RE: [PATCH] PCI: hv: Fix a crash in hv_pci_restore_msi_msg() during hibernation

[Michael Kelley (LINUX)]

From: Dexuan Cui <decui@xxxxxxxxxxxxx> Sent: Tuesday, August 15, 2023 9:00 PM
> 
> > From: Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx>
> > Sent: Tuesday, August 15, 2023 5:35 PM
> > > [...]
> > > For a Linux VM with a NVIDIA GPU running on Hyper-V, before the GPU driver
> > > is installed, hibernating the VM will trigger a panic: if the GPU driver
> > > is not installed and loaded, MSI-X/MSI is not enabled on the device, so
> > > pdev->dev.msi.data is NULL, and msi_lock_descs(&pdev->dev) causes the
> > > NULL pointer dereference. Fix this by checking pdev->dev.msi.data.
> >
> > Is the scenario here a little broader than just the NVIDIA GPU driver?  For
> > any virtual PCI device that is presented in the guest VM as a VMBus device,
> > the driver might not be installed.  There could have been some initial
> > problem getting the driver installed, or it might have been manually
> > uninstalled later in the life of the VM.  Also the host might have rescinded
> > the virtual PCI device and added it back later, creating another opportunity
> > where the driver might not be loaded.  In any case, it seems like we could
> > have the VMBus aspects of the device setup, but not the driver for the
> > device.  This suspend/resume code in pci-hyperv.c is all about handling
> > the VMBus aspects of the device anyway.
> 
> Good point! The bug also affects other PCI devices, e.g. if I unload mlx5_core
> and let the VM with a Mellanox VF NIC hibernate, I hit the same NULL
> pointer dereference.
> 
> > Assuming my thinking is correct, is there some Hyper-V/VMBus setting
> > owned by the pci-hyperv.c driver that would be better to test here than
> > the low-level dev.msi.data pointer?  The MSI code rework that added
> 
> IMO there is no easy and reliable way in Hyper-V/VMBus/pci-hyperv to
> tell if MSI/MSI-X is enabled for a PCI device. We can potentially track the
> MSI/MSI-X irqs in hv_compose_msi_msg() and hv_irq_unmask(), but
> IMO that's not very easy and may be inaccurate.
> 
> > the descriptor lock encapsulates the internals with appropriate accessor
> > functions, and reaching in to directly test dev.msi.data violates that
> > encapsulation.
> 
> I agree.
> 
> Compared with:
> 	if (!pdev->dev.msi.data)
> 		return 0;
> 
> I think it's better to use this:
>             if (!pdev->msi_enabled && !pdev->msix_enabled)
> 		return 0;
> 
> pdev-> msix_enabled has been used in many drivers, e.g.
> 
> "arch/x86/pci/xen.c": xen_pv_teardown_msi_irqs()
> "drivers/hid/intel-ish-hid/ipc/pci-ish.c": ish_probe()
> "drivers/infiniband/hw/vmw_pvrdma/pvrdma_main.c": pvrdma_intr0_handler()
> "drivers/scsi/vmw_pvscsi.c": pvscsi_probe()
> and more.
> 
> So it looks like pdev-> msix_enabled is a legit and stable API.
> I'll post v2 with it. I'll update the changelog accordingly.
> Please let me know if you have concerns about it.

Sounds good.  I agree that what you propose is a better approach.

Michael