Re: [PATCH v3] ubsan: Reintroduce signed overflow sanitizer

[Justin Stitt <justinstitt@google.com>]

Hi,

On Mon, Feb 05, 2024 at 01:37:29AM -0800, Kees Cook wrote:
> In order to mitigate unexpected signed wrap-around[1], bring back the
> signed integer overflow sanitizer. It was removed in commit 6aaa31aeb9cf
> ("ubsan: remove overflow checks") because it was effectively a no-op
> when combined with -fno-strict-overflow (which correctly changes signed
> overflow from being "undefined" to being explicitly "wrap around").
>
> Compilers are adjusting their sanitizers to trap wrap-around and to
> detecting common code patterns that should not be instrumented
> (e.g. "var + offset < var"). Prepare for this and explicitly rename
> the option from "OVERFLOW" to "WRAP".
>
> To annotate intentional wrap-around arithmetic, the add/sub/mul_wrap()
> helpers can be used for individual statements. At the function level,
> the __signed_wrap attribute can be used to mark an entire function as
> expecting its signed arithmetic to wrap around. For a single object file
> the Makefile can use "UBSAN_WRAP_SIGNED_target.o := n" to mark it as
> wrapping, and for an entire directory, "UBSAN_WRAP_SIGNED := n" can be
> used.
>
> Additionally keep these disabled under CONFIG_COMPILE_TEST for now.
>
> Link: https://github.com/KSPP/linux/issues/26 [1]
> Cc: Justin Stitt <justinstitt@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Hao Luo <haoluo@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> v3:
>  - split out signed overflow sanitizer so we can do each separately
> v2: https://lore.kernel.org/all/20240202101311.it.893-kees@kernel.org/
> v1: https://lore.kernel.org/all/20240129175033.work.813-kees@kernel.org/
> ---
>  include/linux/compiler_types.h |  9 ++++-
>  lib/Kconfig.ubsan              | 14 +++++++
>  lib/test_ubsan.c               | 37 ++++++++++++++++++
>  lib/ubsan.c                    | 68 ++++++++++++++++++++++++++++++++++
>  lib/ubsan.h                    |  4 ++
>  scripts/Makefile.lib           |  3 ++
>  scripts/Makefile.ubsan         |  3 ++
>  7 files changed, 137 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 6f1ca49306d2..ee9d272008a5 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -282,11 +282,18 @@ struct ftrace_likely_data {
>  #define __no_sanitize_or_inline __always_inline
>  #endif
>
> +/* Do not trap wrapping arithmetic within an annotated function. */
> +#ifdef CONFIG_UBSAN_SIGNED_WRAP
> +# define __signed_wrap __attribute__((no_sanitize("signed-integer-overflow")))
> +#else
> +# define __signed_wrap
> +#endif
> +
>  /* Section for code which can't be instrumented at all */
>  #define __noinstr_section(section)					\
>  	noinline notrace __attribute((__section__(section)))		\
>  	__no_kcsan __no_sanitize_address __no_profile __no_sanitize_coverage \
> -	__no_sanitize_memory
> +	__no_sanitize_memory __signed_wrap
>
>  #define noinstr __noinstr_section(".noinstr.text")
>
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index 56d7653f4941..129e9bc21877 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -116,6 +116,20 @@ config UBSAN_UNREACHABLE
>  	  This option enables -fsanitize=unreachable which checks for control
>  	  flow reaching an expected-to-be-unreachable position.
>
> +config UBSAN_SIGNED_WRAP
> +	bool "Perform checking for signed arithmetic wrap-around"
> +	default UBSAN
> +	depends on !COMPILE_TEST
> +	depends on $(cc-option,-fsanitize=signed-integer-overflow)
> +	help
> +	  This option enables -fsanitize=signed-integer-overflow which checks
> +	  for wrap-around of any arithmetic operations with signed integers.
> +	  This currently performs nearly no instrumentation due to the
> +	  kernel's use of -fno-strict-overflow which converts all would-be
> +	  arithmetic undefined behavior into wrap-around arithmetic. Future
> +	  sanitizer versions will allow for wrap-around checking (rather than
> +	  exclusively undefined behavior).
> +
>  config UBSAN_BOOL
>  	bool "Perform checking for non-boolean values used as boolean"
>  	default UBSAN
> diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
> index f4ee2484d4b5..276c12140ee2 100644
> --- a/lib/test_ubsan.c
> +++ b/lib/test_ubsan.c
> @@ -11,6 +11,39 @@ typedef void(*test_ubsan_fp)(void);
>  			#config, IS_ENABLED(config) ? "y" : "n");	\
>  	} while (0)
>
> +static void test_ubsan_add_overflow(void)
> +{
> +	volatile int val = INT_MAX;
> +
> +	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
> +	val += 2;
> +}
> +
> +static void test_ubsan_sub_overflow(void)
> +{
> +	volatile int val = INT_MIN;
> +	volatile int val2 = 2;
> +
> +	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
> +	val -= val2;
> +}
> +
> +static void test_ubsan_mul_overflow(void)
> +{
> +	volatile int val = INT_MAX / 2;
> +
> +	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
> +	val *= 3;
> +}
> +
> +static void test_ubsan_negate_overflow(void)
> +{
> +	volatile int val = INT_MIN;
> +
> +	UBSAN_TEST(CONFIG_UBSAN_SIGNED_WRAP);
> +	val = -val;
> +}
> +
>  static void test_ubsan_divrem_overflow(void)
>  {
>  	volatile int val = 16;
> @@ -90,6 +123,10 @@ static void test_ubsan_misaligned_access(void)
>  }
>
>  static const test_ubsan_fp test_ubsan_array[] = {
> +	test_ubsan_add_overflow,
> +	test_ubsan_sub_overflow,
> +	test_ubsan_mul_overflow,
> +	test_ubsan_negate_overflow,

