Re: [Qemu-devel] [PATCH] vfio/pci: do not set the PCIDevice 'has_rom' attribute

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 9 Jul 2018 09:04:47 +0200
Cédric Le Goater <clg@kaod.org> wrote:

> On 07/06/2018 07:16 PM, Alex Williamson wrote:
> > On Fri,  6 Jul 2018 18:36:14 +0200
> > Cédric Le Goater <clg@kaod.org> wrote:
> >   
> >> PCI devices needing a ROM allocate an optional MemoryRegion with
> >> pci_add_option_rom(). pci_del_option_rom() does the cleanup when the
> >> device is destroyed. The only action taken by this routine is to call
> >> vmstate_unregister_ram() which clears the id string of the optional
> >> ROM RAMBlock and now, also flags the RAMBlock as non-migratable. This
> >> was recently added by commit b895de502717 ("migration: discard
> >> non-migratable RAMBlocks"), .
> >>
> >> VFIO devices do their own loading of the PCI option ROM in
> >> vfio_pci_size_rom(). The memory region is switched to an I/O region
> >> and the PCI attribute 'has_rom' is set but the RAMBlock of the ROM
> >> region is not allocated. When the associated PCI device is deleted,
> >> pci_del_option_rom() calls vmstate_unregister_ram() which tries to
> >> flag a NULL RAMBlock, leading to a SEGV.
> >>
> >> It seems that 'has_rom' was set to have memory_region_destroy()
> >> called, but since commit 469b046ead06 ("memory: remove
> >> memory_region_destroy") this is not necessary anymore as the
> >> MemoryRegion is freed automagically.
> >>
> >> Remove the PCIDevice 'has_rom' attribute setting in vfio.
> >>
> >> Signed-off-by: Cédric Le Goater <clg@kaod.org>  
> > 
> > I think the segfault can be attributed to:
> > 
> > fa53a0e53efd ("memory: drop find_ram_block()")
> > 
> > Prior to that vmstate_unregister_ram() called
> > memory_region_get_ram_addr() which would have resulted in
> > RAM_ADDR_INVALID.  This would have been passed to
> > qemu_ram_unset_idstr() which would have used find_ram_block() to lookup
> > the RAMBlock, which would be NULL for the invalid address, safely
> > avoiding any sort of segfault.  
> 
> Yes, but since, commit b895de502717 ("migration: discard non-migratable 
> RAMBlocks") added : 
> 
>  void vmstate_unregister_ram(MemoryRegion *mr, DeviceState *dev)
>  {
>      qemu_ram_unset_idstr(mr->ram_block);
> +    qemu_ram_unset_migratable(mr->ram_block);
>  }
> 
> and qemu_ram_unset_migratable() does not check the block validity.

Ok, yes I see that qemu_ram_unset_idstr() does avoid the NULL pointer
dereference, so I'll make the fixes tag reference to b895de502717.
Thanks,

Alex

> >> ---
> >>
> >>  Tested on a KVM POWER9 pseries machine and a Mellanox MT27710
> >>  Ethernet controller. Performed a couple of plug/unplug, migrated, and
> >>  did a couple more unplug/plug before powering off.
> >>
> >>  The same tests were done with the previous patches which were
> >>  addressing the issue at a different level : 
> >>
> >>    1. [PATCH] exec.c: check RAMBlock validity before changing its flag
> >>       https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg00009.html
> >>
> >>    2. [PATCH] pci: remove pci_del_option_rom()
> >>       https://lists.gnu.org/archive/html/qemu-devel/2018-07/msg01651.html
> >>
> >>  Do we still want to remove pci_del_option_rom() ?
> >>
> >>  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>)
> >>  
> >>  hw/vfio/pci.c |    1 -
> >>  1 file changed, 1 deletion(-)
> >>
> >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >> index a1577dea7fdb..6cbb8fa0549d 100644
> >> --- a/hw/vfio/pci.c
> >> +++ b/hw/vfio/pci.c
> >> @@ -990,7 +990,6 @@ static void vfio_pci_size_rom(VFIOPCIDevice *vdev)
> >>      pci_register_bar(&vdev->pdev, PCI_ROM_SLOT,
> >>                       PCI_BASE_ADDRESS_SPACE_MEMORY, &vdev->pdev.rom);
> >>  
> >> -    vdev->pdev.has_rom = true;
> >>      vdev->rom_read_failed = false;
> >>  }
> >>    
> >   
>