linux-kernel - Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus

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Message-ID: <20220223134627.GO10061@nvidia.com>
Date:   Wed, 23 Feb 2022 09:46:27 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, kvm@...r.kernel.org,
        rafael@...nel.org, David Airlie <airlied@...ux.ie>,
        linux-pci@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Will Deacon <will@...nel.org>,
        Stuart Yoder <stuyoder@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, Li Yang <leoyang.li@....com>,
        iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in
 bus_type

On Wed, Feb 23, 2022 at 01:04:00PM +0000, Robin Murphy wrote:

> 1 - tmp->driver is non-NULL because tmp is already bound.
>   1.a - If tmp->driver->driver_managed_dma == 0, the group must currently be
> DMA-API-owned as a whole. Regardless of what driver dev has unbound from,
> its removal does not release someone else's DMA API (co-)ownership.

This is an uncommon locking pattern, but it does work. It relies on
the mutex being an effective synchronization barrier for an unlocked
store:

				      WRITE_ONCE(dev->driver, NULL)

 mutex_lock(&group->lock)
 READ_ONCE(dev->driver) != NULL and no UAF
 mutex_unlock(&group->lock)

				      mutex_lock(&group->lock)
				      tmp = READ_ONCE(dev1->driver);
				      if (tmp && tmp->blah) [..]
				      mutex_unlock(&group->lock)
 mutex_lock(&group->lock)
 READ_ONCE(dev->driver) == NULL
 mutex_unlock(&group->lock)

				      /* No other CPU can UAF dev->driver */
                                      kfree(driver)

Ie the CPU setting driver cannot pass to the next step without all
other CPUs observing the new value because of the release/acquire built
into the mutex_lock.

It is tricky, and can work in this instance, but the pattern's unlocked
design relies on ordering between the WRITE_ONCE and the locks - and
that ordering in dd.c isn't like that today.

Jason