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

From: Bjorn Helgaas
Date: Mon Jan 06 2020 - 08:55:01 EST

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

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.


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