general protection fault in loopback_pos_update

[syzbot]

Hello,

syzbot hit the following crash on mmots commit
2734fabdcba49bf97498eac1db81f73d76dfd04f (Sat Jan 20 01:03:13 2018 +0000)
pci: test for unexpectedly disabled bridges

So far this crash happened 4 times on linux-next, mmots, upstream.
C reproducer is attached.
syzkaller reproducer is attached.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.

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Reported-by: syzbot+c4227a...@syzkaller.appspotmail.com
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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: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0-rc8-mm1+ #59
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:loopback_pos_update+0x182/0x1580 sound/drivers/aloop.c:479
RSP: 0018:ffff8801db2076d8 EFLAGS: 00010006
RAX: dffffc0000000000 RBX: 0000000000000147 RCX: ffffffff842d37dd
RDX: 0000000000000009 RSI: 00000000a8c1b747 RDI: 0000000000000048
RBP: ffff8801db207768 R08: 1ffff1003b640e2d R09: 000000000000000c
R10: ffff8801db207628 R11: 0000000000000024 R12: 0000000000000000
R13: 00000000ffffbf88 R14: 1ffffffff0d01200 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff8801db200000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000002054bff0 CR3: 0000000006822002 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<IRQ>
loopback_timer_function+0x6d/0x277 sound/drivers/aloop.c:522
call_timer_fn+0x228/0x820 kernel/time/timer.c:1326
expire_timers kernel/time/timer.c:1363 [inline]
__run_timers+0x7ee/0xb70 kernel/time/timer.c:1666
run_timer_softirq+0x4c/0x70 kernel/time/timer.c:1692
__do_softirq+0x2d7/0xb85 kernel/softirq.c:285
invoke_softirq kernel/softirq.c:365 [inline]
irq_exit+0x1cc/0x200 kernel/softirq.c:405
exiting_irq arch/x86/include/asm/apic.h:541 [inline]
smp_apic_timer_interrupt+0x16b/0x700 arch/x86/kernel/apic/apic.c:1052
apic_timer_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:937
</IRQ>
RIP: 0010:native_safe_halt+0x6/0x10 arch/x86/include/asm/irqflags.h:54
RSP: 0018:ffffffff86807c38 EFLAGS: 00000286 ORIG_RAX: ffffffffffffff11
RAX: dffffc0000000000 RBX: 1ffffffff0d00f8a RCX: 0000000000000000
RDX: 1ffffffff0d192a0 RSI: 0000000000000001 RDI: ffffffff868c9500
RBP: ffffffff86807c38 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffffffff86807cf0 R14: ffffffff87044ae0 R15: 0000000000000000
arch_safe_halt arch/x86/include/asm/paravirt.h:93 [inline]
default_idle+0xbf/0x430 arch/x86/kernel/process.c:354
arch_cpu_idle+0xa/0x10 arch/x86/kernel/process.c:345
default_idle_call+0x36/0x90 kernel/sched/idle.c:98
cpuidle_idle_call kernel/sched/idle.c:156 [inline]
do_idle+0x24a/0x3b0 kernel/sched/idle.c:246
cpu_startup_entry+0x104/0x120 kernel/sched/idle.c:351
rest_init+0xed/0xf0 init/main.c:436
start_kernel+0x7f1/0x819 init/main.c:716
x86_64_start_reservations+0x2a/0x2c arch/x86/kernel/head64.c:378
x86_64_start_kernel+0x77/0x7a arch/x86/kernel/head64.c:359
secondary_startup_64+0xa5/0xb0 arch/x86/kernel/head_64.S:237
Code: 00 fc ff df 41 80 3c 06 00 0f 85 e0 11 00 00 49 8d 7f 48 48 b8 00 00
00 00 00 fc ff df 4c 8b 2d f5 57 53 02 48 89 fa 48 c1 ea 03 <80> 3c 02 00
0f 85 b0 11 00 00 4d 89 ec 4d 2b 67 48 4d 89 6f 48
RIP: loopback_pos_update+0x182/0x1580 sound/drivers/aloop.c:479 RSP:
ffff8801db2076d8
---[ end trace 78a3baf5bb92aa31 ]---


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