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Date: Fri, 3 Mar 2023 08:44:33 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Marco Elver <elver@...gle.com>,
Masahiro Yamada <masahiroy@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Nicolas Schier <nicolas@...sle.eu>, Tom Rix <trix@...hat.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Miroslav Benes <mbenes@...e.cz>, linux-kbuild@...r.kernel.org,
llvm@...ts.linux.dev,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] ubsan: Tighten UBSAN_BOUNDS on GCC
On Thu, Mar 02, 2023 at 02:54:45PM -0800, Kees Cook wrote:
> The use of -fsanitize=bounds on GCC will ignore some trailing arrays,
> leaving a gap in coverage. Switch to using -fsanitize=bounds-strict to
> match Clang's stricter behavior.
>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Nicolas Schier <nicolas@...sle.eu>
> Cc: Tom Rix <trix@...hat.com>
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Miroslav Benes <mbenes@...e.cz>
> Cc: linux-kbuild@...r.kernel.org
> Cc: llvm@...ts.linux.dev
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> lib/Kconfig.ubsan | 54 +++++++++++++++++++++++-------------------
> scripts/Makefile.ubsan | 2 +-
> 2 files changed, 30 insertions(+), 26 deletions(-)
>
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index fd15230a703b..9d3e87a0b6d1 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -27,16 +27,27 @@ config UBSAN_TRAP
> the system. For some system builders this is an acceptable
> trade-off.
>
> -config CC_HAS_UBSAN_BOUNDS
> - def_bool $(cc-option,-fsanitize=bounds)
> +config CC_HAS_UBSAN_BOUNDS_STRICT
> + def_bool $(cc-option,-fsanitize=bounds-strict)
> + help
> + The -fsanitize=bounds-strict option is only available on GCC,
> + but uses the more strict handling of arrays that includes knowledge
> + of flexible arrays, which is comparable to Clang's regular
> + -fsanitize=bounds.
>
> config CC_HAS_UBSAN_ARRAY_BOUNDS
> def_bool $(cc-option,-fsanitize=array-bounds)
> + help
> + The -fsanitize=array-bounds option is only available on Clang,
> + and is actually composed of two more specific options,
> + -fsanitize=array-bounds and -fsanitize=local-bounds. However,
> + -fsanitize=local-bounds can only be used when trap mode is
> + enabled. (See also the help for CONFIG_LOCAL_BOUNDS.)
The first sentence does not read right to me, you have array-bounds
twice. I think the first one wants to be just bounds?
> config UBSAN_BOUNDS
> bool "Perform array index bounds checking"
> default UBSAN
> - depends on CC_HAS_UBSAN_ARRAY_BOUNDS || CC_HAS_UBSAN_BOUNDS
> + depends on CC_HAS_UBSAN_ARRAY_BOUNDS || CC_HAS_UBSAN_BOUNDS_STRICT
> help
> This option enables detection of directly indexed out of bounds
> array accesses, where the array size is known at compile time.
> @@ -44,33 +55,26 @@ config UBSAN_BOUNDS
> to the {str,mem}*cpy() family of functions (that is addressed
> by CONFIG_FORTIFY_SOURCE).
>
> -config UBSAN_ONLY_BOUNDS
> - def_bool CC_HAS_UBSAN_BOUNDS && !CC_HAS_UBSAN_ARRAY_BOUNDS
> - depends on UBSAN_BOUNDS
> +config UBSAN_BOUNDS_STRICT
> + def_bool UBSAN_BOUNDS && CC_HAS_UBSAN_BOUNDS_STRICT
> help
> - This is a weird case: Clang's -fsanitize=bounds includes
> - -fsanitize=local-bounds, but it's trapping-only, so for
> - Clang, we must use -fsanitize=array-bounds when we want
> - traditional array bounds checking enabled. For GCC, we
> - want -fsanitize=bounds.
> + GCC's bounds sanitizer. This option is used to select the
> + correct options in Makefile.ubsan.
>
> config UBSAN_ARRAY_BOUNDS
> - def_bool CC_HAS_UBSAN_ARRAY_BOUNDS
> - depends on UBSAN_BOUNDS
> + def_bool UBSAN_BOUNDS && CC_HAS_UBSAN_ARRAY_BOUNDS
> + help
> + Clang's array bounds sanitizer. This option is used to select
> + the correct options in Makefile.ubsan.
>
> config UBSAN_LOCAL_BOUNDS
> - bool "Perform array local bounds checking"
> - depends on UBSAN_TRAP
> - depends on $(cc-option,-fsanitize=local-bounds)
> - help
> - This option enables -fsanitize=local-bounds which traps when an
> - exception/error is detected. Therefore, it may only be enabled
> - with CONFIG_UBSAN_TRAP.
> -
> - Enabling this option detects errors due to accesses through a
> - pointer that is derived from an object of a statically-known size,
> - where an added offset (which may not be known statically) is
> - out-of-bounds.
> + def_bool UBSAN_ARRAY_BOUNDS && UBSAN_TRAP
> + help
> + This option enables Clang's -fsanitize=local-bounds which traps
> + when an access through a pointer that is derived from an object
> + of a statically-known size, where an added offset (which may not
> + be known statically) is out-of-bounds. Since this option is
> + trap-only, it depends on CONFIG_UBSAN_TRAP.
>
> config UBSAN_SHIFT
> bool "Perform checking for bit-shift overflows"
> diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> index 7099c603ff0a..4749865c1b2c 100644
> --- a/scripts/Makefile.ubsan
> +++ b/scripts/Makefile.ubsan
> @@ -2,7 +2,7 @@
>
> # Enable available and selected UBSAN features.
> ubsan-cflags-$(CONFIG_UBSAN_ALIGNMENT) += -fsanitize=alignment
> -ubsan-cflags-$(CONFIG_UBSAN_ONLY_BOUNDS) += -fsanitize=bounds
> +ubsan-cflags-$(CONFIG_UBSAN_BOUNDS_STRICT) += -fsanitize=bounds-strict
> ubsan-cflags-$(CONFIG_UBSAN_ARRAY_BOUNDS) += -fsanitize=array-bounds
> ubsan-cflags-$(CONFIG_UBSAN_LOCAL_BOUNDS) += -fsanitize=local-bounds
> ubsan-cflags-$(CONFIG_UBSAN_SHIFT) += -fsanitize=shift
> --
> 2.34.1
>
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