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Subject: Re: [PATCH] tun: make sure interface usage can not overflow
From: Hannes Frederic Sowa <hannes@redhat.com>
To: Kees Cook <keescook@chromium.org>
Cc: David Laight <David.Laight@aculab.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
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        "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>
Date: Mon, 29 Sep 2014 22:04:58 +0200
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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