Re: WARNING in refcount_dec
From: Willem de Bruijn
Date: Sun Apr 01 2018 - 03:39:40 EST
On Thu, Mar 29, 2018 at 1:16 AM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> (Cc'ing netdev and Willem)
>
> On Wed, Mar 28, 2018 at 12:03 PM, Byoungyoung Lee
> <byoungyoung@xxxxxxxxxx> wrote:
>> Another crash patterns observed: race between (setsockopt$packet_int)
>> and (bind$packet).
>>
>> ------------------------------
>> [ 357.731597] kernel BUG at
>> /home/blee/project/race-fuzzer/kernels/kernel_v4.16-rc3/net/packet/af_packet.c:3107!
>> [ 357.733382] invalid opcode: 0000 [#1] SMP KASAN
>> [ 357.734017] Modules linked in:
>> [ 357.734662] CPU: 1 PID: 3871 Comm: repro.exe Not tainted 4.16.0-rc3 #1
>> [ 357.735791] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>> BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
>> [ 357.737434] RIP: 0010:packet_do_bind+0x88d/0x950
>> [ 357.738121] RSP: 0018:ffff8800b2787b08 EFLAGS: 00010293
>> [ 357.738906] RAX: ffff8800b2fdc780 RBX: ffff880234358cc0 RCX: ffffffff838b244c
>> [ 357.739905] RDX: 0000000000000000 RSI: ffffffff838b257d RDI: 0000000000000001
>> [ 357.741315] RBP: ffff8800b2787c10 R08: ffff8800b2fdc780 R09: 0000000000000000
>> [ 357.743055] R10: 0000000000000001 R11: 0000000000000000 R12: ffff88023352ecc0
>> [ 357.744744] R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000001d00
>> [ 357.746377] FS: 00007f4b43733700(0000) GS:ffff8800b8b00000(0000)
>> knlGS:0000000000000000
>> [ 357.749599] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 357.752096] CR2: 0000000020058000 CR3: 00000002334b8000 CR4: 00000000000006e0
>> [ 357.755045] Call Trace:
>> [ 357.755822] ? compat_packet_setsockopt+0x100/0x100
>> [ 357.757324] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
>> [ 357.758810] packet_bind+0xa2/0xe0
>> [ 357.759640] SYSC_bind+0x279/0x2f0
>> [ 357.760364] ? move_addr_to_kernel.part.19+0xc0/0xc0
>> [ 357.761491] ? __handle_mm_fault+0x25d0/0x25d0
>> [ 357.762449] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>> [ 357.763663] ? __do_page_fault+0x417/0xba0
>> [ 357.764569] ? vmalloc_fault+0x910/0x910
>> [ 357.765405] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>> [ 357.766525] ? mark_held_locks+0x25/0xb0
>> [ 357.767336] ? SyS_socketpair+0x4a0/0x4a0
>> [ 357.768182] SyS_bind+0x24/0x30
>> [ 357.768851] do_syscall_64+0x209/0x5d0
>> [ 357.769650] ? syscall_return_slowpath+0x3e0/0x3e0
>> [ 357.770665] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>> [ 357.771779] ? syscall_return_slowpath+0x260/0x3e0
>> [ 357.772748] ? mark_held_locks+0x25/0xb0
>> [ 357.773581] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>> [ 357.774720] ? retint_user+0x18/0x18
>> [ 357.775493] ? trace_hardirqs_off_caller+0xb5/0x120
>> [ 357.776567] ? trace_hardirqs_off_thunk+0x1a/0x1c
>> [ 357.777512] entry_SYSCALL_64_after_hwframe+0x42/0xb7
>> [ 357.778508] RIP: 0033:0x4503a9
>> [ 357.779156] RSP: 002b:00007f4b43732ce8 EFLAGS: 00000246 ORIG_RAX:
>> 0000000000000031
>> [ 357.780737] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004503a9
>> [ 357.782169] RDX: 0000000000000014 RSI: 0000000020058000 RDI: 0000000000000003
>> [ 357.783710] RBP: 00007f4b43732d10 R08: 0000000000000000 R09: 0000000000000000
>> [ 357.785202] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>> [ 357.786664] R13: 0000000000000000 R14: 00007f4b437339c0 R15: 00007f4b43733700
>> [ 357.788210] Code: c0 fd 48 c7 c2 00 c8 d9 84 be ab 02 00 00 48 c7
>> c7 60 c8 d9 84 c6 05 e7 a2 48 02 01 e8 3f 17 af fd e9 60 fb ff ff e8
>> 43 b3 c0 fd <0f> 0b e8 3c b3 c0 fd 48 8b bd 20 ff ff ff e8 60 1e e7 fd
>> 4c 89
>> [ 357.792260] RIP: packet_do_bind+0x88d/0x950 RSP: ffff8800b2787b08
>> [ 357.793698] ---[ end trace 0c5a2539f0247369 ]---
>> [ 357.794696] Kernel panic - not syncing: Fatal exception
>> [ 357.795918] Kernel Offset: disabled
>> [ 357.796614] Rebooting in 86400 seconds..
