Re: [PATCH v3] PCI: account for sysfs-disabled reset in pci_{slot,bus}_resettable()

[Bjorn Helgaas]

On Fri, Feb 07, 2025 at 02:56:00PM -0600, Nishanth Aravamudan wrote:
> Commit d88f521da3ef ("PCI: Allow userspace to query and set
> device reset mechanism") added support for userspace to disable reset of
> specific PCI devices (by echo'ing "" into reset_method) and
> pci_{slot,bus}_resettable() methods do not check pci_reset_supported()
> to see if userspace has disabled reset.
> 
> __pci_reset_bus()
> 	-> pci_bus_reset(..., PCI_RESET_PROBE)
> 		-> pci_bus_resettable()
> 
> __pci_reset_slot()
> 	-> pci_slot_reset(..., PCI_RESET_PROBE)
> 		-> pci_slot_resettable()
> 
> pci_reset_bus()
> 	-> pci_probe_reset_slot()
> 		-> pci_slot_reset(..., PCI_RESET_PROBE)
> 			-> pci_bus_resettable()
> 	if true:
> 		__pci_reset_slot()
> 	else:
> 		__pci_reset_bus()
> 
> I was able to reproduce this issue with a vfio device passed to a qemu
> guest, where I had disabled PCI reset via sysfs. Both
> vfio_pci_ioctl_get_pci_hot_reset_info() and
> vfio_pci_ioctl_pci_hot_reset() check if either the vdev's slot or bus is
> not resettable by calling pci_probe_reset_slot(). Before my change, this
> ends up ignoring the sysfs file contents and vfio-pci happily ends up
> issuing a reset to that device.
> 
> Add an explicit check of pci_reset_supported() in both
> pci_slot_resettable() and pci_bus_resettable() to ensure both the reset
> status and reset execution are both bypassed if an administrator
> disables it for a device.
> 
> Fixes: d88f521da3ef ("PCI: Allow userspace to query and set device reset mechanism")
> Signed-off-by: Nishanth Aravamudan <naravamudan@xxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Raphael Norwitz <raphael.norwitz@xxxxxxxxxxx>
> Cc: Amey Narkhede <ameynarkhede03@xxxxxxxxx>
> Cc: linux-pci@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Cc: Yishai Hadas <yishaih@xxxxxxxxxx>
> Cc: Shameer Kolothum <shameerali.kolothum.thodi@xxxxxxxxxx>
> Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx

Applied to pci/reset for v6.15, thanks!

> ---
> 
> Changes since v2:
>  - update commit message to include more details
> 
> Changes since v1:
>  - fix capitalization and ()s
>  - clarify same checks are done in reset path
> 
>  drivers/pci/pci.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 869d204a70a3..738d29375ad3 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -5405,6 +5405,8 @@ static bool pci_bus_resettable(struct pci_bus *bus)
>  		return false;
>  
>  	list_for_each_entry(dev, &bus->devices, bus_list) {
> +		if (!pci_reset_supported(dev))
> +			return false;
>  		if (dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
>  		    (dev->subordinate && !pci_bus_resettable(dev->subordinate)))
>  			return false;
> @@ -5481,6 +5483,8 @@ static bool pci_slot_resettable(struct pci_slot *slot)
>  	list_for_each_entry(dev, &slot->bus->devices, bus_list) {
>  		if (!dev->slot || dev->slot != slot)
>  			continue;
> +		if (!pci_reset_supported(dev))
> +			return false;
>  		if (dev->dev_flags & PCI_DEV_FLAGS_NO_BUS_RESET ||
>  		    (dev->subordinate && !pci_bus_resettable(dev->subordinate)))
>  			return false;
> -- 
> 2.34.1
>