Re: [PATCH] pci: Warn if BME cannot be turned off during kexec

[Bjorn Helgaas]

Hi Deepa,

Thanks for the patches.  Since these two patches touch the same piece
of code in pci_device_shutdown(), they conflict with each other.  I
could resolve this myself, but maybe you could make them a series that
applies cleanly together?

Can you also please edit the subject lines so they match the
convention (use "git log --oneline drivers/pci/pci-driver.c" to see
it).

On Sat, Jan 04, 2020 at 02:50:52PM -0800, Deepa Dinamani wrote:
> BME not being off is a security risk, so for whatever
> reason if we cannot disable it, print a warning.

"BME" is not a common term in drivers/pci; can you use "Bus Master
Enable" (to match the PCIe spec) or "PCI_COMMAND_MASTER" (to match the
Linux code)?

Can you also explain why this is a security risk?  It looks like we
disable bus mastering if the device is in D0-D3hot.  If the device is
in D3cold, it's powered off, so we can't read/write config space.  But
if it's in D3cold, the device is powered off, so it can't be a bus
master either, so why would we warn about it?

> Signed-off-by: Deepa Dinamani <deepa.kernel@xxxxxxxxx>
> ---
>  drivers/pci/pci-driver.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index 0454ca0e4e3f..6c866a81f46c 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -491,8 +491,12 @@ static void pci_device_shutdown(struct device *dev)
>  	 * If it is not a kexec reboot, firmware will hit the PCI
>  	 * devices with big hammer and stop their DMA any way.
>  	 */
> -	if (kexec_in_progress && (pci_dev->current_state <= PCI_D3hot))
> -		pci_clear_master(pci_dev);
> +	if (kexec_in_progress) {
> +		if (likely(pci_dev->current_state <= PCI_D3hot))

No need to use "likely" here unless you can measure a difference.  I
doubt this is a performance path.

> +			pci_clear_master(pci_dev);
> +		else
> +			dev_warn(dev, "Unable to turn off BME during kexec");

How often and for what sort of devices would you expect this warning
to be emitted?  If this is a common situation and the user can't do
anything about it, the warnings will clutter the logs and train users
to ignore them.

Bjorn

> +	}
>  }
>  
>  #ifdef CONFIG_PM
> -- 
> 2.17.1
>