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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-22db5102d31sm126707425ad.198.2025.04.30.12.54.10 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 30 Apr 2025 12:54:10 -0700 (PDT) Date: Wed, 30 Apr 2025 12:54:06 -0700 From: William McVicker To: Robin Murphy Cc: joro@8bytes.org, will@kernel.org, Nipun Gupta , Nikhil Agarwal , iommu@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH] cdx: Fix driver_managed_dma check Message-ID: References: <20250425133929.646493-2-robin.murphy@arm.com> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250425133929.646493-2-robin.murphy@arm.com> On 04/25/2025, Robin Murphy wrote: > Since it's not currently safe to take device_lock() in the IOMMU probe > path, that can race against really_probe() setting dev->driver before > attempting to bind. The race itself isn't so bad, since we're only > concerned with dereferencing dev->driver itself anyway, but sadly my > attempt to implement the check with minimal churn leads to a kind of > TOCTOU issue, where dev->driver becomes valid after to_cdx_driver(NULL) > is already computed, and thus the check fails to work as intended. > > Will and I both hit this with the platform bus, but the pattern here is > the same, so fix it for correctness too. Thanks! Reviewed-by: Will McVicker > > Reported-by: Will McVicker > Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path") > Signed-off-by: Robin Murphy > --- > drivers/cdx/cdx.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/drivers/cdx/cdx.c b/drivers/cdx/cdx.c > index 092306ca2541..8aa92a198b71 100644 > --- a/drivers/cdx/cdx.c > +++ b/drivers/cdx/cdx.c > @@ -348,7 +348,7 @@ static void cdx_shutdown(struct device *dev) > > static int cdx_dma_configure(struct device *dev) > { > - struct cdx_driver *cdx_drv = to_cdx_driver(dev->driver); > + const struct device_driver *drv = READ_ONCE(dev->driver); > struct cdx_device *cdx_dev = to_cdx_device(dev); > struct cdx_controller *cdx = cdx_dev->cdx; > u32 input_id = cdx_dev->req_id; > @@ -360,8 +360,8 @@ static int cdx_dma_configure(struct device *dev) > return ret; > } > > - /* @cdx_drv may not be valid when we're called from the IOMMU layer */ > - if (!ret && dev->driver && !cdx_drv->driver_managed_dma) { > + /* @drv may not be valid when we're called from the IOMMU layer */ > + if (!ret && drv && !to_cdx_driver(drv)->driver_managed_dma) { > ret = iommu_device_use_default_domain(dev); > if (ret) > arch_teardown_dma_ops(dev); > -- > 2.39.2.101.g768bb238c484.dirty >