Re: [Qemu-devel] [KVM] segmentation fault happened when reboot VM after hot-uplug virtio NIC

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-09-03 15:22, Zhanghaoyu (A) wrote:
>>> Hi, all
>>>
>>> Segmentation fault happened when reboot VM after hot-unplug virtio NIC, which can be reproduced 100%.
>>> See similar bug report to 
>>> https://bugzilla.redhat.com/show_bug.cgi?id=988256
>>>
>>> test environment:
>>> host: SLES11SP2 (kenrel version: 3.0.58)
>>> qemu: 1.5.1, upstream-qemu (commit 
>>> 545825d4cda03ea292b7788b3401b99860efe8bc)
>>> libvirt: 1.1.0
>>> guest os: win2k8 R2 x64bit or sles11sp2 x64 or win2k3 32bit
>>>
>>> You can reproduce this problem by following steps:
>>> 1. start a VM with virtio NIC(s)
>>> 2. hot-unplug a virtio NIC from the VM 3. reboot the VM, then 
>>> segmentation fault happened during starting period
>>>
>>> the qemu backtrace shown as below:
>>> #0  0x00007ff4be3288d0 in __memcmp_sse4_1 () from /lib64/libc.so.6
>>> #1  0x00007ff4c07f82c0 in patch_hypercalls (s=0x7ff4c15dd610) at 
>>> /mnt/zhanghaoyu/qemu/qemu-1.5.1/hw/i386/kvmvapic.c:549
>>> #2  0x00007ff4c07f84f0 in vapic_prepare (s=0x7ff4c15dd610) at 
>>> /mnt/zhanghaoyu/qemu/qemu-1.5.1/hw/i386/kvmvapic.c:614
>>> #3  0x00007ff4c07f85e7 in vapic_write (opaque=0x7ff4c15dd610, addr=0, data=32, size=2)
>>>     at /mnt/zhanghaoyu/qemu/qemu-1.5.1/hw/i386/kvmvapic.c:651
>>> #4  0x00007ff4c082a917 in memory_region_write_accessor (opaque=0x7ff4c15df938, addr=0, value=0x7ff4bbfe3d00, size=2, 
>>>     shift=0, mask=65535) at 
>>> /mnt/zhanghaoyu/qemu/qemu-1.5.1/memory.c:334
>>> #5  0x00007ff4c082a9ee in access_with_adjusted_size (addr=0, value=0x7ff4bbfe3d00, size=2, access_size_min=1, 
>>>     access_size_max=4, access=0x7ff4c082a89a <memory_region_write_accessor>, opaque=0x7ff4c15df938)
>>>     at /mnt/zhanghaoyu/qemu/qemu-1.5.1/memory.c:364
>>> #6  0x00007ff4c082ae49 in memory_region_iorange_write (iorange=0x7ff4c15dfca0, offset=0, width=2, data=32)
>>>     at /mnt/zhanghaoyu/qemu/qemu-1.5.1/memory.c:439
>>> #7  0x00007ff4c08236f7 in ioport_writew_thunk (opaque=0x7ff4c15dfca0, addr=126, data=32)
>>>     at /mnt/zhanghaoyu/qemu/qemu-1.5.1/ioport.c:219
>>> #8  0x00007ff4c0823078 in ioport_write (index=1, address=126, data=32) 
>>> at /mnt/zhanghaoyu/qemu/qemu-1.5.1/ioport.c:83
>>> #9  0x00007ff4c0823ca9 in cpu_outw (addr=126, val=32) at 
>>> /mnt/zhanghaoyu/qemu/qemu-1.5.1/ioport.c:296
>>> #10 0x00007ff4c0827485 in kvm_handle_io (port=126, data=0x7ff4c0510000, direction=1, size=2, count=1)
>>>     at /mnt/zhanghaoyu/qemu/qemu-1.5.1/kvm-all.c:1485
>>> #11 0x00007ff4c0827e14 in kvm_cpu_exec (env=0x7ff4c15bf270) at 
>>> /mnt/zhanghaoyu/qemu/qemu-1.5.1/kvm-all.c:1634
>>> #12 0x00007ff4c07b6f27 in qemu_kvm_cpu_thread_fn (arg=0x7ff4c15bf270) 
>>> at /mnt/zhanghaoyu/qemu/qemu-1.5.1/cpus.c:759
>>> #13 0x00007ff4be58af05 in start_thread () from /lib64/libpthread.so.0
>>> #14 0x00007ff4be2cd53d in clone () from /lib64/libc.so.6
>>>
>>> If I apply below patch to the upstream qemu, this problem will 
>>> disappear,
>>> ---
>>>  hw/i386/kvmvapic.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/hw/i386/kvmvapic.c b/hw/i386/kvmvapic.c index 
>>> 15beb80..6fff299 100644
>>> --- a/hw/i386/kvmvapic.c
>>> +++ b/hw/i386/kvmvapic.c
>>> @@ -652,11 +652,11 @@ static void vapic_write(void *opaque, hwaddr addr, uint64_t data,
>>>      switch (size) {
>>>      case 2:
>>>          if (s->state == VAPIC_INACTIVE) {
>>> -            rom_paddr = (env->segs[R_CS].base + env->eip) & ROM_BLOCK_MASK;
>>> -            s->rom_state_paddr = rom_paddr + data;
>>> -
>>>              s->state = VAPIC_STANDBY;
>>>          }
>>> +        rom_paddr = (env->segs[R_CS].base + env->eip) & ROM_BLOCK_MASK;
>>> +        s->rom_state_paddr = rom_paddr + data;
>>> +
>>>          if (vapic_prepare(s) < 0) {
>>>              s->state = VAPIC_INACTIVE;
>>>              break;
>>
>> Yes, we need to update the ROM's physical address after the BIOS reshuffled the layout.
>>
>> But I'm not happy with simply updating the address unconditionally. We need to understand the crash first, then make QEMU robust against the guest not issuing this initial write after a ROM region layout change.
>> And finally make it work properly in the normal case.
>>
> The direct cause of crash is trying to access invalid address, which is due to not updating the rom's physical address.
> In my opinion, since hot-plug/unplug involved in, we need to re-calculate rom's physical address for all devices which have rom during starting period when reboot/reset vm,
> is it reasonable to set vapic's state to VAPIC_INACTIVE during vapic's reset?

I checked meanwhile, and it should be sufficient to make the VAPIC state
working again under sane conditions.

Still, this is not yet satisfying /wrt what *precisely* went wrong when
the physical address became incorrect, and if a malicious guest could
exploit this (e.g. by invoking the 16-bit write from an address outside
the VAPIC ROM).

Jan

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