RE: [PATCH v2] video: hyperv_fb: Fix a double free in hvfb_probe

From: Michael Kelley <mikelley@microsoft.com>
To: Lv Yunlong <lyl2019@mail.ustc.edu.cn>,
	KY Srinivasan <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	"wei.liu@kernel.org" <wei.liu@kernel.org>
Cc: "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"linux-fbdev@vger.kernel.org" <linux-fbdev@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2] video: hyperv_fb: Fix a double free in hvfb_probe
Date: Wed, 24 Mar 2021 13:46:39 +0000	[thread overview]
Message-ID: <MWHPR21MB1593F19EE7AD10698582FA78D7639@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20210324103724.4189-1-lyl2019@mail.ustc.edu.cn>

From: Lv Yunlong <lyl2019@mail.ustc.edu.cn> Sent: Wednesday, March 24, 2021 3:37 AM
> 
> In function hvfb_probe in hyperv_fb.c, it calls hvfb_getmem(hdev, info)
> and return err when info->apertures is freed.
> 
> In the error1 label of hvfb_probe, info->apertures will be freed for the
> second time in framebuffer_release(info).
> 
> My patch removes all kfree(info->apertures) instead of set info->apertures
> to NULL. It is because that let framebuffer_release() handle freeing the
> memory flows the fbdev pattern, and less code overall.

Let me suggest some clarifications in the commit message.  It's probably
better not to reference the initial approach of setting info->apertures to
NULL, since there won't be any record of that approach in the commit
history.  Here's what I would suggest:

Function hvfb_probe() calls hvfb_getmem(), expecting upon return that
info->apertures is either NULL or points to memory that should be freed
by framebuffer_release().  But hvfb_getmem() is freeing the memory and
leaving the pointer non-NULL, resulting in a double free if an error
occurs or later if hvfb_remove() is called.

Fix this by removing all kfree(info->apertures) calls in hvfb_getmem().
This will allow framebuffer_release() to free the memory, which follows
the pattern of other fbdev drivers.

Modulo this revision to the commit message, which Wei Liu can
probably incorporate,

Reviewed-by: Michael Kelley <mikelley@microsoft.com>

> 
> Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> ---
>  drivers/video/fbdev/hyperv_fb.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> index c8b0ae676809..4dc9077dd2ac 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -1031,7 +1031,6 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  			PCI_DEVICE_ID_HYPERV_VIDEO, NULL);
>  		if (!pdev) {
>  			pr_err("Unable to find PCI Hyper-V video\n");
> -			kfree(info->apertures);
>  			return -ENODEV;
>  		}
> 
> @@ -1129,7 +1128,6 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  	} else {
>  		pci_dev_put(pdev);
>  	}
> -	kfree(info->apertures);
> 
>  	return 0;
> 
> @@ -1141,7 +1139,6 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  err1:
>  	if (!gen2vm)
>  		pci_dev_put(pdev);
> -	kfree(info->apertures);
> 
>  	return -ENOMEM;
>  }
> --
> 2.25.1
> 