linux-kernel - Re: [PATCH] test_kmod: fix limit check on number of test devices created

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Date:   Tue, 27 Feb 2018 15:24:05 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jessica Yu <jeyu@...hat.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Michal Marek <mmarek@...e.com>, Petr Mladek <pmladek@...e.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Guenter Roeck <linux@...ck-us.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Matt Redfearn <matt.redfearn@...tec.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Colin King <colin.king@...onical.com>,
        Daniel Mentz <danielmentz@...gle.com>,
        David Binderman <dcb314@...mail.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] test_kmod: fix limit check on number of test devices created

On Fri, Feb 23, 2018 at 7:00 PM, Luis R. Rodriguez <mcgrof@...nel.org> wrote:
> As reported by Dan the parentheses is in the wrong place, and since
> unlikely() call returns either 0 or 1 it's never less than zero.
> The second issue is that signed integer overflows like "INT_MAX + 1" are
> undefined behavior.
>
> Since num_test_devs represents the number of devices, we want to stop
> prior to hitting the max, and not rely on the wrap arround at all. So
> just cap at num_test_devs + 1, prior to assigning a new device.
>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> Fixes: d9c6a72d6fa2 ("kmod: add test driver to stress test the module loader")
> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  lib/test_kmod.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/test_kmod.c b/lib/test_kmod.c
> index e372b97eee13..0e5b7a61460b 100644
> --- a/lib/test_kmod.c
> +++ b/lib/test_kmod.c
> @@ -1141,7 +1141,7 @@ static struct kmod_test_device *register_test_dev_kmod(void)
>         mutex_lock(&reg_dev_mutex);
>
>         /* int should suffice for number of devices, test for wrap */
> -       if (unlikely(num_test_devs + 1) < 0) {
> +       if (num_test_devs + 1 == INT_MAX) {
>                 pr_err("reached limit of number of test devices\n");
>                 goto out;
>         }
> --
> 2.16.2
>



-- 
Kees Cook
Pixel Security