linux-kernel - Re: [PATCH] cdx: Fix driver_managed

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Message-ID: <aBJ_3qI6WRXpJ3SL@google.com>
Date: Wed, 30 Apr 2025 12:54:06 -0700
From: William McVicker <willmcvicker@...gle.com>
To: Robin Murphy <robin.murphy@....com>
Cc: joro@...tes.org, will@...nel.org, Nipun Gupta <nipun.gupta@....com>,
	Nikhil Agarwal <nikhil.agarwal@....com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cdx: Fix driver_managed_dma check

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 <willmcvicker@...gle.com>

> 
> Reported-by: Will McVicker <willmcvicker@...gle.com>
> Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>  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
>