Re: [PATCH] fix broken size test in bitmap_find_free_region()

Andrew Morton
2009-02-06T21:30:40Z

On Fri, 6 Feb 2009 13:22:33 +0100 (CET)
Guennadi Liakhovetski <lg@denx.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;