linux-kernel - RE: [PATCH] video/fbdev: Fix a double free in hvfb

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Date:   Tue, 23 Mar 2021 18:52:07 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Lv Yunlong <lyl2019@...l.ustc.edu.cn>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] video/fbdev: Fix a double free in hvfb_probe

From: Lv Yunlong <lyl2019@...l.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@...l.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