_dump+0x160/0x169 kernel/locking/spinlock_debug.c:67
 spin_bug kernel/locking/spinlock_debug.c:75 [inline]
 debug_spin_lock_before kernel/locking/spinlock_debug.c:83 [inline]
 do_raw_spin_lock.cold.3+0x37/0x3c kernel/locking/spinlock_debug.c:112
 __raw_spin_lock include/linux/spinlock_api_smp.h:143 [inline]
 _raw_spin_lock+0x32/0x40 kernel/locking/spinlock.c:144
 spin_lock include/linux/spinlock.h:310 [inline]
 ptr_ring_consume include/linux/ptr_ring.h:335 [inline]
 tun_ring_recv drivers/net/tun.c:2143 [inline]
 tun_do_read+0x18b1/0x29f0 drivers/net/tun.c:2182
 tun_chr_read_iter+0xe5/0x1e0 drivers/net/tun.c:2214
 call_read_iter include/linux/fs.h:1778 [inline]
 new_sync_read fs/read_write.c:406 [inline]
 __vfs_read+0x696/0xa50 fs/read_write.c:418
 vfs_read+0x17f/0x3d0 fs/read_write.c:452
 ksys_pread64+0x174/0x1a0 fs/read_write.c:626
 __do_compat_sys_x86_pread arch/x86/ia32/sys_ia32.c:177 [inline]
 __se_compat_sys_x86_pread arch/x86/ia32/sys_ia32.c:174 [inline]
 __ia32_compat_sys_x86_pread+0xc4/0x130 arch/x86/ia32/sys_ia32.c:174
 do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
 do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7fc0cb9
RSP: 002b:00000000f7fbc0ac EFLAGS: 00000282 ORIG_RAX: 00000000000000b4
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000020000080
RDX: 000000000000006e RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000292 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000


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