ubsan: introduce CONFIG_UBSAN_LOCAL_BOUNDS for Clang

When the kernel is compiled with Clang, -fsanitize=bounds expands to
-fsanitize=array-bounds and -fsanitize=local-bounds.

Enabling -fsanitize=local-bounds with Clang has the unfortunate
side-effect of inserting traps; this goes back to its original intent,
which was as a hardening and not a debugging feature [1].  The same
feature made its way into -fsanitize=bounds, but the traps remained.  For
that reason, -fsanitize=bounds was split into 'array-bounds' and
'local-bounds' [2].

Since 'local-bounds' doesn't behave like a normal sanitizer, enable it
with Clang only if trapping behaviour was requested by
CONFIG_UBSAN_TRAP=y.

Add the UBSAN_BOUNDS_LOCAL config to Kconfig.ubsan to enable the
'local-bounds' option by default when UBSAN_TRAP is enabled.

[1] http://lists.llvm.org/pipermail/llvm-dev/2012-May/049972.html
[2] http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20131021/091536.html

Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: George Popescu <georgepope@android.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: David Brazdil <dbrazdil@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lkml.kernel.org/r/20200922074330.2549523-1-georgepope@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.ubsan

@@ -47,6 +47,20 @@ config UBSAN_BOUNDS
 	  to the {str,mem}*cpy() family of functions (that is addressed
 	  by CONFIG_FORTIFY_SOURCE).
 
+config UBSAN_LOCAL_BOUNDS
+	bool "Perform array local bounds checking"
+	depends on UBSAN_TRAP
+	depends on CC_IS_CLANG
+	depends on !UBSAN_KCOV_BROKEN
+	help
+	  This option enables -fsanitize=local-bounds which traps when an
+	  exception/error is detected. Therefore, it should be enabled only
+	  if trapping is expected.
+	  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.
+
 config UBSAN_MISC
 	bool "Enable all other Undefined Behavior sanity checks"
 	default UBSAN

scripts/Makefile.ubsan

@@ -4,7 +4,15 @@ ifdef CONFIG_UBSAN_ALIGNMENT
 endif
 
 ifdef CONFIG_UBSAN_BOUNDS
-      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      ifdef CONFIG_CC_IS_CLANG
+            CFLAGS_UBSAN += -fsanitize=array-bounds
+      else
+            CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+      endif
+endif
+
+ifdef CONFIG_UBSAN_LOCAL_BOUNDS
+      CFLAGS_UBSAN += -fsanitize=local-bounds
 endif
 
 ifdef CONFIG_UBSAN_MISC