netdev - RE: [PATCH] tun: make sure interface usage can not overflow

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Date:	Tue, 30 Sep 2014 11:18:52 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Hannes Frederic Sowa' <hannes@...hat.com>
CC:	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jason Wang <jasowang@...hat.com>,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Tom Herbert <therbert@...gle.com>,
	Masatake YAMATO <yamato@...hat.com>, Xi Wang <xii@...gle.com>,
	"stephen hemminger" <stephen@...workplumber.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] tun: make sure interface usage can not overflow

From: Hannes Frederic 
> On Di, 2014-09-30 at 08:20 +0000, David Laight wrote:
> > From: Hannes Frederic
> > > 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@...lab.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.
> >
> > You'd have to do something in the header file - nothing in the function
> > body can do that check.
> 
> Sure, it should work. You only need to make sure that gcc inlines the
> function, so the value is constant (it is not enough that gcc knows the
> value range, one specific constant is needed). So the simplest fix for
> this is to specify __tun_chr_ioctl as __always_inline. ;)

You are joking aren't you???

Look at the code.

I'd suggest fixing whatever implements CONFIG_DEBUG_STRICT_USER_COPY_CHECKS
to be less picky.

	David