linux-hardening - Re: [PATCH v2 0/3] Fix FORTIFY=y UBSAN_LOCAL

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Date:   Tue, 6 Sep 2022 09:37:44 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
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>,
        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-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v2 0/3] Fix FORTIFY=y UBSAN_LOCAL_BOUNDS=y

On Fri, Sep 2, 2022 at 1:43 PM Kees Cook <keescook@...omium.org> wrote:
>
> With CONFIG_FORTIFY=y and CONFIG_UBSAN_LOCAL_BOUNDS=y enabled, we
> observe a runtime panic while running Android's Compatibility Test
> Suite's (CTS) android.hardware.input.cts.tests. This is stemming from a
> strlen() call in hidinput_allocate().
>
> __builtin_object_size(str, 0 or 1) has interesting behavior for C
> strings when str is runtime dependent, and all possible values are known
> at compile time; it evaluates to the maximum of those sizes. This causes
> UBSAN_LOCAL_BOUNDS to insert faults for the smaller values, which we
> trip at runtime.
>
> Patch 1 is the actual fix, using a 0-index __builtin_constant_p() check
> to short-circuit the runtime check.
> Patch 2 is a KUnit test to validate this behavior going forward.
> Patch 3 is is a cosmetic cleanup to use SIZE_MAX instead of (size_t)-1

Thanks,
Testing out patch 1/3 against Android's CTS:
https://android-review.googlesource.com/c/kernel/common/+/2206839,
will give formal signoffs/review after a completed test run.

>
> -Kees
>
> v2:
>  - different solution
>  - add KUnit test
>  - expand scope of cosmetic cleanup
> v1: https://lore.kernel.org/lkml/20220830205309.312864-1-ndesaulniers@google.com
>
> Kees Cook (3):
>   fortify: Fix __compiletime_strlen() under UBSAN_BOUNDS_LOCAL
>   fortify: Add KUnit test for FORTIFY_SOURCE internals
>   fortify: Use SIZE_MAX instead of (size_t)-1
>
>  MAINTAINERS                    |  1 +
>  include/linux/fortify-string.h | 29 ++++++-------
>  lib/Kconfig.debug              |  9 ++++
>  lib/Makefile                   |  1 +
>  lib/fortify_kunit.c            | 77 ++++++++++++++++++++++++++++++++++
>  5 files changed, 103 insertions(+), 14 deletions(-)
>  create mode 100644 lib/fortify_kunit.c
>
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers