linux-kernel - Re: [PATCH v2] lib/string_helpers.c: fix infinite loop in string_get

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Date:	Mon, 14 Sep 2015 15:15:58 +0000
From:	James Bottomley <jbottomley@...n.com>
To:	"vkuznets@...hat.com" <vkuznets@...hat.com>
CC:	"linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko@...ux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"kys@...rosoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH v2] lib/string_helpers.c: fix infinite loop in
 string_get_size()

On Mon, 2015-09-14 at 14:57 +0200, Vitaly Kuznetsov wrote:
> Some string_get_size() calls (e.g.:
>  string_get_size(1, 512, STRING_UNITS_10, ..., ...)
>  string_get_size(15, 64, STRING_UNITS_10, ..., ...)
> ) result in an infinite loop. The problem is that if size is equal to
> divisor[units]/blk_size and is smaller than divisor[units] we'll end
> up with size == 0 when we start doing sf_cap calculations:
> 
> For string_get_size(1, 512, STRING_UNITS_10, ..., ...) case:
>    ...
>    remainder = do_div(size, divisor[units]); -> size is 0, remainder is 1
>    remainder *= blk_size; -> remainder is 512
>    ...
>    size *= blk_size; -> size is still 0
>    size += remainder / divisor[units]; -> size is still 0
> 
> The caller causing the issue is sd_read_capacity(), the problem was noticed
> on Hyper-V, such weird size was reported by host when scanning collides
> with device removal. This is probably a separate issue worth fixing, this
> patch is intended to prevent the library routing from infinite looping.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  lib/string_helpers.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/string_helpers.c b/lib/string_helpers.c
> index c98ae81..d0e607c 100644
> --- a/lib/string_helpers.c
> +++ b/lib/string_helpers.c
> @@ -59,7 +59,7 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
>  	}
>  
>  	exp = divisor[units] / (u32)blk_size;

Add a comment here explaining to those who come after why strict greater
than is necessary.  Something like:

/* size must be strictly greater than exp here to ensure
 * that remainder is greater than divisor[units] coming out of the if */

> -	if (size >= exp) {
> +	if (size > exp) {
>  		remainder = do_div(size, divisor[units]);
>  		remainder *= blk_size;
>  		i++;

So when pushed, you finally found the algorithmic problem.  Well done.

Acked-by: James Bottomley <JBottomley@...n.com>

James