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

From: Kees Cook
Date: Tue Nov 19 2019 - 20:06:46 EST

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

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

- 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>
lib/Kconfig.ubsan | 19 +++++++++++++++++++
scripts/Makefile.ubsan | 7 ++++++-
2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index d69e8b21ebae..f5ed2dceef30 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -22,6 +22,25 @@ config UBSAN_TRAP
can just issue a trap. This reduces the kernel size overhead but
turns all warnings into full thread-killing exceptions.

+ bool "Perform array bounds checking"
+ depends on UBSAN
+ default UBSAN
+ help
+ This option enables detection of direct out of bounds array
+ accesses, where the array size is known at compile time. Note
+ that this does not protect character array overflows due to
+ bad calls to the {str,mem}*cpy() family of functions.
+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.
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)

+ CFLAGS_UBSAN += $(call cc-option, -fsanitize=bounds)
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)

CFLAGS_UBSAN += $(call cc-option, -fsanitize-undefined-trap-on-error)