linux-kernel - Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMj1kXH0gRCaoF0NziC870=eSEy0ghi8b0b6A+LMu8PMd58C0Q@mail.gmail.com>
Date:   Tue, 18 Aug 2020 09:10:01 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Joe Perches <joe@...ches.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>, X86 ML <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Marco Elver <elver@...gle.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@...il.com>,
        Andi Kleen <ak@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

On Tue, 18 Aug 2020 at 00:02, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings. This optimization was introduced into
> clang-12. Because the kernel does not provide an implementation of
> stpcpy, we observe linkage failures for almost all targets when building
> with ToT clang.
>
> The interface is unsafe as it does not perform any bounds checking.
> Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
>
> Unlike
> commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> which cited failures with `-fno-builtin-*` flags being retained in LLVM
> LTO, that bug seems to have been fixed by
> https://reviews.llvm.org/D71193, so the above sha can now be reverted in
> favor of `-fno-builtin-bcmp`.
>
> Cc: stable@...r.kernel.org # 4.4

Why does a fix for Clang-12 have to be backported all the way to v4.4?
How does that meet the requirements for stable patches?

> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://reviews.llvm.org/D85963
> Reported-by: Sami Tolvanen <samitolvanen@...gle.com>
> Suggested-by: Dávid Bolvanský <david.bolvansky@...il.com>
> Suggested-by: Kees Cook <keescook@...omium.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 9cac6fde3479..211a1b6f6478 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -959,6 +959,12 @@ ifdef CONFIG_RETPOLINE
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
>  endif
>
> +# The compiler may "libcall optimize" certain function calls into the below
> +# functions, for architectures that don't use -ffreestanding. If we don't plan
> +# to provide implementations of these routines, then prevent the compiler from
> +# emitting calls to what will be undefined symbols.
> +KBUILD_CFLAGS  += -fno-builtin-stpcpy
> +
>  # include additional Makefiles when needed
>  include-y                      := scripts/Makefile.extrawarn
>  include-$(CONFIG_KASAN)                += scripts/Makefile.kasan
> --
> 2.28.0.220.ged08abb693-goog
>