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From:   Kees Cook <keescook@chromium.org>
Date:   Mon, 25 Jun 2018 15:35:54 -0700
Message-ID: <CAGXu5jJGqxKjcWGyAnbkmFebtPor0PEQ+2qpoMCGtjjdYRTHDw@mail.gmail.com>
Subject: Re: [PATCH] add param that allows bootline control of hardened usercopy
To:     Chris von Recklinghausen <crecklin@redhat.com>
Cc:     Laura Abbott <labbott@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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On Mon, Jun 25, 2018 at 3:29 PM, Christoph von Recklinghausen
<crecklin@redhat.com> wrote:
> I have a small set of customers that want CONFIG_HARDENED_USERCOPY
> enabled, and a large number of customers who would be impacted by its
> default behavior (before my change).  The desire was to have the smaller
> number of users need to change their boot lines to get the behavior they
> wanted. Adding CONFIG_HUC_DEFAULT_OFF was an attempt to preserve the
> default behavior of existing users of CONFIG_HARDENED_USERCOPY (default
> enabled) and allowing that to coexist with the desires of the greater
> number of my customers (default disabled).
>
> If folks think that it's better to have it enabled by default and the
> command line option to turn it off I can do that (it is simpler). Does
> anyone else have opinions one way or the other?

I would prefer to isolate the actual problem case, and fix it if
possible. (i.e. try to make the copy fixed-length, etc) Barring that,
yes, a kernel command line to disable the protection would be okay.

Note that the test needs to be inside __check_object_size() otherwise
the inline optimization with __builtin_constant_p() gets broken and
makes everyone slower. :)

-Kees

-- 
Kees Cook
Pixel Security