[PATCH] kbuild: ubsan: skip UBSAN for external modules by default

[Ferran Duarri]

External modules built with KBUILD_EXTMOD set inherit UBSAN sanitizer
flags from the kernel's KBUILD_CFLAGS because is-kernel-object is 'y'
for any obj-m object. This silently breaks third-party modules whose
source code (e.g. VMware vmnet/vmmon) triggers UBSAN UB checks at
runtime, causing packet-forwarding failures and VM instability while
the module itself loads successfully.

Make UBSAN opt-in for external modules: skip the is-kernel-object 'y'
fallback when KBUILD_EXTMOD is set. External modules that explicitly
need UBSAN can still opt in by setting UBSAN_SANITIZE := y.

In-kernel module builds are not affected.
Signed-off-by: Ferran Duarri <ferran.duarri@xxxxxx>
---
 scripts/Makefile.lib | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 0718e39cedda..393af3f970af 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -74,10 +74,10 @@ endif
 
 ifeq ($(CONFIG_UBSAN),y)
 _c_flags += $(if $(patsubst n%,, \
-		$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SANITIZE)$(is-kernel-object)), \
+		$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_SANITIZE)$(if $(KBUILD_EXTMOD),,$(is-kernel-object))), \
 		$(CFLAGS_UBSAN))
 _c_flags += $(if $(patsubst n%,, \
-		$(UBSAN_INTEGER_WRAP_$(target-stem).o)$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_INTEGER_WRAP)$(UBSAN_SANITIZE)$(is-kernel-object)), \
+		$(UBSAN_INTEGER_WRAP_$(target-stem).o)$(UBSAN_SANITIZE_$(target-stem).o)$(UBSAN_INTEGER_WRAP)$(UBSAN_SANITIZE)$(if $(KBUILD_EXTMOD),,$(is-kernel-object))), \
 		$(CFLAGS_UBSAN_INTEGER_WRAP))
 endif