I wouldn't mind also seeing a test_ubsan_div_overflow test case here.

It has some quirky behavior and it'd be nice to test that the sanitizers
properly capture it.

Check out this Godbolt: https://godbolt.org/z/qG5f1j6n1

tl;dr: with -fsanitize=signed-integer-overflow division (/) and
remainder (%) operators still instrument arithmetic even with
-fno-strict-overflow on.

This makes sense as division by 0 and INT_MIN/-1 are UBs that are not
influenced by -fno-strict-overflow.

Really though, the patch is fine and the above test case is optional and
can be shipped later -- as such:

Reviewed-by: Justin Stitt <justinstitt@google.com>

>  	test_ubsan_shift_out_of_bounds,
>  	test_ubsan_out_of_bounds,
>  	test_ubsan_load_invalid_value,
> diff --git a/lib/ubsan.c b/lib/ubsan.c
> index df4f8d1354bb..5fc107f61934 100644
> --- a/lib/ubsan.c
> +++ b/lib/ubsan.c
> @@ -222,6 +222,74 @@ static void ubsan_epilogue(void)
>  	check_panic_on_warn("UBSAN");
>  }
>
> +static void handle_overflow(struct overflow_data *data, void *lhs,
> +			void *rhs, char op)
> +{
> +
> +	struct type_descriptor *type = data->type;
> +	char lhs_val_str[VALUE_LENGTH];
> +	char rhs_val_str[VALUE_LENGTH];
> +
> +	if (suppress_report(&data->location))
> +		return;
> +
> +	ubsan_prologue(&data->location, type_is_signed(type) ?
> +			"signed-integer-overflow" :
> +			"unsigned-integer-overflow");
> +
> +	val_to_string(lhs_val_str, sizeof(lhs_val_str), type, lhs);
> +	val_to_string(rhs_val_str, sizeof(rhs_val_str), type, rhs);
> +	pr_err("%s %c %s cannot be represented in type %s\n",
> +		lhs_val_str,
> +		op,
> +		rhs_val_str,
> +		type->type_name);
> +
> +	ubsan_epilogue();
> +}
> +
> +void __ubsan_handle_add_overflow(void *data,
> +				void *lhs, void *rhs)
> +{
> +
> +	handle_overflow(data, lhs, rhs, '+');
> +}
> +EXPORT_SYMBOL(__ubsan_handle_add_overflow);
> +
> +void __ubsan_handle_sub_overflow(void *data,
> +				void *lhs, void *rhs)
> +{
> +	handle_overflow(data, lhs, rhs, '-');
> +}
> +EXPORT_SYMBOL(__ubsan_handle_sub_overflow);
> +
> +void __ubsan_handle_mul_overflow(void *data,
> +				void *lhs, void *rhs)
> +{
> +	handle_overflow(data, lhs, rhs, '*');
> +}
> +EXPORT_SYMBOL(__ubsan_handle_mul_overflow);
> +
> +void __ubsan_handle_negate_overflow(void *_data, void *old_val)
> +{
> +	struct overflow_data *data = _data;
> +	char old_val_str[VALUE_LENGTH];
> +
> +	if (suppress_report(&data->location))
> +		return;
> +
> +	ubsan_prologue(&data->location, "negation-overflow");
> +
> +	val_to_string(old_val_str, sizeof(old_val_str), data->type, old_val);
> +
> +	pr_err("negation of %s cannot be represented in type %s:\n",
> +		old_val_str, data->type->type_name);
> +
> +	ubsan_epilogue();
> +}
> +EXPORT_SYMBOL(__ubsan_handle_negate_overflow);
> +
> +
>  void __ubsan_handle_divrem_overflow(void *_data, void *lhs, void *rhs)
>  {
>  	struct overflow_data *data = _data;
> diff --git a/lib/ubsan.h b/lib/ubsan.h
> index 5d99ab81913b..0abbbac8700d 100644
> --- a/lib/ubsan.h
> +++ b/lib/ubsan.h
> @@ -124,6 +124,10 @@ typedef s64 s_max;
>  typedef u64 u_max;
>  #endif
>
> +void __ubsan_handle_add_overflow(void *data, void *lhs, void *rhs);
> +void __ubsan_handle_sub_overflow(void *data, void *lhs, void *rhs);
> +void __ubsan_handle_mul_overflow(void *data, void *lhs, void *rhs);
> +void __ubsan_handle_negate_overflow(void *_data, void *old_val);
>  void __ubsan_handle_divrem_overflow(void *_data, void *lhs, void *rhs);
>  void __ubsan_handle_type_mismatch(struct type_mismatch_data *data, void *ptr);
>  void __ubsan_handle_type_mismatch_v1(void *_data, void *ptr);
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 52efc520ae4f..7ce8ecccc65a 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -177,6 +177,9 @@ ifeq ($(CONFIG_UBSAN),y)
>  _c_flags += $(if $(patsubst n%,, \
>  		$(UBSAN_SANITIZE_$(basetarget).o)$(UBSAN_SANITIZE)y), \
>  		$(CFLAGS_UBSAN))
> +_c_flags += $(if $(patsubst n%,, \
> +		$(UBSAN_WRAP_SIGNED_$(basetarget).o)$(UBSAN_SANITIZE_$(basetarget).o)$(UBSAN_WRAP_SIGNED)$(UBSAN_SANITIZE)y), \
> +		$(CFLAGS_UBSAN_WRAP_SIGNED))
>  endif
>
>  ifeq ($(CONFIG_KCOV),y)
> diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> index 7cf42231042b..bc957add0b4d 100644
> --- a/scripts/Makefile.ubsan
> +++ b/scripts/Makefile.ubsan
> @@ -13,3 +13,6 @@ ubsan-cflags-$(CONFIG_UBSAN_ENUM)		+= -fsanitize=enum
>  ubsan-cflags-$(CONFIG_UBSAN_TRAP)		+= $(call cc-option,-fsanitize-trap=undefined,-fsanitize-undefined-trap-on-error)
>
>  export CFLAGS_UBSAN := $(ubsan-cflags-y)
> +
> +ubsan-wrap-signed-cflags-$(CONFIG_UBSAN_SIGNED_WRAP)     += -fsanitize=signed-integer-overflow
> +export CFLAGS_UBSAN_WRAP_SIGNED := $(ubsan-wrap-signed-cflags-y)
> --
> 2.34.1
>

Thanks
Justin