Re: [PATCH RESEND] PCI: s390: Fix use-after-free of PCI bus resources with s390 per-function hotplug

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Fri, 2023-02-24 at 05:19 +0100, Lukas Wunner wrote:
> On Thu, Feb 23, 2023 at 01:53:45PM -0600, Bjorn Helgaas wrote:
> > Hmm.  Good question.  Off the top of my head, I can't explain the
> > difference between pci_rescan_remove_lock and pci_bus_sem, so I'm
> > confused, too.  I added Lukas in case he has a ready explanation.
> 
> pci_bus_sem is a global lock which protects the "devices" list of all
> pci_bus structs.
> 
> We do have a bunch of places left where the "devices" list is accessed
> without holding pci_bus_sem, though I've tried to slowly eliminate
> them.
> 
> pci_rescan_remove_lock is a global "big kernel lock" which serializes
> any device addition and removal.
> 
> pci_rescan_remove_lock is known to be far too course-grained and thus
> deadlock-prone, particularly if hotplug ports are nested (as is the
> case with Thunderbolt).  It needs to be split up into several smaller
> locks which protect e.g. allocation of resources of a bus (bus numbers
> or MMIO / IO space) and whatever else needs to be protected.  It's just
> that nobody has gotten around to identify what exactly needs to be
> protected, adding the new locks and removing pci_rescan_remove_lock.
> 
> Thanks,
> 
> Lukas

Thanks for the insights. So from that description I think it might make
sense to do this fix patch with the pci_rescan_remove_lock so it can be
backported. Then we can take the opportunity to add a lock specific to
the allocation/freeing of resources which would then replace at least
this new directly and clearly resource related use of
pci_rescan_remove_lock and potentially others we find.
What do you think?

Thanks,
Niklas