linux-kernel - Re: [PATCH] media: v4l2: Fix memleak in videobuf_read

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Date:   Thu, 7 Jan 2021 10:57:08 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Dinghao Liu <dinghao.liu@....edu.cn>, kjlu@....edu
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michel Lespinasse <walken@...gle.com>,
        Ricardo Cerqueira <v4l@...queira.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: v4l2: Fix memleak in videobuf_read_one

On 05/01/2021 08:59, Dinghao Liu wrote:
> When videobuf_waiton() fails, we should execute clean
> functions to prevent memleak. It's the same when
> __videobuf_copy_to_user() fails.
> 
> Fixes: 7a7d9a89d0307 ("V4L/DVB (6251): Replace video-buf to a more generic approach")
> Signed-off-by: Dinghao Liu <dinghao.liu@....edu.cn>
> ---
>  drivers/media/v4l2-core/videobuf-core.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/videobuf-core.c b/drivers/media/v4l2-core/videobuf-core.c
> index 606a271bdd2d..0709b75d11cd 100644
> --- a/drivers/media/v4l2-core/videobuf-core.c
> +++ b/drivers/media/v4l2-core/videobuf-core.c
> @@ -924,8 +924,12 @@ ssize_t videobuf_read_one(struct videobuf_queue *q,
>  
>  	/* wait until capture is done */
>  	retval = videobuf_waiton(q, q->read_buf, nonblocking, 1);
> -	if (0 != retval)
> +	if (retval != 0) {
> +		q->ops->buf_release(q, q->read_buf);
> +		kfree(q->read_buf);
> +		q->read_buf = NULL;
>  		goto done;
> +	}

I'm fairly certain that this is wrong: if waiton returns an error, then
that means that the wait is either interrupted or that we are in non-blocking
mode and no buffer has arrived yet. In that case you just go to done since
there is nothing to clean up.

>  
>  	CALL(q, sync, q, q->read_buf);
>  
> @@ -940,8 +944,12 @@ ssize_t videobuf_read_one(struct videobuf_queue *q,
>  
>  	/* Copy to userspace */
>  	retval = __videobuf_copy_to_user(q, q->read_buf, data, count, nonblocking);
> -	if (retval < 0)
> +	if (retval < 0) {
> +		q->ops->buf_release(q, q->read_buf);
> +		kfree(q->read_buf);
> +		q->read_buf = NULL;
>  		goto done;

I'm not sure about this either: if userspace gave a crappy pointer and this
copy_to_user fails, then that doesn't mean you should release the buffer.
The next read() might have a valid pointer or, more likely, the application
exits or crashes and everything is cleaned up when the filehandle is closed.

> +	}
>  
>  	q->read_off += retval;
>  	if (q->read_off == q->read_buf->size) {
> 

Do you have actual proof that this is a memleak? I don't want to mess around
with the old videobuf unless you can show me that there is a real bug.

Regards,

	Hans