linux-kernel - Re: [PATCH] dma-debug: Avoid NULL dereference when checking sync

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Message-Id: <20161107163858.cc3c523690ced2ed2a10e7db@linux-foundation.org>
Date:   Mon, 7 Nov 2016 16:38:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Maarten ter Huurne <maarten@...ewalker.org>
Cc:     Vinod Koul <vinod.koul@...el.com>, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-debug: Avoid NULL dereference when checking sync

On Mon, 31 Oct 2016 16:11:41 +0100 Maarten ter Huurne <maarten@...ewalker.org> wrote:

> check_sync() calls bucket_find_contain(), which in turn calls
> dma_get_max_seg_size(), which dereferences the device pointer.
> 
> ...
>
> --- a/lib/dma-debug.c
> +++ b/lib/dma-debug.c
> @@ -1223,6 +1223,11 @@ static void check_sync(struct device *dev,
>  	struct hash_bucket *bucket;
>  	unsigned long flags;
>  
> +	if (!ref->dev) {
> +		err_printk(dev, NULL, "DMA-API: device driver passes NULL for device to DMA sync function; cannot check usage\n");
> +		return;
> +	}
> +
>  	bucket = get_hash_bucket(ref, &flags);
>  
>  	entry = bucket_find_contain(&bucket, ref, &flags);

Well.

a) who is calling check_sync() without having filled in ref.dev? 
   How does this occur?  Is this a bug in the caller?

b) why does check_sync() have a `dev' argument anyway?  Is there any
   situation in which that will differ from ref.dev?