[PATCH 1/3] ubsan: Add trap instrumentation option

[Kees Cook <keescook@chromium.org>]

The Undefined Behavior Sanitizer can operate in two modes: warning
reporting mode via lib/ubsan.c handler calls, or trap mode, which uses
__builtin_trap() as the handler. Using lib/ubsan.c means the kernel
image is about 5% larger (due to all the debugging text and reporting
structures to capture details about the warning conditions). Using the
trap mode, the image size changes are much smaller, though at the loss
of the "warning only" mode.

In order to give greater flexibility to system builders that want
minimal changes to image size and are prepared to deal with kernel
threads being killed, this introduces CONFIG_UBSAN_TRAP. The resulting
image sizes comparison:

   text    data     bss       dec       hex     filename
19533663   6183037  18554956  44271656  2a38828 vmlinux.stock
19991849   7618513  18874448  46484810  2c54d4a vmlinux.ubsan
19712181   6284181  18366540  44362902  2a4ec96 vmlinux.ubsan-trap

CONFIG_UBSAN=y:      image +4.8% (text +2.3%, data +18.9%)
CONFIG_UBSAN_TRAP=y: image +0.2% (text +0.9%, data +1.6%)

Suggested-by: Elena Petrova <lenaptr@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 lib/Kconfig.ubsan      | 15 +++++++++++++--
 lib/Makefile           |  2 ++
 scripts/Makefile.ubsan |  9 +++++++--
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 0e04fcb3ab3d..d69e8b21ebae 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -5,12 +5,23 @@ config ARCH_HAS_UBSAN_SANITIZE_ALL
 config UBSAN
 	bool "Undefined behaviour sanity checker"
 	help
-	  This option enables undefined behaviour sanity checker
+	  This option enables undefined behaviour sanity checker.
 	  Compile-time instrumentation is used to detect various undefined
-	  behaviours in runtime. Various types of checks may be enabled
+	  behaviours at runtime. Various types of checks may be enabled
 	  via boot parameter ubsan_handle
 	  (see: Documentation/dev-tools/ubsan.rst).
 
+config UBSAN_TRAP
+	bool "On Sanitizer warnings, stop the offending kernel thread"
+	depends on UBSAN
+	depends on $(cc-option, -fsanitize-undefined-trap-on-error)
+	help
+	  Building kernels with Sanitizer features enabled tends to grow
+	  the kernel size by over 5%, due to adding all the debugging
+	  text on failure paths. To avoid this, Sanitizer instrumentation
+	  can just issue a trap. This reduces the kernel size overhead but
+	  turns all warnings into full thread-killing exceptions.
+
 config UBSAN_SANITIZE_ALL
 	bool "Enable instrumentation for the entire kernel"
 	depends on UBSAN
diff --git a/lib/Makefile b/lib/Makefile
index c5892807e06f..bc498bf0f52d 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -272,7 +272,9 @@ quiet_cmd_build_OID_registry = GEN     $@
 clean-files	+= oid_registry_data.c
 
 obj-$(CONFIG_UCS2_STRING) += ucs2_string.o
+ifneq ($(CONFIG_UBSAN_TRAP),y)
 obj-$(CONFIG_UBSAN) += ubsan.o
+endif
 
 UBSAN_SANITIZE_ubsan.o := n
 KASAN_SANITIZE_ubsan.o := n
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 019771b845c5..668a91510bfe 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -1,5 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0
 ifdef CONFIG_UBSAN
+
+ifdef CONFIG_UBSAN_ALIGNMENT
+      CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
+endif
+
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=shift)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=integer-divide-by-zero)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=unreachable)
@@ -9,8 +14,8 @@ ifdef CONFIG_UBSAN
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=bool)
       CFLAGS_UBSAN += $(call cc-option, -fsanitize=enum)
 
-ifdef CONFIG_UBSAN_ALIGNMENT
-      CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
+ifdef CONFIG_UBSAN_TRAP
+      CFLAGS_UBSAN += $(call cc-option, -fsanitize-undefined-trap-on-error)
 endif
 
       # -fsanitize=* options makes GCC less smart than usual and
-- 
2.17.1