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

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Date:   Tue, 28 Feb 2023 10:08:45 +0100
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Lukas Wunner <lukas@...ner.de>, Bjorn Helgaas <helgaas@...nel.org>
Cc:     Gerd Bayer <gbayer@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Pierre Morel <pmorel@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH RESEND] PCI: s390: Fix use-after-free of PCI bus
 resources with s390 per-function hotplug

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