RE: [PATCH] video/fbdev: 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] video/fbdev: Fix a double free in hvfb_probe
Date: Tue, 23 Mar 2021 18:52:07 +0000	[thread overview]
Message-ID: <MWHPR21MB1593E96D24957865DB46D3DBD7649@MWHPR21MB1593.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20210323073350.17697-1-lyl2019@mail.ustc.edu.cn>

From: Lv Yunlong <lyl2019@mail.ustc.edu.cn> Sent: Tuesday, March 23, 2021 12:34 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 twice
> by framebuffer_release(info).
> 
> My patch sets info->apertures to NULL after it was freed to avoid
> double free.
> 
> Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> ---
>  drivers/video/fbdev/hyperv_fb.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> index c8b0ae676809..2fc9b507e73a 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -1032,6 +1032,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  		if (!pdev) {
>  			pr_err("Unable to find PCI Hyper-V video\n");
>  			kfree(info->apertures);
> +			info->apertures = NULL;
>  			return -ENODEV;
>  		}
> 
> @@ -1130,6 +1131,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  		pci_dev_put(pdev);
>  	}
>  	kfree(info->apertures);
> +	info->apertures = NULL;
> 
>  	return 0;
> 
> @@ -1142,6 +1144,7 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info
> *info)
>  	if (!gen2vm)
>  		pci_dev_put(pdev);
>  	kfree(info->apertures);
> +	info->apertures = NULL;
> 
>  	return -ENOMEM;
>  }
> --
> 2.25.1
> 

While I think this works, a slightly better solution might be to remove
all calls to kfree(info->apertures) in hvfb_getmem(),  and just let
framebuffer_release() handle freeing the memory.  That's what is
done in other drivers that follow the fbdev pattern, and it's less
code overall.  

Michael