>>
>> On Wed, Mar 28, 2018 at 1:19 AM, Byoungyoung Lee <byoungyoung@xxxxxxxxxx> wrote:
>>> We report the crash: WARNING in refcount_dec
>>>
>>> This crash has been found in v4.16-rc3 using RaceFuzzer (a modified
>>> version of Syzkaller), which we describe more at the end of this
>>> report. Our analysis shows that the race occurs when invoking two
>>> syscalls concurrently, (setsockopt$packet_int) and
>>> (setsockopt$packet_rx_ring).
>>>
>>> C repro code : https://kiwi.cs.purdue.edu/static/race-fuzzer/repro-refcount_dec.c
>>> kernel config: https://kiwi.cs.purdue.edu/static/race-fuzzer/kernel-config-v4.16-rc3
>
>
> I tried your reproducer, no luck here.
>
Are both crashes with the same reproducer?
It races setsockopt PACKET_RX_RING with PACKET_VNET_HDR.
There have been previous bug fixes for other setsockopts racing with
ring creation. The change would be
@@ -3763,14 +3763,19 @@ packet_setsockopt(struct socket *sock, int
level, int optname, char __user *optv
if (sock->type != SOCK_RAW)
return -EINVAL;
- if (po->rx_ring.pg_vec || po->tx_ring.pg_vec)
- return -EBUSY;
if (optlen < sizeof(val))
return -EINVAL;
if (copy_from_user(&val, optval, sizeof(val)))
return -EFAULT;
- po->has_vnet_hdr = !!val;
+ lock_sock(sk);
+ if (po->rx_ring.pg_vec || po->tx_ring.pg_vec) {
+ ret = -EBUSY;
+ } else {
+ po->has_vnet_hdr = !!val;
+ ret = 0;
+ }
+ release_sock(sk);
return 0;
}
But I do not immediately see why these concurrent operations
would be unsafe.
The program races a lot more complex operations, like bind and
close. So the specific setsockopt may be a red herring.
I'm traveling; haven't been able to setup your fuzzer and run the
repro locally yet.
>
>
>>>
>>> ---------------------------------------
>>> [ 305.838560] refcount_t: decrement hit 0; leaking memory.
