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

From: Wei Liu <wei.liu@kernel.org>
To: Michael Kelley <mikelley@microsoft.com>
Cc: 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>,
	"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: Thu, 25 Mar 2021 13:30:36 +0000	[thread overview]
Message-ID: <20210325133036.rbriezf3v32wofkl@liuwe-devbox-debian-v2> (raw)
In-Reply-To: <MWHPR21MB1593F19EE7AD10698582FA78D7639@MWHPR21MB1593.namprd21.prod.outlook.com>

On Wed, Mar 24, 2021 at 01:46:39PM +0000, Michael Kelley wrote:
> 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,
> 

Yes. I surely can incorporate the changes.

I will also add the Fixes tag.

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