Message-ID: <5bf4ca0f643bcd59f5761cdd29403433046a9995.camel@linux.ibm.com>
Subject: Re: [PATCH RESEND] PCI: s390: Fix use-after-free of PCI bus
 resources with s390 per-function hotplug
From:   Niklas Schnelle <schnelle@linux.ibm.com>
To:     Lukas Wunner <lukas@wunner.de>, Bjorn Helgaas <helgaas@kernel.org>
Cc:     Gerd Bayer <gbayer@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Pierre Morel <pmorel@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org
Date:   Tue, 28 Feb 2023 10:08:45 +0100
In-Reply-To: <20230224041918.GB26067@wunner.de>
References: <1793b3cd12921b7a3fa8b3ee7e20b7cf1df1eca1.camel@linux.ibm.com>
         <20230223195345.GA3805039@bhelgaas> <20230224041918.GB26067@wunner.de>
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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.
>=20
> pci_bus_sem is a global lock which protects the "devices" list of all
> pci_bus structs.
>=20
> 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.
>=20
> pci_rescan_remove_lock is a global "big kernel lock" which serializes
> any device addition and removal.
>=20
> 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.
>=20
> Thanks,
>=20
> 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