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

[Jan Kiszka]

On 2013-09-03 15:57, Jan Kiszka wrote:
> 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).

OK, this is the unfortunate chain of events:

rom_state_paddr becomes invalid, so vapic_map_rom_writable, called by
vapic_prepare during the 16-bit write, accesses an invalid region to
grab the ROM size from the ROM image. This size becomes 0, and the
g_malloc in patch_hypercalls return NULL as we try to allocate 0 bytes.
So memcmp uses a NULL pointer as base for walking the ROM image. I'll
send a separate patch to catch this case.

Other invalid ROM size values cannot cause harm. At most, 128K is
allocated, and random guest memory is read.

Jan

-- 
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