| Message ID | 20240123002814.1396804-6-keescook@chromium.org (mailing list archive) |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | overflow: Refactor open-coded arithmetic wrap-around | expand |
On Tue, Jan 23, 2024 at 1:28 AM Kees Cook <keescook@chromium.org> wrote: > > Because the kernel is built with -fno-strict-overflow, signed and pointer > arithmetic is defined to always wrap around instead of "overflowing" > (which would either be elided due to being undefined behavior or would > wrap around, which led to very weird bugs in the kernel). By elided I guess you also mean assumed to not happen and thus the usual chain-of-logic magic? > So, the config options are added back as CONFIG_UBSAN_SIGNED_WRAP and > CONFIG_UBSAN_UNSIGNED_WRAP. Since the kernel has several places that > explicitly depend on wrap-around behavior (e.g. counters, atomics, etc), > also introduce the __signed_wrap and __unsigned_wrap function attributes > for annotating functions where wrapping is expected and should not > be caught. This will allow us to distinguish in the kernel between > intentional and unintentional cases of arithmetic wrap-around. Sounds good -- it seems to go in the direction of Rust, i.e. to have a way to mark expected wrap-arounds so that we can start catching the unintended ones. > + depends on !COMPILE_TEST > + depends on $(cc-option,-fsanitize=signed-integer-overflow) Maybe this line goes above the other, to be consistent with the unsigned case? (or the other way around) > + depends on !X86_32 # avoid excessive stack usage on x86-32/clang > + depends on !COMPILE_TEST > + help > + This option enables -fsanitize=unsigned-integer-overflow which checks > + for wrap-around of any arithmetic operations with unsigned integers. This > + currently causes x86 to fail to boot. Is it related to the excessive stack usage? In that case, users would not reach the point to see this description, right? If so, I guess it could be removed from the `help` and moved into the comment above or similar. > +static void test_ubsan_sub_overflow(void) > +{ > + volatile int val = INT_MIN; > + volatile unsigned int uval = 0; > + volatile int val2 = 2; In the other tests you use a constant instead of `val2`, I am curious if there is a reason for it? Thanks! Cheers, Miguel
On January 22, 2024 6:24:14 PM PST, Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> wrote: >On Tue, Jan 23, 2024 at 1:28 AM Kees Cook <keescook@chromium.org> wrote: >> >> Because the kernel is built with -fno-strict-overflow, signed and pointer >> arithmetic is defined to always wrap around instead of "overflowing" >> (which would either be elided due to being undefined behavior or would >> wrap around, which led to very weird bugs in the kernel). > >By elided I guess you also mean assumed to not happen and thus the >usual chain-of-logic magic? Yes. We removed this bad behavior by using -fno-strict-overflow, and we will want to keep it enabled. > >> So, the config options are added back as CONFIG_UBSAN_SIGNED_WRAP and >> CONFIG_UBSAN_UNSIGNED_WRAP. Since the kernel has several places that >> explicitly depend on wrap-around behavior (e.g. counters, atomics, etc), >> also introduce the __signed_wrap and __unsigned_wrap function attributes >> for annotating functions where wrapping is expected and should not >> be caught. This will allow us to distinguish in the kernel between >> intentional and unintentional cases of arithmetic wrap-around. > >Sounds good -- it seems to go in the direction of Rust, i.e. to have a >way to mark expected wrap-arounds so that we can start catching the >unintended ones. Yup! That's the plan. > >> + depends on !COMPILE_TEST >> + depends on $(cc-option,-fsanitize=signed-integer-overflow) > >Maybe this line goes above the other, to be consistent with the >unsigned case? (or the other way around) Sure, I can move it around. > >> + depends on !X86_32 # avoid excessive stack usage on x86-32/clang >> + depends on !COMPILE_TEST >> + help >> + This option enables -fsanitize=unsigned-integer-overflow which checks >> + for wrap-around of any arithmetic operations with unsigned integers. This >> + currently causes x86 to fail to boot. > >Is it related to the excessive stack usage? In that case, users would >not reach the point to see this description, right? If so, I guess it >could be removed from the `help` and moved into the comment above or >similar. The stack usage is separate. (This may even be fixed in modern Clang; this comes from the original version of this Kconfig.) The not booting part is separate and has not been tracked down yet. > >> +static void test_ubsan_sub_overflow(void) >> +{ >> + volatile int val = INT_MIN; >> + volatile unsigned int uval = 0; >> + volatile int val2 = 2; > >In the other tests you use a constant instead of `val2`, I am curious >if there is a reason for it? I wondered the same -- they were this way when they were removed, so I just restored them as they were. :) -Kees
On Tue, Jan 23, 2024 at 5:45 AM Kees Cook <kees@kernel.org> wrote: > > Yes. We removed this bad behavior by using -fno-strict-overflow, and we will want to keep it enabled. Yeah, I only meant that the wording of the commit seems to say there is something special about the "overflowing behavior", i.e. I was expecting just UB with the usual implications, but given the extra text in the parenthesis, I wondered while reading it if there was something different/special going on. > The stack usage is separate. (This may even be fixed in modern Clang; this comes from the original version of this Kconfig.) The not booting part is separate and has not been tracked down yet. I see. Thanks! In any case, if the sentence means only 32-bit x86, users couldn't still see it. But since this was already in the revert now that I take a look, I guess ignore this :) > I wondered the same -- they were this way when they were removed, so I just restored them as they were. :) Makes sense :) Cheers, Miguel
diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst index 270f3af13b86..aebd7c6cd2fc 100644 --- a/Documentation/process/deprecated.rst +++ b/Documentation/process/deprecated.rst @@ -141,6 +141,10 @@ replaced with a type max subtraction test instead:: ... if (INT_MAX - var < offset) ... +For inline helpers that are performing wrapping arithmetic, the entire +function can be annotated as intentionally wrapping by adding the +`__signed_wrap` or `__unsigned_wrap` function attribute. + simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull() ---------------------------------------------------------------------- The simple_strtol(), simple_strtoll(), diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index d27b58fddfaa..d24f43fc79c6 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -282,11 +282,23 @@ struct ftrace_likely_data { #define __no_sanitize_or_inline __always_inline #endif +/* Allow 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 +#ifdef CONFIG_UBSAN_UNSIGNED_WRAP +# define __unsigned_wrap __attribute__((no_sanitize("unsigned-integer-overflow"))) +#else +# define __unsigned_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 __unsigned_wrap #define noinstr __noinstr_section(".noinstr.text") diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan index 59e21bfec188..a7003e5bd2a1 100644 --- a/lib/Kconfig.ubsan +++ b/lib/Kconfig.ubsan @@ -116,6 +116,25 @@ 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. + +config UBSAN_UNSIGNED_WRAP + bool "Perform checking for unsigned arithmetic wrap-around" + depends on $(cc-option,-fsanitize=unsigned-integer-overflow) + depends on !X86_32 # avoid excessive stack usage on x86-32/clang + depends on !COMPILE_TEST + help + This option enables -fsanitize=unsigned-integer-overflow which checks + for wrap-around of any arithmetic operations with unsigned integers. This + currently causes x86 to fail to boot. + 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 2062be1f2e80..84d8092d6c32 100644 --- a/lib/test_ubsan.c +++ b/lib/test_ubsan.c @@ -11,6 +11,51 @@ 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_WRAP); + val += 2; + + UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_WRAP); + 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_WRAP); + val -= val2; + + UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_WRAP); + 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_WRAP); + val *= 3; + + UBSAN_TEST(CONFIG_UBSAN_UNSIGNED_WRAP); + uval *= 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 +135,10 @@ 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, 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.ubsan b/scripts/Makefile.ubsan index 4749865c1b2c..de4fc0ae448a 100644 --- a/scripts/Makefile.ubsan +++ b/scripts/Makefile.ubsan @@ -8,6 +8,8 @@ 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_WRAP) += -fsanitize=signed-integer-overflow +ubsan-cflags-$(CONFIG_UBSAN_UNSIGNED_WRAP) += -fsanitize=unsigned-integer-overflow ubsan-cflags-$(CONFIG_UBSAN_BOOL) += -fsanitize=bool ubsan-cflags-$(CONFIG_UBSAN_ENUM) += -fsanitize=enum ubsan-cflags-$(CONFIG_UBSAN_TRAP) += -fsanitize-undefined-trap-on-error
Effectively revert commit 6aaa31aeb9cf ("ubsan: remove overflow checks"), to allow the kernel to be built with the "*-overflow" sanitizers again. This gives developers a chance to experiment[1][2][3] with the instrumentation again, while dealing with the impact of -fno-strict-oveflow. Notably, the naming of the options is adjusted to use the name "WRAP" instead of "OVERFLOW". In the strictest sense, arithmetic "overflow" happens when a result exceeds the storage of the type, and is considered by the C standard and compilers to be undefined behavior for signed and pointer types (without -fno-strict-overflow). Unsigned arithmetic overflow is defined as always wrapping around. Because the kernel is built with -fno-strict-overflow, signed and pointer arithmetic is defined to always wrap around instead of "overflowing" (which would either be elided due to being undefined behavior or would wrap around, which led to very weird bugs in the kernel). So, the config options are added back as CONFIG_UBSAN_SIGNED_WRAP and CONFIG_UBSAN_UNSIGNED_WRAP. Since the kernel has several places that explicitly depend on wrap-around behavior (e.g. counters, atomics, etc), also introduce the __signed_wrap and __unsigned_wrap function attributes for annotating functions where wrapping is expected and should not be caught. This will allow us to distinguish in the kernel between intentional and unintentional cases of arithmetic wrap-around. Additionally keep these disabled under CONFIG_COMPILE_TEST for now. Link: https://github.com/KSPP/linux/issues/26 [1] Link: https://github.com/KSPP/linux/issues/27 [2] Link: https://github.com/KSPP/linux/issues/344 [3] Cc: Justin Stitt <justinstitt@google.com> Cc: Miguel Ojeda <ojeda@kernel.org> Cc: Nathan Chancellor <nathan@kernel.org> Cc: Nick Desaulniers <ndesaulniers@google.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Marco Elver <elver@google.com> Cc: Hao Luo <haoluo@google.com> Cc: Przemek Kitszel <przemyslaw.kitszel@intel.com> Signed-off-by: Kees Cook <keescook@chromium.org> --- Documentation/process/deprecated.rst | 4 ++ include/linux/compiler_types.h | 14 +++++- lib/Kconfig.ubsan | 19 ++++++++ lib/test_ubsan.c | 49 ++++++++++++++++++++ lib/ubsan.c | 68 ++++++++++++++++++++++++++++ lib/ubsan.h | 4 ++ scripts/Makefile.ubsan | 2 + 7 files changed, 159 insertions(+), 1 deletion(-)