linux-kernel - Re: [PATCH] PCI/PM: Prevent runtime suspend before devices are fully initialized

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Message-ID: <aPJ4pZFENCTx9yhy@google.com>
Date: Fri, 17 Oct 2025 10:11:01 -0700
From: Brian Norris <briannorris@...omium.org>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
	linux-pci@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] PCI/PM: Prevent runtime suspend before devices are fully
 initialized

Hi Rafael,

On Fri, Oct 17, 2025 at 11:45:14AM +0200, Rafael J. Wysocki wrote:
> On Fri, Oct 17, 2025 at 1:28 AM Brian Norris <briannorris@...omium.org> wrote:
> >
> > PCI devices are created via pci_scan_slot() and similar, and are
> > promptly configured for runtime PM (pci_pm_init()). They are initially
> > prevented from suspending by way of pm_runtime_forbid(); however, it's
> > expected that user space may override this via sysfs [1].
> >
> > Now, sometime after initial scan, a PCI device receives its BAR
> > configuration (pci_assign_unassigned_bus_resources(), etc.).
> >
> > If a PCI device is allowed to suspend between pci_scan_slot() and
> > pci_assign_unassigned_bus_resources(), then pci-driver.c will
> > save/restore incorrect BAR configuration for the device, and the device
> > may cease to function.
> >
> > This behavior races with user space, since user space may enable runtime
> > PM [1] as soon as it sees the device, which may be before BAR
> > configuration.
> >
> > Prevent suspending in this intermediate state by holding a runtime PM
> > reference until the device is fully initialized and ready for probe().
> >
> > [1] echo auto > /sys/bus/pci/devices/.../power/control
> >
> > Cc: <stable@...r.kernel.org>
> > Signed-off-by: Brian Norris <briannorris@...omium.org>
> > ---
> >
> >  drivers/pci/bus.c | 7 +++++++
> >  drivers/pci/pci.c | 6 ++++++
> >  2 files changed, 13 insertions(+)
> >
> > diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> > index f26aec6ff588..227a8898acac 100644
> > --- a/drivers/pci/bus.c
> > +++ b/drivers/pci/bus.c
> > @@ -14,6 +14,7 @@
> >  #include <linux/of.h>
> >  #include <linux/of_platform.h>
> >  #include <linux/platform_device.h>
> > +#include <linux/pm_runtime.h>
> >  #include <linux/proc_fs.h>
> >  #include <linux/slab.h>
> >
> > @@ -375,6 +376,12 @@ void pci_bus_add_device(struct pci_dev *dev)
> >                 put_device(&pdev->dev);
> >         }
> >
> > +       /*
> > +        * Now that resources are assigned, drop the reference we grabbed in
> > +        * pci_pm_init().
> > +        */
> > +       pm_runtime_put_noidle(&dev->dev);
> > +
> >         if (!dn || of_device_is_available(dn))
> >                 pci_dev_allow_binding(dev);
> >
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index b14dd064006c..06a901214f2c 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -3226,6 +3226,12 @@ void pci_pm_init(struct pci_dev *dev)
> >         pci_pm_power_up_and_verify_state(dev);
> >         pm_runtime_forbid(&dev->dev);
> >         pm_runtime_set_active(&dev->dev);
> > +       /*
> > +        * We cannot allow a device to suspend before its resources are
> > +        * configured. Otherwise, we may allow saving/restoring unexpected BAR
> > +        * configuration.
> > +        */
> > +       pm_runtime_get_noresume(&dev->dev);
> >         pm_runtime_enable(&dev->dev);
> 
> So runtime PM should not be enabled here, should it?

Hmm, I suppose not. Does that imply it would be a better solution to
simply defer pm_runtime_enable() to pci_bus_add_device() or some similar
point? I'll give that a shot, since that seems like a simpler and
cleaner solution.

Thanks,
Brian

> >  }
> >
> > --