Re: [Qemu-devel] [Fix PATCH] Qemu/Xen: Fix early freeing MSIX MMIO memory region

[Stefano Stabellini]

On Tue, 6 Oct 2015, Paolo Bonzini wrote:
> On 05/10/2015 18:53, Stefano Stabellini wrote:
> >> This patch is to fix the issue via moving MSIX MMIO memory region into
> >> struct XenPCIPassthroughState and free it together with pt device's obj.
> > 
> > Given that all the MSI-X related info are in XenPTMSIX, I would prefer
> > to keep the mmio memory region there, if possible.
> > 
> > Couldn't you just unhook msix->mmio from XenPCIPassthroughState's object
> > in xen_pt_msix_delete?  Calling object_property_del_child or
> > object_unparent?
> 
> This is the right thing to do, but there are two separate things to fix.
> 
> One is the use-after-free of msix->mmio, the other is that freeing 
> s->msix and in general xen_pt_config_delete should be done from the 
> .instance_finalize callback.  This is documented in docs/memory.txt.
> 
> This is an attempt at a patch (not even compiled):
> 
> diff --git a/hw/xen/xen_pt_msi.c b/hw/xen/xen_pt_msi.c
> index e3d7194..e476bac 100644
> --- a/hw/xen/xen_pt_msi.c
> +++ b/hw/xen/xen_pt_msi.c
> @@ -610,7 +610,7 @@ error_out:
>      return rc;
>  }
>  
> -void xen_pt_msix_delete(XenPCIPassthroughState *s)
> +void xen_pt_msix_unmap(XenPCIPassthroughState *s)
>  {
>      XenPTMSIX *msix = s->msix;
>  
> @@ -627,6 +627,17 @@ void xen_pt_msix_delete(XenPCIPassthroughState *s)
>      }
>  
>      memory_region_del_subregion(&s->bar[msix->bar_index], &msix->mmio);
> +}
> +
> +void xen_pt_msix_delete(XenPCIPassthroughState *s)
> +{
> +    XenPTMSIX *msix = s->msix;
> +
> +    if (!msix) {
> +        return;
> +    }
> +
> +    object_unparent(&msix->mmio);
>  
>      g_free(s->msix);
>      s->msix = NULL;
> 
> where xen_pt_config_unmap would be called from xen_pt_destroy, and the call
> to xen_pt_config_delete would be moved to xen_pci_passthrough_info's
> instance_finalize member.

Thanks for the explanation and the code. It makes sense to me.

Lan, could you please write up a patch based on this approach and test
it?