[PATCH v2 2/3] ubsan: Split "bounds" checker from other options

[Kees Cook]

In order to do kernel builds with the bounds checker individually
available, introduce CONFIG_UBSAN_BOUNDS, with the remaining options
under CONFIG_UBSAN_MISC.

For example, using this, we can start to expand the coverage syzkaller is
providing. Right now, all of UBSan is disabled for syzbot builds because
taken as a whole, it is too noisy. This will let us focus on one feature
at a time.

For the bounds checker specifically, this provides a mechanism to
eliminate an entire class of array overflows with close to zero
performance overhead (I cannot measure a difference). In my (mostly)
defconfig, enabling bounds checking adds ~4200 checks to the kernel.
Performance changes are in the noise, likely due to the branch predictors
optimizing for the non-fail path.

Some notes on the bounds checker:

- it does not instrument {mem,str}*()-family functions, it only
  instruments direct indexed accesses (e.g. "foo[i]"). Dealing with
  the {mem,str}*()-family functions is a work-in-progress around
  CONFIG_FORTIFY_SOURCE[1].

- it ignores flexible array members, including the very old single
  byte (e.g. "int foo[1];") declarations. (Note that GCC's
  implementation appears to ignore _all_ trailing arrays, but Clang only
  ignores empty, 0, and 1 byte arrays[2].)

[1] https://github.com/KSPP/linux/issues/6
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92589

Suggested-by: Elena Petrova <lenaptr@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
---
 lib/Kconfig.ubsan      | 20 ++++++++++++++++++++
 scripts/Makefile.ubsan |  7 ++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 9deb655838b0..9b9f76d1a3f7 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -25,6 +25,26 @@ config UBSAN_TRAP
 	  the system. For some system builders this is an acceptable
 	  trade-off.
 
+config UBSAN_BOUNDS
+	bool "Perform array index bounds checking"
+	depends on UBSAN
+	default UBSAN
+	help
+	  This option enables detection of directly indexed out of bounds
+	  array accesses, where the array size is known at compile time.
+	  Note that this does not protect array overflows via bad calls
+	  to the {str,mem}*cpy() family of functions (that is addressed
+	  by CONFIG_FORTIFY_SOURCE).
+
+config UBSAN_MISC
+	bool "Enable all other Undefined Behavior sanity checks"
+	depends on UBSAN
+	default UBSAN
+	help
+	  This option enables all sanity checks that don't have their
+	  own Kconfig options. Disable this if you only want to have
+	  individually selected checks.
+
 config UBSAN_SANITIZE_ALL
 	bool "Enable instrumentation for the entire kernel"
 	depends on UBSAN
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 668a91510bfe..5b15bc425ec9 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -5,14 +5,19 @@ ifdef CONFIG_UBSAN_ALIGNMENT
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
 endif
 
+ifdef CONFIG_UBSAN_BOUNDS
+      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
+endif
+
+ifdef CONFIG_UBSAN_MISC
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=shift)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=integer-divide-by-zero)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=unreachable)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=signed-integer-overflow)
-      CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=object-size)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=bool)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=enum)
+endif
 
 ifdef CONFIG_UBSAN_TRAP
       CFLAGS_UBSAN += $(call cc-option, -fsanitize-undefined-trap-on-error)
-- 
2.17.1