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

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Date: Mon, 29 Apr 2024 13:10:52 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Li Zhijian <lizhijian@...itsu.com>
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,
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cxl/region: Fix potential invalid pointer dereference

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.

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;

  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  }