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

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Message-ID: <87d1xl1htk.fsf@vitty.brq.redhat.com>
Date:	Mon, 14 Sep 2015 11:16:55 +0200
From:	Vitaly Kuznetsov <vkuznets@...hat.com>
To:	James Bottomley <jbottomley@...n.com>
Cc:	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"linux\@rasmusvillemoes.dk" <linux@...musvillemoes.dk>,
	"andriy.shevchenko\@linux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"kys\@microsoft.com" <kys@...rosoft.com>
Subject: Re: [PATCH] lib/string_helpers.c: fix infinite loop in string_get_size()

James Bottomley <jbottomley@...n.com> writes:

> On Thu, 2015-09-10 at 16:08 -0700, Andrew Morton wrote:
>> On Fri,  4 Sep 2015 14:56:33 +0200 Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>> 
>> > string_get_size(1, 512, 0, ..., ...) call results in an infinite loop. The
>> > problem is that if size == 0 when we start calculating sf_cap this loop
>> > will never end.
>> > 
>> > The caller causing the issue is sd_read_capacity(), the problem was noticed
>> > on Hyper-V.
>> 
>> When fixing bugs, please provide enough info for others to be able to
>> understand which kernel version(s) need the fix.  In this case: what
>> end-user action triggers this bug?  (iow, how does sdkp->capacity
>> become zero?)
>
> Any more details.  The attached programme, which is cut straight out of
> the algorithm in string_helpers.c and modified for a C environment
> slightly (only in do_div and the typedefs) produces this
>
> hello
> STRING IS 512 B
>
> With your input, so I don't think the problem is where you think it
> is.

Sorry for delayed reply, I was traveling.

Please change
string_get_size(1, 512, STRING_UNITS_2, buf, sizeof(buf));

to
string_get_size(1, 512, STRING_UNITS_10, buf, sizeof(buf));

in your test.c program to see the issue, it will enter the infinite loop
as well.

Regardless to Hyper-V I think such library function shouldn't do such
nasty things (but I'll try to investigate why such small size was
reported).

-- 
  Vitaly
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