RE: [PATCH] PCI: hv: fix reading of PCI_INTERRUPT_LINE and PCI_INTERRUPT_PIN

[Michael Kelley]

From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Thursday, June 20, 2024 6:48 PM
> 
> The intent of the code snippet is to always return 0 for both fields.
> The check is wrong though. Fix that.
> 
> This is discovered by this call in VFIO:
> 
>     pci_read_config_byte(vdev->pdev, PCI_INTERRUPT_PIN, &pin);
> 
> The old code does not set *val to 0 because the second half of the check is
> incorrect.
> 
> Fixes: 4daace0d8ce85 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V
> VMs")
> Cc: stable@xxxxxxxxxx
> Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> ---
>  drivers/pci/controller/pci-hyperv.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 5992280e8110..eec087c8f670 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -1130,8 +1130,8 @@ static void _hv_pcifront_read_config(struct hv_pci_dev
> *hpdev, int where,
>  		   PCI_CAPABILITY_LIST) {
>  		/* ROM BARs are unimplemented */
>  		*val = 0;
> -	} else if (where >= PCI_INTERRUPT_LINE && where + size <=
> -		   PCI_INTERRUPT_PIN) {
> +	} else if ((where == PCI_INTERRUPT_LINE || where == PCI_INTERRUPT_PIN) &&
> +		   size == 1) {

Any reason not to continue the pattern of the rest of the function,
and do the following to fix the bug?

   	} else if (where >= PCI_INTERRUPT_LINE && where + size <= 
  		   PCI_MIN_GNT) {

Your fix doesn't allow PCI_INTERRUPT_LINE and PCI_INTERRUPT_PIN
to be read together as a 2-byte access, though I don't know if that
matters.

I have a slight preference for the more consistent approach, but
don't really object to what you've done.  Treat my idea as a
suggestion to consider, but if you want to go with your approach,
that's OK too.

Michael

>  		/*
>  		 * Interrupt Line and Interrupt PIN are hard-wired to zero
>  		 * because this front-end only supports message-signaled
> --
> 2.43.0
>