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 08:55:33 -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 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.

-- 
Josh