>>> [ 305.839669] WARNING: CPU: 0 PID: 3867 at
>>> /home/blee/project/race-fuzzer/kernels/kernel_v4.16-rc3/lib/refcount.c:228
>>> refcount_dec+0x62/0x70
>>> [ 305.841441] Modules linked in:
>>> [ 305.841883] CPU: 0 PID: 3867 Comm: repro.exe Not tainted 4.16.0-rc3 #1
>>> [ 305.842803] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>>> BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
>>> [ 305.844345] RIP: 0010:refcount_dec+0x62/0x70
>>> [ 305.845005] RSP: 0018:ffff880224d374f8 EFLAGS: 00010286
>>> [ 305.845802] RAX: 000000000000002c RBX: 0000000000000000 RCX: ffffffff81538991
>>> [ 305.846768] RDX: 0000000000000000 RSI: ffffffff813cd761 RDI: 0000000000000005
>>> [ 305.847748] RBP: ffff880224d37500 R08: ffff88023169a440 R09: 0000000000000000
>>> [ 305.848748] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88023473ad40
>>> [ 305.849738] R13: ffff88023473b368 R14: 0000000000000001 R15: 0000000000000000
>>> [ 305.850733] FS: 0000000000c6e940(0000) GS:ffff8800b8a00000(0000)
>>> knlGS:0000000000000000
>>> [ 305.851837] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [ 305.852652] CR2: 00007fb120571db8 CR3: 0000000005422000 CR4: 00000000000006f0
>>> [ 305.853619] Call Trace:
>>> [ 305.854086] __unregister_prot_hook+0x15f/0x1d0
>>> [ 305.854722] packet_release+0x77a/0x7a0
>>> [ 305.855335] ? mark_held_locks+0x25/0xb0
>>> [ 305.855883] ? packet_lookup_frame+0x110/0x110
>>> [ 305.856576] ? __lock_is_held+0x39/0xc0
>>> [ 305.857109] ? note_gp_changes+0x300/0x300
>>> [ 305.857745] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
>>> [ 305.858548] ? locks_remove_file+0x31b/0x420
>>> [ 305.859138] ? fcntl_setlk+0xad0/0xad0
>>> [ 305.859743] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.860534] ? fsnotify_first_mark+0x2c0/0x2c0
>>> [ 305.861234] sock_release+0x53/0xe0
>>> [ 305.861711] ? sock_alloc_file+0x2c0/0x2c0
>>> [ 305.862334] sock_close+0x16/0x20
>>> [ 305.862801] __fput+0x246/0x4e0
>>> [ 305.863310] ? fput+0x130/0x130
>>> [ 305.863743] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.864604] ____fput+0x15/0x20
>>> [ 305.865046] task_work_run+0x1a5/0x200
>>> [ 305.865636] ? kmem_cache_free+0x25c/0x2d0
>>> [ 305.866194] ? task_work_cancel+0x1a0/0x1a0
>>> [ 305.866829] ? switch_task_namespaces+0x9e/0xb0
>>> [ 305.867458] do_exit+0xacf/0x10d0
>>> [ 305.868023] ? mm_update_next_owner+0x650/0x650
>>> [ 305.868642] ? __pv_queued_spin_lock_slowpath+0xbf0/0xbf0
>>> [ 305.869427] ? trace_hardirqs_on_caller+0x136/0x2a0
>>> [ 305.870102] ? trace_hardirqs_on+0xd/0x10
>>> [ 305.870701] ? wake_up_new_task+0x41c/0x650
>>> [ 305.871324] ? to_ratio+0x20/0x20
>>> [ 305.871816] ? lock_release+0x530/0x530
>>> [ 305.872341] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.873161] ? match_held_lock+0x7e/0x360
>>> [ 305.873777] ? trace_hardirqs_off+0x10/0x10
>>> [ 305.874359] ? put_pid+0x111/0x140
>>> [ 305.874897] ? task_active_pid_ns+0x70/0x70
>>> [ 305.875500] ? find_held_lock+0xca/0xf0
>>> [ 305.876118] ? do_group_exit+0x1f9/0x260
