linux-kernel - Re: [PATCH 2/2] kasan: use fortified strings for hwaddress sanitizer

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Date:   Mon, 18 Oct 2021 12:57:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     linux-hardening@...r.kernel.org,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] kasan: use fortified strings for hwaddress sanitizer

On Wed, Oct 13, 2021 at 05:00:06PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> GCC has separate macros for -fsanitize=kernel-address and
> -fsanitize=kernel-hwaddress, and the check in the arm64 string.h
> gets this wrong, which leads to string functions not getting
> fortified with gcc. The newly added tests find this:
> 
> warning: unsafe memchr() usage lacked '__read_overflow' warning in /git/arm-soc/lib/test_fortify/read_overflow-memchr.c
> warning: unsafe memchr_inv() usage lacked '__read_overflow' symbol in /git/arm-soc/lib/test_fortify/read_overflow-memchr_inv.c
> warning: unsafe memcmp() usage lacked '__read_overflow' warning in /git/arm-soc/lib/test_fortify/read_overflow-memcmp.c
> warning: unsafe memscan() usage lacked '__read_overflow' symbol in /git/arm-soc/lib/test_fortify/read_overflow-memscan.c
> warning: unsafe memcmp() usage lacked '__read_overflow2' warning in /git/arm-soc/lib/test_fortify/read_overflow2-memcmp.c
> warning: unsafe memcpy() usage lacked '__read_overflow2' symbol in /git/arm-soc/lib/test_fortify/read_overflow2-memcpy.c
> warning: unsafe memmove() usage lacked '__read_overflow2' symbol in /git/arm-soc/lib/test_fortify/read_overflow2-memmove.c
> warning: unsafe memcpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-memcpy.c
> warning: unsafe memmove() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-memmove.c
> warning: unsafe memset() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-memset.c
> warning: unsafe strcpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strcpy-lit.c
> warning: unsafe strcpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strcpy.c
> warning: unsafe strlcpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strlcpy-src.c
> warning: unsafe strlcpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strlcpy.c
> warning: unsafe strncpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strncpy-src.c
> warning: unsafe strncpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strncpy.c
> warning: unsafe strscpy() usage lacked '__write_overflow' symbol in /git/arm-soc/lib/test_fortify/write_overflow-strscpy.c
> 

What is the build config that trips these warnings?

In trying to understand this, I see in arch/arm64/include/asm/string.h:

#if (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)) && \
        !defined(__SANITIZE_ADDRESS__)

other architectures (like arm32) do:

#if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)

so it's okay because it's not getting touched by the hwaddress sanitizer?
e.g. I see:

config CC_HAS_KASAN_GENERIC
        def_bool $(cc-option, -fsanitize=kernel-address)

config CC_HAS_KASAN_SW_TAGS
        def_bool $(cc-option, -fsanitize=kernel-hwaddress)

> Add a workaround to include/linux/compiler_types.h so we always
> define __SANITIZE_ADDRESS__ for either mode, as we already do
> for clang.

Where is the clang work-around? (Or is this a statement that clang,
under -fsanitize=kernel-hwaddress, already sets __SANITIZE_ADDRESS__ by
default?

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  include/linux/compiler_types.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index aad6f6408bfa..2f2776fffefe 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -178,6 +178,13 @@ struct ftrace_likely_data {
>   */
>  #define noinline_for_stack noinline
>  
> +/*
> + * Treat __SANITIZE_HWADDRESS__ the same as __SANITIZE_ADDRESS__ in the kernel
> + */
> +#ifdef __SANITIZE_HWADDRESS__
> +#define __SANITIZE_ADDRESS__
> +#endif

Should this go into compiler-gcc.h instead?

> +
>  /*
>   * Sanitizer helper attributes: Because using __always_inline and
>   * __no_sanitize_* conflict, provide helper attributes that will either expand
> -- 
> 2.29.2
> 

-- 
Kees Cook