FROMLIST: ubsan: Split "bounds" checker from other options

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

Bug: 136249967
Link: https://lore.kernel.org/kernel-hardening/[email protected]/
Suggested-by: Elena Petrova <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Andrey Ryabinin <[email protected]>
Signed-off-by: Elena Petrova <[email protected]>
Change-Id: I1f79faea7386af1bc50faaf8b399ea6448611d5a
Signed-off-by: UtsavBalar1231 <[email protected]>

lib/Kconfig.ubsan

@@ -23,6 +23,25 @@ config UBSAN_TRAP
 	  can just issue a trap. This reduces the kernel size overhead but
 	  turns all warnings into full thread-killing exceptions.
 
+config UBSAN_BOUNDS
+	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.
+
 config UBSAN_SANITIZE_ALL
 	bool "Enable instrumentation for the entire kernel"
 	depends on UBSAN

scripts/Makefile.ubsan

@@ -5,15 +5,20 @@ 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=vla-bound)
       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)