linux-kernel - Re: [PATCH v2 6/8] media: platform: amd: isp4 video node and buffers handling added

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Message-ID: <aIVNj1DLab18eArC@sultan-box>
Date: Sat, 26 Jul 2025 14:50:07 -0700
From: Sultan Alsawaf <sultan@...neltoast.com>
To: "Du, Bin" <bin.du@....com>
Cc: mchehab@...nel.org, hverkuil@...all.nl,
	laurent.pinchart+renesas@...asonboard.com,
	bryan.odonoghue@...aro.org, sakari.ailus@...ux.intel.com,
	prabhakar.mahadev-lad.rj@...renesas.com,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	pratap.nirujogi@....com, benjamin.chan@....com, king.li@....com,
	gjorgji.rosikopulos@....com, Phil.Jawich@....com,
	Dominic.Antony@....com
Subject: Re: [PATCH v2 6/8] media: platform: amd: isp4 video node and buffers
 handling added

On Sat, Jul 26, 2025 at 02:41:41PM -0700, Sultan Alsawaf wrote:
> On Fri, Jul 25, 2025 at 05:22:41PM +0800, Du, Bin wrote:
> > > > +		dev_warn(buf->dev, "ignore buffer free, refcount %u > 0",
> > > > +			 refcount_read(&buf->refcount));
> > > 
> > > This refcount_read() is a possible use-after-free because `buf` is accessed
> > > after isp4vid_vb2_put() puts its reference to `buf`. So something else could put
> > > the last reference to `buf` and free it after this refcount dec but before the
> > > refcount_read(). Maybe just remove this dev_warn() entirely?
> > > 
> > The warning is important to debug mem related issue, plan to keep it but
> > without accessing buf or buf->refcount here. Do you think it acceptible?
> 
> Yes, that sounds good. So something like this:
> `dev_warn(buf->dev, "ignore buffer free, refcount > 0");`

Sorry, to fix the dev_warn() we need to make a copy of buf->dev first:

--- a/drivers/media/platform/amd/isp4/isp4_video.c
+++ b/drivers/media/platform/amd/isp4/isp4_video.c
@@ -584,8 +584,9 @@ static void isp4vid_vb2_put(void *buf_priv)
 {
 	struct isp4vid_vb2_buf *buf = (struct isp4vid_vb2_buf *)buf_priv;
 	struct amdgpu_bo *bo = (struct amdgpu_bo *)buf->bo;
+	struct device *dev = buf->dev;
 
-	dev_dbg(buf->dev,
+	dev_dbg(dev,
 		"release isp user bo 0x%llx size %ld refcount %d is_expbuf %d",
 		buf->gpu_addr, buf->size,
 		buf->refcount.refs.counter, buf->is_expbuf);
@@ -601,8 +602,7 @@ static void isp4vid_vb2_put(void *buf_priv)
 		kfree(buf);
 		buf = NULL;
 	} else {
-		dev_warn(buf->dev, "ignore buffer free, refcount %u > 0",
-			 refcount_read(&buf->refcount));
+		dev_warn(dev, "ignore buffer free, refcount > 0\n");
 	}
 }
 
--

Sultan