linux-kernel - Re: [PATCH] mtd: blktrans: fix integer overflow

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Date:	Wed, 2 Sep 2015 10:02:49 -0700
From:	Brian Norris <computersforpeace@...il.com>
To:	Peng Fan <van.freenix@...il.com>
Cc:	dwmw2@...radead.org, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: blktrans: fix integer overflow

On Fri, Aug 21, 2015 at 10:57:31PM +0800, Peng Fan wrote:
> In drivers/mtd/mtd_blkdevs.c:
> 406	set_capacity(gd, (new->size * tr->blksize) >> 9);
> The type of new->size is unsigned long and the type of tr->blksize is int,
> the result of 'new->size * tr->blksize' may exceed ULONG_MAX on 32bit
> machines.
> 
> I use nand chip MT29F32G08CBADBWP which is 4GB and the parameters passed
> to kernel is 'mtdparts=gpmi-nand:-(user)', the whole nand chip will be
> treated as a 4GB mtd partition. new->size is 0x800000 and tr->blksize is
> 0x200, 'new->size * tr->blksize' however is 0. This is what we do not want
> to see.
> 
> Change the type of entry size of mtd_blktrans_dev to unsigned long long
> to fix the overflow issue.
> 
> Signed-off-by: Peng Fan <van.freenix@...il.com>
> Cc: David Woodhouse <dwmw2@...radead.org>
> Cc: Brian Norris <computersforpeace@...il.com>
> ---
>  include/linux/mtd/blktrans.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/mtd/blktrans.h b/include/linux/mtd/blktrans.h
> index e93837f..3853d74 100644
> --- a/include/linux/mtd/blktrans.h
> +++ b/include/linux/mtd/blktrans.h
> @@ -38,7 +38,7 @@ struct mtd_blktrans_dev {
>  	struct mutex lock;
>  	int devnum;
>  	bool bg_stop;
> -	unsigned long size;
> +	unsigned long long size;

Sorry, you can't just do that. Some builds won't have right arithmetic
instructions (or compiler helpers) to do certain 64-bit operations, so you
either need to go and fix all the blktrans users to be more careful (e.g., do
bit shifts instead of division/modulo), or else find another solution. See, on
an ARM32 build:

  LD      init/built-in.o
drivers/built-in.o: In function `nftl_add_mtd':
:(.text+0x1369dc): undefined reference to `__aeabi_uldivmod'
:(.text+0x1369f4): undefined reference to `__aeabi_uldivmod'
:(.text+0x136a70): undefined reference to `__aeabi_uldivmod'
drivers/built-in.o: In function `inftl_add_mtd':
:(.text+0x137eb0): undefined reference to `__aeabi_uldivmod'
:(.text+0x137ec8): undefined reference to `__aeabi_uldivmod'
drivers/built-in.o::(.text+0x137f4c): more undefined references to `__aeabi_uldivmod' follow

>  	int readonly;
>  	int open;
>  	struct kref ref;

One possibility, since you only point to a single computation that
overflows, is to just fix the overflow locally. It's not like the 'size'
(which represents number of sectors) is actually ever overflowing a
32-bit integer. It's just the multiplication that overflows. So you
could cast to 64-bit arithmetic just for the multiplication. e.g.:

	set_capcity(gd, ((u64)new->size * tr->blksize) >> 9);

Or some other creative solution.

Then, we don't have to address this problem till we start seeing 2TB
MTDs!

Brian
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