Re: [PATCH] ubsan: remove overflow checks

[Kees Cook <keescook@chromium.org>]

On Wed, Feb 10, 2021 at 02:23:48AM +0300, Andrey Ryabinin wrote:
> Since GCC 8.0 -fsanitize=signed-integer-overflow doesn't work with -fwrapv.
> -fwrapv makes signed overflows defines and GCC essentially disables
> ubsan checks. On GCC < 8.0 -fwrapv doesn't have influence on
> -fsanitize=signed-integer-overflow setting, so it kinda works
> but generates false-positves and violates uaccess rules:
> 
> lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
> 
> Disable signed overflow checks to avoid these problems.
> Remove unsigned overflow checks as well.
> Unsigned overflow appeared as side effect of the commit
>  cdf8a76fda4a ("ubsan: move cc-option tests into Kconfig"),
> but it never worked (kernel doesn't boot). And unsigned overflows
> are allowed by C standard, so it just pointless.
> 
> Signed-off-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>

NAK, please don't remove the entire thing. I want this to work again
with -fwrapv, and it's not entirely broken under Clang. But the feature
shouldn't be removed from the kernel.

I'd prefer Peter's fix instead.

-Kees

> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> ---
>  lib/Kconfig.ubsan      | 17 -----------
>  lib/test_ubsan.c       | 49 ------------------------------
>  lib/ubsan.c            | 68 ------------------------------------------
>  scripts/Makefile.ubsan |  2 --
>  4 files changed, 136 deletions(-)
> 
> diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
> index 3a0b1c930733..e5372a13511d 100644
> --- a/lib/Kconfig.ubsan
> +++ b/lib/Kconfig.ubsan
> @@ -112,23 +112,6 @@ config UBSAN_UNREACHABLE
>  	  This option enables -fsanitize=unreachable which checks for control
>  	  flow reaching an expected-to-be-unreachable position.
>  
> -config UBSAN_SIGNED_OVERFLOW
> -	bool "Perform checking for signed arithmetic overflow"
> -	default UBSAN
> -	depends on $(cc-option,-fsanitize=signed-integer-overflow)
> -	help
> -	  This option enables -fsanitize=signed-integer-overflow which checks
> -	  for overflow of any arithmetic operations with signed integers.
> -
> -config UBSAN_UNSIGNED_OVERFLOW
> -	bool "Perform checking for unsigned arithmetic overflow"
> -	depends on $(cc-option,-fsanitize=unsigned-integer-overflow)
> -	depends on !X86_32 # avoid excessive stack usage on x86-32/clang
> -	help
> -	  This option enables -fsanitize=unsigned-integer-overflow which checks
> -	  for overflow of any arithmetic operations with unsigned integers. This
> -	  currently causes x86 to fail to boot.
> -
>  config UBSAN_OBJECT_SIZE
>  	bool "Perform checking for accesses beyond the end of objects"
>  	default UBSAN
> diff --git a/lib/test_ubsan.c b/lib/test_ubsan.c
> index 5e5d9355ef49..7e7bbd0f3fd2 100644
> --- a/lib/test_ubsan.c
> +++ b/lib/test_ubsan.c
> @@ -11,51 +11,6 @@ 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;
> -	volatile unsigned int uval = UINT_MAX;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
> -	val += 2;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
> -	uval += 2;
> -}
> -
> -static void test_ubsan_sub_overflow(void)
> -{
> -	volatile int val = INT_MIN;
> -	volatile unsigned int uval = 0;
> -	volatile int val2 = 2;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
> -	val -= val2;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
> -	uval -= val2;
> -}
> -
> -static void test_ubsan_mul_overflow(void)
> -{
> -	volatile int val = INT_MAX / 2;
> -	volatile unsigned int uval = UINT_MAX / 2;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
> -	val *= 3;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_OVERFLOW);
> -	uval *= 3;
> -}
> -
> -static void test_ubsan_negate_overflow(void)
> -{
> -	volatile int val = INT_MIN;
> -
> -	UBSAN_TEST(CONFIG_UBSAN_SIGNED_OVERFLOW);
> -	val = -val;
> -}
> -
>  static void test_ubsan_divrem_overflow(void)
>  {
>  	volatile int val = 16;
> @@ -155,10 +110,6 @@ static void test_ubsan_object_size_mismatch(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,
>  	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 bec38c64d6a6..26229973049d 100644
> --- a/lib/ubsan.c
> +++ b/lib/ubsan.c
> @@ -163,74 +163,6 @@ static void ubsan_epilogue(void)
>  	}
>  }
>  
> -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/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
> index 0e53a93e8f15..9e2092fd5206 100644
> --- a/scripts/Makefile.ubsan
> +++ b/scripts/Makefile.ubsan
> @@ -8,8 +8,6 @@ ubsan-cflags-$(CONFIG_UBSAN_LOCAL_BOUNDS)	+= -fsanitize=local-bounds
>  ubsan-cflags-$(CONFIG_UBSAN_SHIFT)		+= -fsanitize=shift
>  ubsan-cflags-$(CONFIG_UBSAN_DIV_ZERO)		+= -fsanitize=integer-divide-by-zero
>  ubsan-cflags-$(CONFIG_UBSAN_UNREACHABLE)	+= -fsanitize=unreachable
> -ubsan-cflags-$(CONFIG_UBSAN_SIGNED_OVERFLOW)	+= -fsanitize=signed-integer-overflow
> -ubsan-cflags-$(CONFIG_UBSAN_UNSIGNED_OVERFLOW)	+= -fsanitize=unsigned-integer-overflow
>  ubsan-cflags-$(CONFIG_UBSAN_OBJECT_SIZE)	+= -fsanitize=object-size
>  ubsan-cflags-$(CONFIG_UBSAN_BOOL)		+= -fsanitize=bool
>  ubsan-cflags-$(CONFIG_UBSAN_ENUM)		+= -fsanitize=enum
> -- 
> 2.26.2
> 

-- 
Kees Cook