Re: [PATCH v2 2/6] ubsan: Reintroduce signed and unsigned overflow sanitizers

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 compilers adjust their sanitizers
to deal with the impact of -fno-strict-oveflow (i.e. moving from
"overflow" checking to "wrap-around" checking).

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 could 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, crypto,
etc), also introduce the __signed_wrap and __unsigned_wrap function
attributes for annotating functions where wrapping is expected and should
not be instrumented. 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 <redacted>
Cc: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Kees Cook <redacted>
---
 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 +
 6 files changed, 155 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6f1ca49306d2..e585614f3152 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.