>>> [ 305.876650] ? lock_downgrade+0x380/0x380
>>> [ 305.877297] ? task_clear_jobctl_pending+0xb5/0xd0
>>> [ 305.877951] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>>> [ 305.878725] ? do_raw_spin_unlock+0x112/0x1a0
>>> [ 305.879309] ? do_raw_spin_trylock+0x100/0x100
>>> [ 305.879969] ? mark_held_locks+0x25/0xb0
>>> [ 305.880505] ? force_sig+0x30/0x30
>>> [ 305.881054] ? _raw_spin_unlock_irq+0x27/0x50
>>> [ 305.881671] ? trace_hardirqs_on_caller+0x136/0x2a0
>>> [ 305.882412] do_group_exit+0xfb/0x260
>>> [ 305.882945] ? SyS_exit+0x30/0x30
>>> [ 305.883442] ? find_mergeable_anon_vma+0x90/0x90
>>> [ 305.884103] ? SyS_read+0x1c0/0x1c0
>>> [ 305.884785] ? mark_held_locks+0x25/0xb0
>>> [ 305.885503] ? do_syscall_64+0xb2/0x5d0
>>> [ 305.886093] ? do_group_exit+0x260/0x260
>>> [ 305.886741] SyS_exit_group+0x1d/0x20
>>> [ 305.887255] do_syscall_64+0x209/0x5d0
>>> [ 305.887888] ? syscall_return_slowpath+0x3e0/0x3e0
>>> [ 305.888611] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>>> [ 305.889420] ? syscall_return_slowpath+0x260/0x3e0
>>> [ 305.890188] ? mark_held_locks+0x25/0xb0
>>> [ 305.890724] ? entry_SYSCALL_64_after_hwframe+0x52/0xb7
>>> [ 305.891556] ? trace_hardirqs_off_caller+0xb5/0x120
>>> [ 305.892265] ? trace_hardirqs_off_thunk+0x1a/0x1c
>>> [ 305.892939] entry_SYSCALL_64_after_hwframe+0x42/0xb7
>>> [ 305.893676] RIP: 0033:0x44d989
>>> [ 305.894100] RSP: 002b:00000000007fff38 EFLAGS: 00000202 ORIG_RAX:
>>> 00000000000000e7
>>> [ 305.895158] RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000044d989
>>> [ 305.896174] RDX: 00007fb120d739c0 RSI: 00007fb120572700 RDI: 0000000000000001
>>> [ 305.897161] RBP: 00000000007fff60 R08: 00007fb120572700 R09: 0000000000000000
>>> [ 305.898128] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
>>> [ 305.899464] R13: 000000000040d270 R14: 000000000040d300 R15: 0000000000000000
>>> [ 305.900823] Code: b6 1d 81 9a ef 03 31 ff 89 de e8 ca a3 67 ff 84
>>> db 75 df e8 f1 a2 67 ff 48 c7 c7 60 8f 83 84 c6 05 61 9a ef 03 01 e8
>>> ee 5f 49 ff <0f> 0b eb c3 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 41
>>> 57 49
>>> [ 305.904324] ---[ end trace 360c084b02d93021 ]---
>>> [ 305.919117] ------------[ cut here ]------------
>>> [ 305.920120] refcount_t: underflow; use-after-free.
>>> [ 305.921335] WARNING: CPU: 0 PID: 3867 at
>>> /home/blee/project/race-fuzzer/kernels/kernel_v4.16-rc3/lib/refcount.c:187
>>> refcount_sub_and_test+0x1ec/0x200
>>> [ 305.923927] Modules linked in:
>>> [ 305.924611] CPU: 0 PID: 3867 Comm: repro.exe Tainted: G W
>>> 4.16.0-rc3 #1
>>> [ 305.925987] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>>> BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
>>> [ 305.928119] RIP: 0010:refcount_sub_and_test+0x1ec/0x200
>>> [ 305.929124] RSP: 0018:ffff880224d374a0 EFLAGS: 00010282
>>> [ 305.930161] RAX: 0000000000000026 RBX: 0000000000000000 RCX: ffffffff813c9644
>>> [ 305.931504] RDX: 0000000000000000 RSI: ffffffff813cd761 RDI: ffff880224d37018
