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

Du, Bin
2025-07-29T06:08:16Z

Many thanks Sultan, will try your modifications in this and other mails
together and let you know the result

On 7/27/2025 5:41 AM, Sultan Alsawaf wrote:
> On Fri, Jul 25, 2025 at 05:22:41PM +0800, Du, Bin wrote:
>>>> +static struct dma_buf *isp4vid_vb2_get_dmabuf(struct vb2_buffer *vb,
>>>> + void *buf_priv,
>>>> + unsigned long flags)
>>>> +{
>>>> + struct isp4vid_vb2_buf *buf = buf_priv;
>>>> + struct dma_buf *dbuf;
>>>> +
>>>> + if (buf->dbuf) {
>>>> + dev_dbg(buf->dev,
>>>> + "dbuf already created, reuse implicit dbuf\n");
>>>> + dbuf = buf->dbuf;
>>>
>>> The dmabuf is reused here without taking a reference to it. When the get_dmabuf
>>> memop is called by vb2_core_expbuf(), it assumes that a reference to the dmabuf
>>> is acquired. So you need to add `get_dma_buf(dbuf)` here.
>> After test, we found we can't add get_dma_buf(dbuf) here because it will
>> make cheese APP fail to open camera with following error:
>> amdgpu: [drm] *ERROR* failed to alloc gart kernel buffer (-28)
>
> I see, it's because buf->is_expbuf is set to true even for the implicit dbuf, so
> the initial reference on the implicit dbuf is never put, causing a leak.
>
> Also, the refcount increment in isp4vid_vb2_get_dmabuf() is done every time even
> when reusing the existing dbuf, but releasing the dbuf will only do a single
> refcount decrement. This also causes a leak.
>
> And, isp4vid_get_dmabuf() may fail but isp4vid_vb2_get_dmabuf() doesn't check
> the return value, so there may be another leak when isp4vid_get_dmabuf() fails
> because of the refcount increment. The refcount increment and setting of
> buf->is_expbuf to true should only be done on success.
>
> I have fixed all of these isp4vid_vb2_get_dmabuf() issues in the following diff,
> please try it:
>
> --- a/drivers/media/platform/amd/isp4/isp4_video.c
> +++ b/drivers/media/platform/amd/isp4/isp4_video.c
> @@ -476,18 +476,22 @@ static struct dma_buf *isp4vid_vb2_get_dmabuf(struct vb2_buffer *vb,
> unsigned long flags)
> {
> struct isp4vid_vb2_buf *buf = buf_priv;
> - struct dma_buf *dbuf;
> + struct dma_buf *dbuf = buf->dbuf;
>
> - if (buf->dbuf) {
> + if (dbuf) {
> dev_dbg(buf->dev,
> - "dbuf already created, reuse implicit dbuf\n");
> - dbuf = buf->dbuf;
> + "dbuf already created, reuse %s dbuf\n",
> + buf->is_expbuf ? "exported" : "implicit");
> + get_dma_buf(dbuf);
> } else {
> dbuf = isp4vid_get_dmabuf(vb, buf_priv, flags);
> + if (!dbuf)
> + return NULL;
> +
> dev_dbg(buf->dev, "created new dbuf\n");
> + buf->is_expbuf = true;
> + refcount_inc(&buf->refcount);
> }
> - buf->is_expbuf = true;
> - refcount_inc(&buf->refcount);
>
> dev_dbg(buf->dev, "buf exported, refcount %d\n",
> buf->refcount.refs.counter);
> --
>
>>>> + 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");`
>
> Sultan