Re: [Qemu-devel] [PATCH] exec.c: check RAMBlock validity before changing its flag

["Cédric Le Goater" <clg@kaod.org>]

On 07/02/2018 05:57 AM, Peter Xu wrote:
> On Sun, Jul 01, 2018 at 07:19:53PM +0200, Cédric Le Goater wrote:
>> When a PCI device is unplugged, the PCI memory regions are deleted
>> before the optional ROM RAMBlock is flagged non-migratable. But, when
>> this is done, the RAMBlock has already been cleared from the region,
>> leading to a segv.
>>
>> Fix the issue by testing the RAMBlock before flagging it, as it is
>> done in qemu_ram_unset_idstr()
>>
>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>> ---
>>
>>  I caught this bug while deleting a passthrough device from a pseries
>>  machine. Here is the stack:
>>    
>>    #0  qemu_ram_unset_migratable (rb=0x0) at /home/legoater/work/qemu/qemu-xive-3.0.git/exec.c:1994
>>    #1  0x000000010072def0 in vmstate_unregister_ram (mr=0x101796af0, dev=<optimized out>)
>>    #2  0x0000000100694e5c in pci_del_option_rom (pdev=0x101796330)
>>    #3  pci_qdev_unrealize (dev=<optimized out>, errp=<optimized out>)
>>    #4  0x00000001005ff910 in device_set_realized (obj=0x101796330, value=<optimized out>, errp=0x0)
>>    #5  0x00000001007a487c in property_set_bool (obj=0x101796330, v=<optimized out>, name=<optimized out>, 
>>    #6  0x00000001007a7878 in object_property_set (obj=0x101796330, v=0x7fff70033110, 
>>    #7  0x00000001007aaf1c in object_property_set_qobject (obj=0x101796330, value=<optimized out>, 
>>    #8  0x00000001007a7b90 in object_property_set_bool (obj=0x101796330, value=<optimized out>, 
>>    #9  0x00000001005fcdd8 in device_unparent (obj=0x101796330)
>>    #10 0x00000001007a6dd0 in object_finalize_child_property (obj=<optimized out>, name=<optimized out>, 
>>    #11 0x00000001007a50c0 in object_property_del_child (obj=0x10111f800, child=0x101796330, 
>>    #12 0x0000000100425cc0 in spapr_phb_remove_pci_device_cb (dev=0x101796330)
>>    #13 0x0000000100427974 in spapr_drc_release (drc=0x1017e2df0)
>>    #14 0x0000000100429098 in spapr_drc_detach (drc=0x1017e2df0)
>>    #15 0x00000001004294e0 in drc_isolate_physical (drc=0x1017e2df0)
>>    #16 0x000000010042a50c in rtas_set_isolation_state (state=0, idx=<optimized out>)
>>
>>  May be we should call pci_del_option_rom() before
>>  pci_unregister_io_regions() ?
> 
> This seems to make more sense to me.
> 
> Meanwhile I assume the name pci_del_option_rom() is a bit misleading -
> it's not really deleting the ROM but unregistering the ROM only.
> Instead IIUC it's pci_unregister_io_regions() which deleted that. So
> maybe we can either rename the function pci_del_option_rom(), or we
> can pick the ROM destruction out of pci_unregister_io_regions() and
> put it into pci_del_option_rom() to make sure it's done as the last
> step?

So it is a little more complex than I thought. 

The PCI device is a vfio PCI device and the PCI ROM region is initialized 
in vfio_pci_size_rom() with memory_region_init_io(), which does not 
allocate the RAMBlock, but has_rom is still set to true. 

When the device is deleted, pci_del_option_rom() is called and with it, vmstate_unregister_ram() because has_rom is set to true. Leading to the
SEGV.

I am not sure how to handle this case. It seems that the realize routine 
of VFIOPCIDevice is hijacking a little the PCIDevice layer.


C.