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

[Cédric Le Goater]

On 07/04/2018 04:26 AM, Peter Xu wrote:
> On Tue, Jul 03, 2018 at 02:45:24PM +0200, Cédric Le Goater wrote:
>> 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 <address@hidden>
>>>> ---
>>>>
>>>>  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.
> 
> Indeed.
> 
> Then now I'm a bit confused on who actually deleted the ROM memory
> region that was created when pci_add_option_rom() was called.  It
> seems to be leaked.
> 
> AFAIU the rest of the memory regions of the BARs (0-5) are managed by
> specific device emulation code, however this ROM memory region is
> managed by PCI subsystem.  Not sure whether that means we should
> destroy the region in PCI subsystem too, e.g. in pci_del_option_rom().
> 
> And now I see this patch might be a valid fix for the VFIO-specific
> issue (though we might comment that a bit somewhere).

yes. I will send a v2 with an updated commit log.

Thanks,

C.