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 09:42:42 -0400
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 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. ;)

So this will trigger when the object size is known but the copy length
is non-const?

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.

-Kees

-- 
Kees Cook
Nexus Security