[Syzkaller & bisect] There is BUG: soft lockup after sendmsg syscall in v6.8-rc4

[Pengfei Xu]

Hi Kuniyuki Iwashima and kernel experts,

Greeting!
There is BUG: soft lockup after sendmsg syscall in v6.8-rc4 in guest.

All detailed info: https://github.com/xupengfe/syzkaller_logs/tree/main/240220_161242_softlockup
Syzkaller reproduced code: https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/repro.c
Syzkaller syscall reproduced steps: https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/repro.prog
Kconfig(need make olddefconfig): https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/kconfig_origin
Bisect info: https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/bisect_info.log
v6.8-rc4 issue dmesg: https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/841c35169323cd833294798e58b9bf63fa4fa1de_dmesg.log
bzImage_v6.8-rc4: https://github.com/xupengfe/syzkaller_logs/raw/main/240220_161242_softlockup/bzImage_v6.8-rc4.tar.gz

Bisected and found first bad commit:
"
1279f9d9dec2 af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.
"

After reverted above commit on top of v6.8-rc4 kernel, this issue was gone.

Syzkaller repro.report: https://github.com/xupengfe/syzkaller_logs/blob/main/240220_161242_softlockup/repro.report
"
watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [gdbus:349]
Modules linked in:
irq event stamp: 25162
hardirqs last  enabled at (25161): [<ffffffff855dff2e>] irqentry_exit+0x3e/0xa0 kernel/entry/common.c:351
hardirqs last disabled at (25162): [<ffffffff855dded4>] sysvec_apic_timer_interrupt+0x14/0xc0 arch/x86/kernel/apic/apic.c:1076
softirqs last  enabled at (25140): [<ffffffff8127fcc8>] invoke_softirq kernel/softirq.c:427 [inline]
softirqs last  enabled at (25140): [<ffffffff8127fcc8>] __irq_exit_rcu+0xa8/0x110 kernel/softirq.c:632
softirqs last disabled at (25135): [<ffffffff8127fcc8>] invoke_softirq kernel/softirq.c:427 [inline]
softirqs last disabled at (25135): [<ffffffff8127fcc8>] __irq_exit_rcu+0xa8/0x110 kernel/softirq.c:632
CPU: 0 PID: 349 Comm: gdbus Not tainted 6.8.0-rc4-2024-02-12-intel-next-e92619743fcb+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
RIP: 0010:__sanitizer_cov_trace_pc+0x38/0x70 kernel/kcov.c:207
Code: 65 8b 05 73 89 a0 7e 48 89 e5 48 8b 75 08 a9 00 01 ff 00 74 0f f6 c4 01 74 35 8b 82 e4 1d 00 00 85 c0 74 2b 8b 82 c0 1d 00 00 <83> f8 02 75 20 48 8b 8a c8 1d 00 00 8b 92 c4 1d 00 00 48 8b 01 48
RSP: 0018:ffff88800b48f7b0 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff888013018000 RCX: 1ffffffff12150bb
RDX: ffff888011ab8000 RSI: ffffffff8515e235 RDI: ffff888013018770
RBP: ffff88800b48f7b0 R08: 0000000000000001 R09: fffffbfff120f86e
R10: ffffffff8907c377 R11: 0000000000000001 R12: ffff888013018000
R13: dffffc0000000000 R14: ffff888013018630 R15: ffff88800b48f840
FS:  0000000000000000(0000) GS:ffff88806ca00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fffcc5fd648 CR3: 0000000006c82000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 <IRQ>
 </IRQ>
 <TASK>
 unix_gc+0x465/0xfd0 net/unix/garbage.c:319
 unix_release_sock+0xb8c/0xe80 net/unix/af_unix.c:683
 unix_release+0x9c/0x100 net/unix/af_unix.c:1064
 __sock_release+0xb6/0x280 net/socket.c:659
 sock_close+0x27/0x40 net/socket.c:1421
 __fput+0x284/0xb70 fs/file_table.c:376
 ____fput+0x1f/0x30 fs/file_table.c:404
 task_work_run+0x19d/0x2b0 kernel/task_work.c:180
 exit_task_work include/linux/task_work.h:38 [inline]
 do_exit+0xb51/0x28c0 kernel/exit.c:871
 do_group_exit+0xe5/0x2c0 kernel/exit.c:1020
 get_signal+0x2715/0x27d0 kernel/signal.c:2893
 arch_do_signal_or_restart+0x8e/0x7e0 arch/x86/kernel/signal.c:311
 exit_to_user_mode_loop kernel/entry/common.c:105 [inline]
 exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
 __syscall_exit_to_user_mode_work kernel/entry/common.c:201 [inline]
 syscall_exit_to_user_mode+0x129/0x190 kernel/entry/common.c:212
 do_syscall_64+0x82/0x150 arch/x86/entry/common.c:89
 entry_SYSCALL_64_after_hwframe+0x6e/0x76
RIP: 0033:0x7fe8f8b4296f
Code: Unable to access opcode bytes at 0x7fe8f8b42945.
RSP: 002b:00007fe8d7dff9c0 EFLAGS: 00000293 ORIG_RAX: 0000000000000007
RAX: fffffffffffffdfc RBX: 00007fe8f8f73071 RCX: 00007fe8f8b4296f
RDX: 00000000ffffffff RSI: 0000000000000002 RDI: 00007fe8a801f3f0
RBP: 00007fe8a801f3f0 R08: 0000000000000000 R09: 00007fe8d7dff850
R10: 00007ffec9196080 R11: 0000000000000293 R12: 0000000000000002
R13: 0000000000000002 R14: 00007fe8d7dffa30 R15: 00007fe8a801c4c0
 </TASK>
"

Thanks!

---

If you don't need the following environment to reproduce the problem or if you
already have one reproduced environment, please ignore the following information.

How to reproduce:
git clone https://gitlab.com/xupengfe/repro_vm_env.git
cd repro_vm_env
tar -xvf repro_vm_env.tar.gz
cd repro_vm_env; ./start3.sh  // it needs qemu-system-x86_64 and I used v7.1.0
  // start3.sh will load bzImage_2241ab53cbb5cdb08a6b2d4688feb13971058f65 v6.2-rc5 kernel
  // You could change the bzImage_xxx as you want
  // Maybe you need to remove line "-drive if=pflash,format=raw,readonly=on,file=./OVMF_CODE.fd \" for different qemu version
You could use below command to log in, there is no password for root.
ssh -p 10023 root@localhost

After login vm(virtual machine) successfully, you could transfer reproduced
binary to the vm by below way, and reproduce the problem in vm:
gcc -pthread -o repro repro.c
scp -P 10023 repro root@localhost:/root/

Get the bzImage for target kernel:
Please use target kconfig and copy it to kernel_src/.config
make olddefconfig
make -jx bzImage           //x should equal or less than cpu num your pc has

Fill the bzImage file into above start3.sh to load the target kernel in vm.


Tips:
If you already have qemu-system-x86_64, please ignore below info.
If you want to install qemu v7.1.0 version:
git clone https://github.com/qemu/qemu.git
cd qemu
git checkout -f v7.1.0
mkdir build
cd build
yum install -y ninja-build.x86_64
yum -y install libslirp-devel.x86_64
./configure --target-list=x86_64-softmmu --enable-kvm --enable-vnc --enable-gtk --enable-sdl --enable-usb-redir --enable-slirp
make
make install

Best Regards,
Thanks!