Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus_type

From: Jason Gunthorpe <jgg@nvidia.com>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>,
	Christoph Hellwig <hch@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Joerg Roedel <joro@8bytes.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	kvm@vger.kernel.org, rafael@kernel.org,
	David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Stuart Yoder <stuyoder@gmail.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH v6 02/11] driver core: Add dma_cleanup callback in bus_type
Date: Wed, 23 Feb 2022 09:46:27 -0400	[thread overview]
Message-ID: <20220223134627.GO10061@nvidia.com> (raw)
In-Reply-To: <880a269d-d39d-bab3-8d19-b493e874ec99@arm.com>

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