linux-kernel - Re: [PATCH] cxl/region: Fix potential double free of region device in delete_region

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Message-ID: <aHo6qXHUubfq9vKL@rric.localdomain>
Date: Fri, 18 Jul 2025 14:14:33 +0200
From: Robert Richter <rrichter@....com>
To: Ma Ke <make24@...as.ac.cn>
Cc: dave@...olabs.net, jonathan.cameron@...wei.com, dave.jiang@...el.com,
	alison.schofield@...el.com, vishal.l.verma@...el.com,
	ira.weiny@...el.com, dan.j.williams@...el.com, ming.li@...omail.com,
	bwidawsk@...nel.org, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	stable@...r.kernel.org
Subject: Re: [PATCH] cxl/region: Fix potential double free of region device
 in delete_region_store

On 18.07.25 10:29:40, Ma Ke wrote:
> cxl_find_region_by_name() uses device_find_child_by_name() to locate a
> region device by name. This function implicitly increments the
> device's reference count before returning the pointer by calling
> device_find_child(). However, in delete_region_store(), after calling
> devm_release_action() which synchronously executes
> unregister_region(), an additional explicit put_device() is invoked.
> The unregister_region() callback already contains a put_device() call
> to decrement the reference count. This results in two consecutive
> decrements of the same device's reference count. First decrement
> occurs in unregister_region() via its put_device() call. Second
> decrement occurs in delete_region_store() via the explicit
> put_device(). We should remove the additional put_device().
> 
> As comment of device_find_child() says, 'NOTE: you will need to drop
> the reference with put_device() after use'.
> 
> Found by code review.
> 
> Cc: stable@...r.kernel.org
> Fixes: 779dd20cfb56 ("cxl/region: Add region creation support")
> Signed-off-by: Ma Ke <make24@...as.ac.cn>
> ---
>  drivers/cxl/core/region.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
> index 6e5e1460068d..eacf726cf463 100644
> --- a/drivers/cxl/core/region.c
> +++ b/drivers/cxl/core/region.c
> @@ -2672,7 +2672,6 @@ static ssize_t delete_region_store(struct device *dev,
>  		return PTR_ERR(cxlr);
>  
>  	devm_release_action(port->uport_dev, unregister_region, cxlr);
> -	put_device(&cxlr->dev);

cxl_find_region_by_name() increments the ref count in addition. So the
ref count could be e.g. 2. This put_device() here only decrements it
once to have the same ref count as it was before calling the find. The
put_device() in unregister_region clears the ref count, which causes
the release function to be called.

So, no, we need both and we may not change the code.

Thanks,

-Robert

>  
>  	return len;
>  }
> -- 
> 2.25.1
>