pci/aspm: Fix null pointer dereference during re-enumeration of PCI bus in pcie_aspm_init_link_state even though ASPM is off

Hi Jan,

On Wed, Jan 04, 2017 at 12:41:36PM +0100, Jan Rüth wrote:
> On 03/01/2017 21:53, Bjorn Helgaas wrote:
> > On Tue, Jan 03, 2017 at 09:04:36AM +0100, Jan Rüth wrote:
> >> This patch fixes a null pointer dereference during PCI bus enumeration
> >> when ASPM is off. On an IBM x3850 8664 this bug causes the kernel to
> >> halt, this behavior did not appear in 3.10, so this is a regression.
> >>
> >> pcie_aspm_sanity_check should only be called if ASPM is on.
> > 
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=187731
> > 
> >> Signed-off-by: Jan Rueth <rueth@comsys.rwth-aachen.de>
> >> ---
> >>  drivers/pci/pcie/aspm.c | +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> >> index f981129..e758b56 100644
> >> --- a/drivers/pci/pcie/aspm.c
> >> +++ b/drivers/pci/pcie/aspm.c
> >> @@ -552,11 +552,12 @@ static struct pcie_link_state
> >> *alloc_pcie_link_state(struct pci_dev *pdev)
> >>  void pcie_aspm_init_link_state(struct pci_dev *pdev)
> >>  {
> >>         struct pcie_link_state *link;
> >> -       int blacklist = !!pcie_aspm_sanity_check(pdev);
> >> -
> >> +       int blacklist;
> >>         if (!aspm_support_enabled)
> >>                 return;
> >>
> >> +       blacklist = !!pcie_aspm_sanity_check(pdev);
> >> +
> >>         if (pdev->link_state)
> >>                 return;
> > 
> > For some reason this doesn't apply for me (complains about a malformed
> > patch), but I can apply it by hand easily enough.
> > 
> > However, this path is a little obtuse, and I'd like to understand
> > what's going on better.  We currently have:
> > 
> >   pci_scan_slot
> >     if (bus->self && nr)
> >       pcie_aspm_init_link_state(bus->self)
> >         pcie_aspm_sanity_check
> > 	  list_for_each_entry(child, &pdev->subordinate->devices, ...)
> > 
> > I assume the null pointer is pdev->subordinate, so maybe we're calling
> > pcie_aspm_init_link_state() for a bridge where we weren't able to
> > create a child bus ("pdev->subordinate")?
> 
> At the time when I debugged it could trace it back to the loop but I
> can't remember which part actually broke. But I can add some debug
> output and crash it again.

I'm pretty sure the null pointer would be pdev->subordinate, and your
patch certainly avoids dereferencing that as long as you boot with
"pcie_aspm=off".  But I think if you boot with your patch but without
"pcie_aspm=off", we'll probably dereference that null pointer again.

The only way I can see to have a null pdev->subordinate pointer is via
SR-IOV VF devices, and you should have some of those.  But I think we
should see the messages from pci_setup_device() for them, and I don't
see them in the dmesg log in the bugzilla, so I'm still confused about
this.

Why are you using "pcie_aspm=off"?  If it's due to Linux ASPM bugs, I
want to fix those, too.  If it's due to hardware problems on your
platform, maybe we can add some sort of quirk to do this
automatically.

Can you please apply the patches below (you should be able to feed
this whole email to "patch" at once), collect the complete dmesg logs
(both with and without "pcie_aspm=off"), and attach them to the
bugzilla?

Bjorn


commit 50deebe0077acdfafe08501b7da220d256743e36
Author: Jan Rüth <rueth@comsys.rwth-aachen.de>
Date:   Tue Jan 3 14:09:50 2017 -0600

    PCI/ASPM: Avoid ASPM sanity checking when ASPM is off
    
    Previously we called pcie_aspm_sanity_check() even if ASPM was disabled,
    which is unnecessary.  Delay the pcie_aspm_sanity_check() until we know
    that we should do some ASPM configuration.
    
    As a side effect, this avoids a null pointer dereference in
    pcie_aspm_sanity_check(), which we saw on IBM x3850 8664 (see bugzilla
    below).
    
    [bhelgaas: changelog, delay call a little more]
    Link: https://bugzilla.kernel.org/show_bug.cgi?id=187731
    Signed-off-by: Jan Rüth <rueth@comsys.rwth-aachen.de>
    Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
    CC: stable@vger.kernel.org

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