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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Mon, 27 Jun 2022 18:37:06 +0200
Message-ID: <CAJZ5v0hfdnRg0EqG2Zcp9=Kjq+P1NC45iudatisVL_G=QjOC+A@mail.gmail.com>
Subject: Re: [PATCH] PCI/ACPI: do not reference a pci device after it has been released
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@kernel.org>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Len Brown <lenb@kernel.org>,
        Linux PCI <linux-pci@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        whitehat002 <hackyzh002@gmail.com>
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On Mon, Jun 27, 2022 at 5:07 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Thu, Apr 28, 2022 at 10:30:38PM +0200, Rafael J. Wysocki wrote:
> > On Thu, Apr 28, 2022 at 10:15 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > >
> > > On Thu, Apr 28, 2022 at 6:22 PM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Thu, Apr 28, 2022 at 10:58:58AM -0500, Bjorn Helgaas wrote:
> > > > > On Thu, Apr 28, 2022 at 04:28:53PM +0200, Greg Kroah-Hartman wrote:
> > > > > > In acpi_get_pci_dev(), the debugging message for when a PCI bridge is
> > > > > > not found uses a pointer to a pci device whose reference has just been
> > > > > > dropped.  The chance that this really is a device that is now been
> > > > > > removed from the system is almost impossible to happen, but to be safe,
> > > > > > let's print out the debugging message based on the acpi root device
> > > > > > which we do have a valid reference to at the moment.
> > > > >
> > > > > This code was added by 497fb54f578e ("ACPI / PCI: Fix NULL pointer
> > > > > dereference in acpi_get_pci_dev() (rev. 2)").  Not sure if it's worth
> > > > > a Fixes: tag.
> > > >
> > > > Can't hurt, I'll add it for the v2 based on this review.
> > > >
> > > > >
> > > > > acpi_get_pci_dev() is used by only five callers, three of which are
> > > > > video/backlight related.  I'm always skeptical of one-off interfaces
> > > > > like this, but I don't know enough to propose any refactoring or other
> > > > > alternatives.
> > > > >
> > > > > I'll leave this for Rafael, but if I were applying I would silently
> > > > > touch up the subject to match convention:
> > > > >
> > > > >   PCI/ACPI: Do not reference PCI device after it has been released
> > > >
> > > > Much simpler, thanks.
> > > >
> > > > >
> > > > > > Cc: Bjorn Helgaas <bhelgaas@google.com>
> > > > > > Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> > > > > > Cc: Len Brown <lenb@kernel.org>
> > > > > > Cc: linux-pci@vger.kernel.org
> > > > > > Cc: linux-acpi@vger.kernel.org
> > > > > > Reported-by: whitehat002 <hackyzh002@gmail.com>
> > > > > > Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > > > > ---
> > > > > >  drivers/acpi/pci_root.c | 3 ++-
> > > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/drivers/acpi/pci_root.c b/drivers/acpi/pci_root.c
> > > > > > index 6f9e75d14808..ecda378dbc09 100644
> > > > > > --- a/drivers/acpi/pci_root.c
> > > > > > +++ b/drivers/acpi/pci_root.c
> > > > > > @@ -303,7 +303,8 @@ struct pci_dev *acpi_get_pci_dev(acpi_handle handle)
> > > > > >              * case pdev->subordinate will be NULL for the parent.
> > > > > >              */
> > > > > >             if (!pbus) {
> > > > > > -                   dev_dbg(&pdev->dev, "Not a PCI-to-PCI bridge\n");
> > > > > > +                   dev_dbg(&root->device->dev,
> > > > > > +                           "dev %d, function %d is not a PCI-to-PCI bridge\n", dev, fn);
> > > > >
> > > > > This should use "%02x.%d" to be consistent with the dev_set_name() in
> > > > > pci_setup_device().
> > > >
> > > > Ah, missed that, will change it and send out a new version tomorrow.
> > >
> > > I would make the change below (modulo the gmail-induced wthite space
> > > breakage), though.
> >
> > That said ->
> >
> > > ---
> > >  drivers/acpi/pci_root.c |    5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > Index: linux-pm/drivers/acpi/pci_root.c
> > > ===================================================================
> > > --- linux-pm.orig/drivers/acpi/pci_root.c
> > > +++ linux-pm/drivers/acpi/pci_root.c
> > > @@ -295,8 +295,6 @@ struct pci_dev *acpi_get_pci_dev(acpi_ha
> > >              break;
> > >
> > >          pbus = pdev->subordinate;
> > > -        pci_dev_put(pdev);
> > > -
> > >          /*
> > >           * This function may be called for a non-PCI device that has a
> > >           * PCI parent (eg. a disk under a PCI SATA controller).  In that
> > > @@ -304,9 +302,12 @@ struct pci_dev *acpi_get_pci_dev(acpi_ha
> > >           */
> > >          if (!pbus) {
> > >              dev_dbg(&pdev->dev, "Not a PCI-to-PCI bridge\n");
> > > +            pci_dev_put(pdev);
> > >              pdev = NULL;
> > >              break;
> > >          }
> > > +
> > > +        pci_dev_put(pdev);
> >
> > -> we are going to use pbus after this and it is pdev->subordinate
> > which cannot survive without pdev AFAICS.
> >
> > Are we not concerned about this case?
>
> Good point.
>
> whitehat002, any ideas?  You found this issue but it really looks like
> it is not anything that can ever be hit, so how far do you want to go to
> unwind it?

I have an idea, sorry for the delay here.

I should be ready to post something tomorrow.

Cheers!