linux-kernel - Re: [PATCH 1/2] cxl/region: Fix potential invalid pointer dereference

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Date: Mon, 29 Apr 2024 10:25:35 +0000
From: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To: Dan Carpenter <dan.carpenter@...aro.org>
CC: "dave@...olabs.net" <dave@...olabs.net>, "jonathan.cameron@...wei.com"
	<jonathan.cameron@...wei.com>, "dave.jiang@...el.com" <dave.jiang@...el.com>,
	"alison.schofield@...el.com" <alison.schofield@...el.com>,
	"vishal.l.verma@...el.com" <vishal.l.verma@...el.com>, "ira.weiny@...el.com"
	<ira.weiny@...el.com>, "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] cxl/region: Fix potential invalid pointer dereference



On 29/04/2024 18:10, Dan Carpenter wrote:
> On Mon, Apr 29, 2024 at 09:31:53AM +0800, Li Zhijian wrote:
>> construct_region() could return a PTR_ERR() which cannot be derefernced.
>> Moving the dereference behind the error checking to make sure the
>> pointer is valid.
>>
> 
> No, this patch is unnecessary.

Agree,


> 
> drivers/cxl/core/region.c
>    3080          /*
>    3081           * Ensure that if multiple threads race to construct_region() for @hpa
>    3082           * one does the construction and the others add to that.
>    3083           */
>    3084          mutex_lock(&cxlrd->range_lock);
>    3085          region_dev = device_find_child(&cxlrd->cxlsd.cxld.dev, hpa,
>    3086                                         match_region_by_range);
>    3087          if (!region_dev) {
>    3088                  cxlr = construct_region(cxlrd, cxled);
>    3089                  region_dev = &cxlr->dev;
>                                       ^^^^^^^^^^^
> This is not a dereference, it's just pointer math.  In in this case it's
> the same as saying:
> 
> 		region_dev = (void *)cxlr;


You are right, a equivalent code could be:
		region_dev = ((char *)cxlr) + offsetof(struct cxl_region, dev);


Thanks


> 
>    3090          } else
>    3091                  cxlr = to_cxl_region(region_dev);
>    3092          mutex_unlock(&cxlrd->range_lock);
>    3093
>    3094          rc = PTR_ERR_OR_ZERO(cxlr);
>                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This check means that if cxlr is an error pointer then we will clean up
> and return an error.
> 
> regards,
> dan carpenter
> 
>    3095          if (rc)
>    3096                  goto out;
>    3097
>    3098          attach_target(cxlr, cxled, -1, TASK_UNINTERRUPTIBLE);
>    3099
>    3100          down_read(&cxl_region_rwsem);
>    3101          p = &cxlr->params;
>    3102          attach = p->state == CXL_CONFIG_COMMIT;
>    3103          up_read(&cxl_region_rwsem);
>    3104
>    3105          if (attach) {
>    3106                  /*
>    3107                   * If device_attach() fails the range may still be active via
>    3108                   * the platform-firmware memory map, otherwise the driver for
>    3109                   * regions is local to this file, so driver matching can't fail.
>    3110                   */
>    3111                  if (device_attach(&cxlr->dev) < 0)
>    3112                          dev_err(&cxlr->dev, "failed to enable, range: %pr\n",
>    3113                                  p->res);
>    3114          }
>    3115
>    3116          put_device(region_dev);
>    3117  out:
>    3118          put_device(cxlrd_dev);
>    3119          return rc;
>    3120  }
> 
>