linux-kernel - Re: [PATCH 2/2] mm/usercopy: enable usercopy size checking for modern versions of gcc

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Date:   Fri, 26 Aug 2016 16:00:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: Re: [PATCH 2/2] mm/usercopy: enable usercopy size checking for
 modern versions of gcc

On Fri, Aug 26, 2016 at 03:56:27PM -0500, Josh Poimboeuf wrote:
> On Fri, Aug 26, 2016 at 08:55:33AM -0500, Josh Poimboeuf wrote:
> > On Fri, Aug 26, 2016 at 09:42:42AM -0400, Kees Cook wrote:
> > > On Thu, Aug 25, 2016 at 11:27 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > > > On Thu, Aug 25, 2016 at 10:14:36PM -0400, Kees Cook wrote:
> > > >> Okay, right. __builtin_object_size() is totally fine, I absolutely
> > > >> misspoke: it's the resolution of const value ranges. I wouldn't expect
> > > >> gcc to warn here, though, since "copy + 1" isn't a const value...
> > > >
> > > > Look at the code again :-)
> > > >
> > > > __copy_to_user_overflow(), which does the "provably correct" warning, is
> > > > "called" when the copy size is non-const (and the object size is const).
> > > > So "copy + 1" being non-const is consistent with the warning.
> > > 
> > > Right, yes. Man, this is hard to read. All the names are the same. ;)
> > 
> > Yeah, agreed.  The code is way too cryptic.
> > 
> > > So this will trigger when the object size is known but the copy length
> > > is non-const?
> > 
> > Right.
> > 
> > > When I played with re-enabling this in the past, I didn't hit very
> > > many false positives. I sent a bunch of patches a few months back for
> > > legitimate problems that this warning pointed out, so I'm a bit
> > > cautious to just entirely drop it.
> > 
> > Ah, I didn't realize that.  We should definitely keep
> > DEBUG_STRICT_USER_COPY_CHECKS then.  Though it would be *really* nice to
> > find a way to associate some kind of whitelist with it to separate the
> > wheat from all the chaff.
> 
> Ok, so I could drop patch 1/2 and then resubmit 2/2 with an updated
> patch header.
> 
> There's one problem with that though.  It's going to annoy a lot of
> people who do allyesconfig/allmodconfig builds because
> DEBUG_STRICT_USER_COPY_CHECKS adds several fake warnings.
> 
> Anybody know if there's a way to disable an option for
> allyesconfig/allmodconfig?

Hm, I guess that wouldn't be good enough anyway because the build bot
randconfig builds woudn't be happy with the warnings either.  Not sure
how to keep the feature around without littering the landscape with
false positives...

-- 
Josh