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

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

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.

TL;DR, I'll add the above commit with a Fixes: tag for stable and
downstream releases, looks good otherwise.  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;
>  }
>