>>> [ 305.932942] RBP: ffff880224d37538 R08: ffff88023169a440 R09: 0000000000000000
>>> [ 305.934365] R10: 0000000000000000 R11: 0000000000000000 R12: 00000000ffffffff
>>> [ 305.935734] R13: ffff88023473adc0 R14: 0000000000000001 R15: 1ffff100449a6e96
>>> [ 305.937114] FS: 0000000000c6e940(0000) GS:ffff8800b8a00000(0000)
>>> knlGS:0000000000000000
>>> [ 305.938668] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>> [ 305.939768] CR2: 00007fb120571db8 CR3: 0000000005422000 CR4: 00000000000006f0
>>> [ 305.941212] Call Trace:
>>> [ 305.941689] ? refcount_inc+0x70/0x70
>>> [ 305.942216] ? skb_dequeue+0xa5/0xc0
>>> [ 305.942713] refcount_dec_and_test+0x1a/0x20
>>> [ 305.943295] packet_release+0x702/0x7a0
>>> [ 305.943816] ? mark_held_locks+0x25/0xb0
>>> [ 305.944378] ? packet_lookup_frame+0x110/0x110
>>> [ 305.945021] ? __lock_is_held+0x39/0xc0
>>> [ 305.945561] ? note_gp_changes+0x300/0x300
>>> [ 305.946132] ? __sanitizer_cov_trace_const_cmp8+0x18/0x20
>>> [ 305.946866] ? locks_remove_file+0x31b/0x420
>>> [ 305.947464] ? fcntl_setlk+0xad0/0xad0
>>> [ 305.948000] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.948781] ? fsnotify_first_mark+0x2c0/0x2c0
>>> [ 305.949386] sock_release+0x53/0xe0
>>> [ 305.949866] ? sock_alloc_file+0x2c0/0x2c0
>>> [ 305.950437] sock_close+0x16/0x20
>>> [ 305.950906] __fput+0x246/0x4e0
>>> [ 305.951360] ? fput+0x130/0x130
>>> [ 305.951807] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.952620] ____fput+0x15/0x20
>>> [ 305.953071] task_work_run+0x1a5/0x200
>>> [ 305.953585] ? kmem_cache_free+0x25c/0x2d0
>>> [ 305.954143] ? task_work_cancel+0x1a0/0x1a0
>>> [ 305.954714] ? switch_task_namespaces+0x9e/0xb0
>>> [ 305.955334] do_exit+0xacf/0x10d0
>>> [ 305.955801] ? mm_update_next_owner+0x650/0x650
>>> [ 305.956431] ? __pv_queued_spin_lock_slowpath+0xbf0/0xbf0
>>> [ 305.957157] ? trace_hardirqs_on_caller+0x136/0x2a0
>>> [ 305.957811] ? trace_hardirqs_on+0xd/0x10
>>> [ 305.958360] ? wake_up_new_task+0x41c/0x650
>>> [ 305.958937] ? to_ratio+0x20/0x20
>>> [ 305.959391] ? lock_release+0x530/0x530
>>> [ 305.959924] ? trace_event_raw_event_sched_switch+0x320/0x320
>>> [ 305.960693] ? match_held_lock+0x7e/0x360
>>> [ 305.961244] ? trace_hardirqs_off+0x10/0x10
>>> [ 305.961810] ? put_pid+0x111/0x140
>>> [ 305.962277] ? task_active_pid_ns+0x70/0x70
>>> [ 305.962862] ? find_held_lock+0xca/0xf0
>>> [ 305.963396] ? do_group_exit+0x1f9/0x260
>>> [ 305.963933] ? lock_downgrade+0x380/0x380
>>> [ 305.964508] ? task_clear_jobctl_pending+0xb5/0xd0
>>> [ 305.965147] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>>> [ 305.965871] ? do_raw_spin_unlock+0x112/0x1a0
>>> [ 305.966459] ? do_raw_spin_trylock+0x100/0x100
>>> [ 305.967060] ? mark_held_locks+0x25/0xb0
>>> [ 305.967592] ? force_sig+0x30/0x30
>>> [ 305.968135] ? _raw_spin_unlock_irq+0x27/0x50
>>> [ 305.968741] ? trace_hardirqs_on_caller+0x136/0x2a0
>>> [ 305.969470] do_group_exit+0xfb/0x260
>>> [ 305.969987] ? SyS_exit+0x30/0x30
