Re: [PATCH] device-dax: Fix refcount leak in __devm_create_dev_dax() error path

[Dan Williams]

Guangshuo Li wrote:
> After device_initialize(), the lifetime of the embedded struct device is
> expected to be managed through the device core reference counting.
> 
> In __devm_create_dev_dax(), several failure paths after
> device_initialize() free dev_dax directly instead of releasing the
> device reference with put_device(). This bypasses the normal device
> lifetime rules and may leave the reference count of the embedded struct
> device unbalanced, resulting in a refcount leak and potentially leading
> to a use-after-free.

Please do not list "theoretical" problems as justification. Point to
real problems.

> Fix this by assigning dev->type before device_initialize(), so the
> release callback is available for put_device(), and use put_device() in
> the post-initialization error paths. Keep dev_dax range cleanup explicit
> in the error path.

I see a more straightforward way to address just the practical problem
that also incorporates the other feedback I have below. Can you spot
that and fixup the changelog to address the practical impact?

> Fixes: c2f3011ee697f ("device-dax: add an allocation interface for device-dax instances")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Guangshuo Li <lgs201920130244@xxxxxxxxx>
> ---
>  drivers/dax/bus.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
> index fde29e0ad68b..8753115cd371 100644
> --- a/drivers/dax/bus.c
> +++ b/drivers/dax/bus.c
> @@ -1453,6 +1453,7 @@ static struct dev_dax *__devm_create_dev_dax(struct dev_dax_data *data)
>  	}
>  
>  	dev = &dev_dax->dev;
> +	dev->type = &dev_dax_type;
>  	device_initialize(dev);
>  	dev_set_name(dev, "dax%d.%d", dax_region->id, dev_dax->id);
>  
> @@ -1499,7 +1500,6 @@ static struct dev_dax *__devm_create_dev_dax(struct dev_dax_data *data)
>  	dev->devt = inode->i_rdev;
>  	dev->bus = &dax_bus_type;
>  	dev->parent = parent;
> -	dev->type = &dev_dax_type;
>  
>  	rc = device_add(dev);
>  	if (rc) {
> @@ -1523,14 +1523,21 @@ static struct dev_dax *__devm_create_dev_dax(struct dev_dax_data *data)
>  
>  err_alloc_dax:
>  	kfree(dev_dax->pgmap);
> +	dev_dax->pgmap = NULL;
> +
>  err_pgmap:
>  	free_dev_dax_ranges(dev_dax);
> +	put_device(dev);
> +	return ERR_PTR(rc);
> +
>  err_range:
> -	free_dev_dax_id(dev_dax);
> +	put_device(dev);
> +	return ERR_PTR(rc);

Please no gotos with early returns, that makes a mess.