Date:   Wed, 19 Oct 2022 09:48:27 -0700
From:   Alexander Potapenko <glider@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, kasan-dev@...glegroups.com,
        linux-mm@...ck.org
Subject: Re: -Wmacro-redefined in include/linux/fortify-string.h

On Wed, Oct 19, 2022 at 8:37 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> Hi all,
>
> I am seeing the following set of warnings when building an x86_64
> configuration that has CONFIG_FORTIFY_SOURCE=y and CONFIG_KMSAN=y:

I was also looking into this issue recently, because people start
running into it: https://github.com/google/kmsan/issues/89

I have a solution that redefines __underlying_memXXX to __msan_memXXX
under __SANITIZE_MEMORY__ in fortify-string.h and skips `#define
memXXX __msan_memXXX` in string_64.h, making KMSAN kinda work with
FORTIFY_SOURCE.
Dunno if that's necessary though: KMSAN is a debugging tool anyway,
and supporting it in fortify-string.h sounds excessive.

So I'm fine with disabling FORTIFY_STRING under KMSAN, unless someone objects.

>   In file included from scripts/mod/devicetable-offsets.c:3:
>   In file included from ./include/linux/mod_devicetable.h:13:
>   In file included from ./include/linux/uuid.h:12:
>   In file included from ./include/linux/string.h:253:
>   ./include/linux/fortify-string.h:496:9: error: 'memcpy' macro redefined [-Werror,-Wmacro-redefined]
>   #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                  \
>           ^
>   ./arch/x86/include/asm/string_64.h:17:9: note: previous definition is here
>   #define memcpy __msan_memcpy
>           ^
>   In file included from scripts/mod/devicetable-offsets.c:3:
>   In file included from ./include/linux/mod_devicetable.h:13:
>   In file included from ./include/linux/uuid.h:12:
>   In file included from ./include/linux/string.h:253:
>   ./include/linux/fortify-string.h:500:9: error: 'memmove' macro redefined [-Werror,-Wmacro-redefined]
>   #define memmove(p, q, s)  __fortify_memcpy_chk(p, q, s,                 \
>           ^
>   ./arch/x86/include/asm/string_64.h:73:9: note: previous definition is here
>   #define memmove __msan_memmove
>           ^
>   2 errors generated.
>
> I can see that commit ff901d80fff6 ("x86: kmsan: use __msan_ string
> functions where possible.") appears to include a fix up for this warning
> with memset() but not memcpy() or memmove(). If I apply a similar fix up
> like so:
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 4029fe368a4f..718ee17b31e3 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -493,6 +493,7 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
>   * __struct_size() vs __member_size() must be captured here to avoid
>   * evaluating argument side-effects further into the macro layers.
>   */
> +#ifndef CONFIG_KMSAN
>  #define memcpy(p, q, s)  __fortify_memcpy_chk(p, q, s,                 \
>                 __struct_size(p), __struct_size(q),                     \
>                 __member_size(p), __member_size(q),                     \
> @@ -501,6 +502,7 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
>                 __struct_size(p), __struct_size(q),                     \
>                 __member_size(p), __member_size(q),                     \
>                 memmove)
> +#endif
>
>  extern void *__real_memscan(void *, int, __kernel_size_t) __RENAME(memscan);
>  __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
>
> Then the instances of -Wmacro-redefined disappear but the fortify tests
> no longer pass for somewhat obvious reasons:
>
>   warning: unsafe memcpy() usage lacked '__read_overflow2' symbol in lib/test_fortify/read_overflow2-memcpy.c
>   warning: unsafe memmove() usage lacked '__read_overflow2' symbol in lib/test_fortify/read_overflow2-memmove.c
>   warning: unsafe memcpy() usage lacked '__read_overflow2_field' symbol in lib/test_fortify/read_overflow2_field-memcpy.c
>   warning: unsafe memmove() usage lacked '__read_overflow2_field' symbol in lib/test_fortify/read_overflow2_field-memmove.c
>   warning: unsafe memcpy() usage lacked '__write_overflow' symbol in lib/test_fortify/write_overflow-memcpy.c
>   warning: unsafe memmove() usage lacked '__write_overflow' symbol in lib/test_fortify/write_overflow-memmove.c
>   warning: unsafe memset() usage lacked '__write_overflow' symbol in lib/test_fortify/write_overflow-memset.c
>   warning: unsafe memcpy() usage lacked '__write_overflow_field' symbol in lib/test_fortify/write_overflow_field-memcpy.c
>   warning: unsafe memmove() usage lacked '__write_overflow_field' symbol in lib/test_fortify/write_overflow_field-memmove.c
>   warning: unsafe memset() usage lacked '__write_overflow_field' symbol in lib/test_fortify/write_overflow_field-memset.c
>
> Should CONFIG_KMSAN depend on CONFIG_FORTIFY_SOURCE=n like so? It seems
> like the two features are incompatible if I am reading ff901d80fff6
> correctly.
>
> diff --git a/lib/Kconfig.kmsan b/lib/Kconfig.kmsan
> index b2489dd6503f..6a681621e3c5 100644
> --- a/lib/Kconfig.kmsan
> +++ b/lib/Kconfig.kmsan
> @@ -11,7 +11,7 @@ config HAVE_KMSAN_COMPILER
>  config KMSAN
>         bool "KMSAN: detector of uninitialized values use"
>         depends on HAVE_ARCH_KMSAN && HAVE_KMSAN_COMPILER
> -       depends on SLUB && DEBUG_KERNEL && !KASAN && !KCSAN
> +       depends on SLUB && DEBUG_KERNEL && !KASAN && !KCSAN && !FORTIFY_SOURCE
>         select STACKDEPOT
>         select STACKDEPOT_ALWAYS_INIT
>         help
>
> or is there a different obvious fix that I am missing?
>
> Cheers,
> Nathan



--
Alexander Potapenko
Software Engineer

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