3.9 / 3.10: Reliable host crashes

[Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>]

Hi all,

I found a way to reliably crash my host system:

1) Boot guest VM with init=/bin/bash

2) In guest VM: echo b >/proc/sysrq-trigger

3) Try to reboot the guest -> crashes the host during kernel initialization


When I checked the logs I first thought it would be a KSM issue:

> Jul 10 15:33:37 fsdevel7 kernel: [  507.995602] br0: port 3(tap2) entered disabled state
> Jul 10 15:33:37 fsdevel7 kernel: [  508.043454] BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
> Jul 10 15:33:37 fsdevel7 kernel: [  508.043487] IP: [<ffffffff811825b9>] get_ksm_page+0x39/0x130

and


> Jul 10 15:34:49 fsdevel7 kernel: [  580.274709] RIP: 0010:[<ffffffff8116d694>]  [<ffffffff8116d694>] anon_vma_clone+0x94/0x1a0
> Jul 10 15:34:49 fsdevel7 kernel: [  580.274710] RSP: 0018:ffff880406495cf0  EFLAGS: 00010286

> Jul 10 15:34:49 fsdevel7 kernel: [  580.274721] Call Trace:
> Jul 10 15:34:49 fsdevel7 kernel: [  580.274724]  [<ffffffff8116d868>] anon_vma_fork+0x38/0x130
> Jul 10 15:34:49 fsdevel7 kernel: [  580.274728]  [<ffffffff8105770f>] dup_mmap+0x1bf/0x420


But then I simply disabled KSM in rc.local
(echo 0 > /sys/kernel/mm/ksm/run) before starting the VMs. Now the 
system still crashes, but now without any logs message.
At least sysrq-b still works, but I don' see any
"SysRq : Emergency Sync" messages in log files. So log messages are not 
written properly anymore. Unfortunately I don't have a serial console on 
my desktop system.

I don't have the time now, but the next step is probably to get a 
crash-dump and to see if that has something useful. And I'm also going 
to run the VM from a tty without X, maybe it prints something to the 
console.
Anything else you might want? Or already any idea?
Crashes do not happen with 3.8-ubuntu, but very reliably with 3.9.9 or 
3.10.1 (host).

Here's my kvm command line:

> qemu-system-x86_64                                                      \
>         -m 8192                                                         \
>         -machine type=pc,accel=kvm,kernel_irqchip=on                    \
>         -netdev type=tap,script=${kvm_ifup},downscript=${kvm_ifdown},ifname=$iface,vhost=on,id=guest0   \
>         -device virtio-net-pci,netdev=guest0,mac=52:54:00:12:34:11      \
>         -boot c                                                         \
>         -drive file=${FILE},if=${DISKIF},cache=unsafe                   \
>         -drive file=${META},if=${DISKIF},cache=writeback                \
>         -drive file=${STORAGE},if=${DISKIF},cache=writeback             \
>         -enable-kvm                                                     \
>         -vga vmware                                                     \
>         -cpu host                                                       \
>         -smp 4                                                          \
>         "$@"



Thanks,
Bernd