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SubjectWARNING in port_delete
Reporting the crash: WARNING in port_delete

This crash has been found in v4.18-rc3 using RaceFuzzer (a modified
version of Syzkaller), which we descrbie more at the end of this
report. Our analysis shows that the race occurs when invoking two close
syscalls concurrently.


The executed program is as follows:
r0 = open("/dev/snd/seq", 0x0, 0x0)
ioctl(r0, SNDRV_SEQ_IOCTL_CREATE_PORT, {{0x80}, "706f72943000000000000000000000000000000000000000000000000000000000233f770800000000000000000000000000000000000000001000", 0xffffffffffffffff, 0xfffffffffffffbff})
r1 = openat(AT_FDCWD, "/dev/sequencer2", 0x8402, 0x0)
close(r0)
close(r1)

and two threads executed the program as follows:
Thread0 Thread1
open("/dev/snd/seq")
ioctl(r0, SNDRV_SEQ_IOCTL_CREATE_PORT)
openat("/dev/sequencer2")
close(r0) close(r1)

(Note that two close() syscalls were exeuted after openat() syscall, and
two close() syscalls were executed concurrently.)


Crash log:
==================================================================
WARNING: CPU: 1 PID: 32519 at /home/daeryong/workspace/race-fuzzer/kernels_repo/kernel_v4.18-rc3/sound/core/seq/seq_ports.c:275 port_delete+0xde/0xf0 sound/core/seq/seq_ports.c:275
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 32519 Comm: syz-executor0 Not tainted 4.18.0-rc3 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x16e/0x22c lib/dump_stack.c:113
panic+0x1a8/0x3a7 kernel/panic.c:184
__warn+0x191/0x1a0 kernel/panic.c:536
report_bug+0x132/0x1b0 lib/bug.c:186
fixup_bug.part.10+0x28/0x50 arch/x86/kernel/traps.c:178
fixup_bug arch/x86/kernel/traps.c:247 [inline]
do_error_trap+0x284/0x2c0 arch/x86/kernel/traps.c:296
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:port_delete+0xde/0xf0 sound/core/seq/seq_ports.c:275
Code: 00 00 e8 b5 68 d2 fd 8b 83 58 01 00 00 85 c0 75 1d e8 86 5c ae fd 48 89 df e8 3e 52 d2 fd 31 c0 5b 41 5c 5d c3 e8 72 5c ae fd <0f> 0b eb c8 e8 69 5c ae fd 0f 0b eb da 0f 1f 44 00 00 55 48 89 e5
RSP: 0018:ffff8801d635f900 EFLAGS: 00010216
RAX: 0000000000040000 RBX: ffff8801da196900 RCX: ffffffff839b86ee
RDX: 0000000000005e94 RSI: ffffc90002640000 RDI: ffff8801da196978
RBP: ffff8801d635f910 R08: 0000000000000098 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff839ba820
R13: ffff8801da196950 R14: ffff8801ed5bc080 R15: ffff8801da196958
snd_seq_delete_port+0x2b0/0x2d0 sound/core/seq/seq_ports.c:303
snd_seq_ioctl_delete_port+0x5b/0xa0 sound/core/seq/seq_clientmgr.c:1325
snd_seq_kernel_client_ctl+0xd4/0xf0 sound/core/seq/seq_clientmgr.c:2361
snd_seq_event_port_detach+0xcb/0x120 sound/core/seq/seq_ports.c:705
delete_port+0x3a/0x70 sound/core/seq/oss/seq_oss_init.c:354
snd_seq_oss_release+0x7c/0x90 sound/core/seq/oss/seq_oss_init.c:433
odev_release+0x40/0x60 sound/core/seq/oss/seq_oss.c:153
__fput+0x234/0x470 fs/file_table.c:209
____fput+0x15/0x20 fs/file_table.c:243
task_work_run+0x15a/0x1c0 kernel/task_work.c:113
tracehook_notify_resume include/linux/tracehook.h:192 [inline]
exit_to_usermode_loop+0x2a3/0x2b0 arch/x86/entry/common.c:166
prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
do_syscall_64+0x485/0x4b0 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x456469
Code: 1d ba fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb b9 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fe9a8936b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 000000000072bfa0 RCX: 0000000000456469
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000014
RBP: 0000000000000054 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fe9a89376d4
R13: 00000000ffffffff R14: 00000000006f5880 R15: 0000000000000000
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


= 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.

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