linux-hardening - Re: [PATCH v2 1/3] fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS

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Message-ID: <202209071613.A08F0F9225@keescook>
Date:   Wed, 7 Sep 2022 16:18:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        David Gow <davidgow@...gle.com>,
        Yury Norov <yury.norov@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Sander Vanheule <sander@...nheule.net>,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Isabella Basso <isabbasso@...eup.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Eric Biggers <ebiggers@...gle.com>,
        Hannes Reinecke <hare@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] fortify: Fix __compiletime_strlen() under
 UBSAN_BOUNDS_LOCAL

On Tue, Sep 06, 2022 at 07:36:46PM -0700, Nick Desaulniers wrote:
> On Fri, Sep 2, 2022 at 1:43 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > Co-developed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> 
> That's overly generous of you!

Well, it was a lot of work to track down, and you wrote it up that way,
I just moved things around a little bit. :)

> Anyways, the disassembly LGTM and the bot also came back green.
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Tested-by: Android Treehugger Robot
> Link: https://android-review.googlesource.com/c/kernel/common/+/2206839

Thank you!

> Another thought, Nikita suggested that you could also compare mode 1 vs mode 3:
> https://github.com/llvm/llvm-project/issues/57510#issuecomment-1235126343

Yeah, it could work (I tried this as well), but I think the better
approach is checking index 0.

> That said, since mode 3 returns 0 for "unknown" I'd imagine that
> wouldn't be pretty since it wouldn't be a direct comparison against
> __p_size.

Yeah -- it is a little weird. I might come back to this if we get more
glitches like this in the future.

-- 
Kees Cook