From mboxrd@z Thu Jan  1 00:00:00 1970
From: Hannes Frederic Sowa <hannes@redhat.com>
Subject: Re: [PATCH] tun: make sure interface usage can not overflow
Date: Mon, 29 Sep 2014 22:04:58 +0200
Message-ID: <1412021098.14757.6.camel@localhost>
References: <20140928232753.GA31180@www.outflux.net>
	 <063D6719AE5E284EB5DD2968C1650D6D1749FC70@AcuExch.aculab.com>
	 <CAGXu5jKL13qdCBet3yqpc3vLYh_LifBACzL+NP8n-A+E8RMZXw@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Cc: David Laight <David.Laight@aculab.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jason Wang <jasowang@redhat.com>,
	Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Tom Herbert <therbert@google.com>,
	Masatake YAMATO <yamato@redhat.com>, Xi Wang <xii@google.com>,
	stephen hemminger <stephen@networkplumber.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
To: Kees Cook <keescook@chromium.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <CAGXu5jKL13qdCBet3yqpc3vLYh_LifBACzL+NP8n-A+E8RMZXw@mail.gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: netdev.vger.kernel.org

On Mo, 2014-09-29 at 12:41 -0700, Kees Cook wrote:
> On Mon, Sep 29, 2014 at 4:04 AM, David Laight <David.Laight@aculab.com> wrote:
> > From: Kees Cook
> >> This makes the size argument a const, since it is always populated by
> >> the caller.
> >
> > There is almost no point making parameters 'const.
> > ('const foo *' makes sense).
> >
> >> Additionally double-checks to make sure the copy_from_user
> >> can never overflow, keeping CONFIG_DEBUG_STRICT_USER_COPY_CHECKS happy:
> >>
> >>    In function 'copy_from_user',
> >>        inlined from '__tun_chr_ioctl' at drivers/net/tun.c:1871:7:
> >>        ... copy_from_user() buffer size is not provably correct
> >
> > If 'ifreq_len' could be too big then you want to error the ioctl, not panic.
> > If it can't be too big you don't need the check.
> 
> The ifreq_len comes from the callers, and is the output of "sizeof"
> which is const. Changing the function parameter to "const" means any
> changes made in the future where the incoming value isn't const, the
> compiler will throw a warning.

Hmmm, I think you want something like BUILD_BUG_ON(!
__builtin_constant_p(var)). const in function argument only ensures that
the value cannot be modified in the function.

Bye,
Hannes