linux-kernel - Re: [PATCH] fix broken size test in bitmap_find_free

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Date:	Fri, 6 Feb 2009 13:29:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Guennadi Liakhovetski <lg@...x.de>
Cc:	linux-kernel@...r.kernel.org, lethal@...ux-sh.org,
	adrian@...golddream.dyndns.info, lkmladrian@...il.com,
	linux-sh@...r.kernel.org, penberg@...helsinki.fi,
	dbaryshkov@...il.com, penguin-kernel@...ove.sakura.ne.jp,
	hannes@...xchg.org
Subject: Re: [PATCH] fix broken size test in bitmap_find_free_region()

On Fri, 6 Feb 2009 13:22:33 +0100 (CET)
Guennadi Liakhovetski <lg@...x.de> wrote:

> This loop and test in bitmap_find_free_region()
> 
> 	for (pos = 0; pos < bits; pos += (1 << order))
> 		if (__reg_op(bitmap, pos, order, REG_OP_ISFREE))
> 			break;
> 	if (pos == bits)
> 		return -ENOMEM;
> 
> can only return an error (-ENOMEM) if bits is a multiple of (1 << order), 
> which is, for instance, true, if bits is (also) a power of 2. This 
> is not necessarily the case with dma_alloc_from_coherent(). A failure to 
> recognise too large a request leads in dma_alloc_from_coherent() to 
> accessing beyond available memory, and to writing beyond the bitmap.
> 

Do we have any reports of dma_alloc_from_coherent() actually behaving
in that way?


> ---
> 
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 1338469..d49c37f 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -953,7 +953,7 @@ int bitmap_find_free_region(unsigned long *bitmap, int bits, int order)
>  	for (pos = 0; pos < bits; pos += (1 << order))
>  		if (__reg_op(bitmap, pos, order, REG_OP_ISFREE))
>  			break;
> -	if (pos == bits)
> +	if (pos + (1 << order) > bits)
>  		return -ENOMEM;
>  	__reg_op(bitmap, pos, order, REG_OP_ALLOC);
>  	return pos;
--
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