>>> [ 305.970505] ? find_mergeable_anon_vma+0x90/0x90
>>> [ 305.971126] ? SyS_read+0x1c0/0x1c0
>>> [ 305.971718] ? mark_held_locks+0x25/0xb0
>>> [ 305.972259] ? do_syscall_64+0xb2/0x5d0
>>> [ 305.972843] ? do_group_exit+0x260/0x260
>>> [ 305.973374] SyS_exit_group+0x1d/0x20
>>> [ 305.973932] do_syscall_64+0x209/0x5d0
>>> [ 305.974452] ? syscall_return_slowpath+0x3e0/0x3e0
>>> [ 305.975149] ? __sanitizer_cov_trace_const_cmp4+0x16/0x20
>>> [ 305.975941] ? syscall_return_slowpath+0x260/0x3e0
>>> [ 305.976669] ? mark_held_locks+0x25/0xb0
>>> [ 305.977206] ? entry_SYSCALL_64_after_hwframe+0x52/0xb7
>>> [ 305.977978] ? trace_hardirqs_off_caller+0xb5/0x120
>>> [ 305.978690] ? trace_hardirqs_off_thunk+0x1a/0x1c
>>> [ 305.979381] entry_SYSCALL_64_after_hwframe+0x42/0xb7
>>> [ 305.980114] RIP: 0033:0x44d989
>>> [ 305.980531] RSP: 002b:00000000007fff38 EFLAGS: 00000202 ORIG_RAX:
>>> 00000000000000e7
>>> [ 305.981664] RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 000000000044d989
>>> [ 305.982655] RDX: 00007fb120d739c0 RSI: 00007fb120572700 RDI: 0000000000000001
>>> [ 305.983654] RBP: 00000000007fff60 R08: 00007fb120572700 R09: 0000000000000000
>>> [ 305.984656] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
>>> [ 305.985707] R13: 000000000040d270 R14: 000000000040d300 R15: 0000000000000000
>>> [ 305.986724] Code: b6 1d 18 9b ef 03 31 ff 89 de e8 60 a4 67 ff 84
>>> db 75 1a e8 87 a3 67 ff 48 c7 c7 00 8f 83 84 c6 05 f8 9a ef 03 01 e8
>>> 84 60 49 ff <0f> 0b 31 db e9 2b ff ff ff 66 66 2e 0f 1f 84 00 00 00 00
>>> 00 55
>>> [ 305.990106] ---[ end trace 360c084b02d93022 ]---
>>> [ 305.998636] IPVS: ftp: loaded support on port[0] = 21
>>> ---------------------------------------
>>>
>>> = About RaceFuzzer
>>>
>>> RaceFuzzer is a customized version of Syzkaller, specifically tailored
>>> to find race condition bugs in the Linux kernel. While we leverage
>>> many different technique, the notable feature of RaceFuzzer is in
>>> leveraging a custom hypervisor (QEMU/KVM) to interleave the
>>> scheduling. In particular, we modified the hypervisor to intentionally
>>> stall a per-core execution, which is similar to supporting per-core
>>> breakpoint functionality. This allows RaceFuzzer to force the kernel
>>> to deterministically trigger racy condition (which may rarely happen
>>> in practice due to randomness in scheduling).
>>>
>>> RaceFuzzer's C repro always pinpoints two racy syscalls. Since C
>>> repro's scheduling synchronization should be performed at the user
>>> space, its reproducibility is limited (reproduction may take from 1
>>> second to 10 minutes (or even more), depending on a bug). This is
>>> because, while RaceFuzzer precisely interleaves the scheduling at the
>>> kernel's instruction level when finding this bug, C repro cannot fully
>>> utilize such a feature. Please disregard all code related to
>>> "should_hypercall" in the C repro, as this is only for our debugging
>>> purposes using our own hypervisor.