[PATCH] bus: fsl_mc: Fix driver_managed_dma check

[PATCH] bus: fsl_mc: Fix driver_managed_dma check

[Robin Murphy]

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_fsl_mc_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.

Reported-by: Will McVicker <willmcvicker@google.com>
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 drivers/bus/fsl-mc/fsl-mc-bus.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
index a8be8cf246fb..67031136ef66 100644
--- a/drivers/bus/fsl-mc/fsl-mc-bus.c
+++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
@@ -139,9 +139,9 @@ static int fsl_mc_bus_uevent(const struct device *dev, struct kobj_uevent_env *e
 
 static int fsl_mc_dma_configure(struct device *dev)
 {
+	const struct device_driver *drv = READ_ONCE(dev->driver);
 	struct device *dma_dev = dev;
 	struct fsl_mc_device *mc_dev = to_fsl_mc_device(dev);
-	struct fsl_mc_driver *mc_drv = to_fsl_mc_driver(dev->driver);
 	u32 input_id = mc_dev->icid;
 	int ret;
 
@@ -153,8 +153,8 @@ static int fsl_mc_dma_configure(struct device *dev)
 	else
 		ret = acpi_dma_configure_id(dev, DEV_DMA_COHERENT, &input_id);
 
-	/* @mc_drv may not be valid when we're called from the IOMMU layer */
-	if (!ret && dev->driver && !mc_drv->driver_managed_dma) {
+	/* @drv may not be valid when we're called from the IOMMU layer */
+	if (!ret && drv && !to_fsl_mc_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

Re: [PATCH] bus: fsl_mc: Fix driver_managed_dma check

[William McVicker]

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_fsl_mc_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@google.com>

> 
> Reported-by: Will McVicker <willmcvicker@google.com>
> Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> ---
>  drivers/bus/fsl-mc/fsl-mc-bus.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
> index a8be8cf246fb..67031136ef66 100644
> --- a/drivers/bus/fsl-mc/fsl-mc-bus.c
> +++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
> @@ -139,9 +139,9 @@ static int fsl_mc_bus_uevent(const struct device *dev, struct kobj_uevent_env *e
>  
>  static int fsl_mc_dma_configure(struct device *dev)
>  {
> +	const struct device_driver *drv = READ_ONCE(dev->driver);
>  	struct device *dma_dev = dev;
>  	struct fsl_mc_device *mc_dev = to_fsl_mc_device(dev);
> -	struct fsl_mc_driver *mc_drv = to_fsl_mc_driver(dev->driver);
>  	u32 input_id = mc_dev->icid;
>  	int ret;
>  
> @@ -153,8 +153,8 @@ static int fsl_mc_dma_configure(struct device *dev)
>  	else
>  		ret = acpi_dma_configure_id(dev, DEV_DMA_COHERENT, &input_id);
>  
> -	/* @mc_drv may not be valid when we're called from the IOMMU layer */
> -	if (!ret && dev->driver && !mc_drv->driver_managed_dma) {
> +	/* @drv may not be valid when we're called from the IOMMU layer */
> +	if (!ret && drv && !to_fsl_mc_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
> 

Re: [PATCH] bus: fsl_mc: Fix driver_managed_dma check

[Christophe Leroy]

On Fri, 25 Apr 2025 14:39:28 +0100, 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_fsl_mc_driver(NULL)
> is already computed, and thus the check fails to work as intended.
> 
> [...]

Applied, thanks!

[1/1] bus: fsl_mc: Fix driver_managed_dma check
      commit: 152f33ee30ee6a7f4c15bedd7529dc5945315547

Best regards,
-- 
Christophe Leroy <christophe.leroy@csgroup.eu>