kbuild: Use objtree for module signing key path

[ Upstream commit af61da281f52aba0c5b090bafb3a31c5739850ff ]

When building out-of-tree modules with CONFIG_MODULE_SIG_FORCE=y,
module signing fails because the private key path uses $(srctree)
while the public key path uses $(objtree). Since signing keys are
generated in the build directory during kernel compilation, both
paths should use $(objtree) for consistency.

This causes SSL errors like:
  SSL error:02001002:system library:fopen:No such file or directory
  sign-file: /kernel-src/certs/signing_key.pem

The issue occurs because:
- sig-key uses: $(srctree)/certs/signing_key.pem (source tree)
- cmd_sign uses: $(objtree)/certs/signing_key.x509 (build tree)

But both keys are generated in $(objtree) during the build.

This complements commit 25ff08aa43e37 ("kbuild: Fix signing issue for
external modules") which fixed the scripts path and public key path,
but missed the private key path inconsistency.

Fixes out-of-tree module signing for configurations with separate
source and build directories (e.g., O=/kernel-out).

Signed-off-by: Mikhail Malyshev <mike.malyshev@gmail.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20251015163452.3754286-1-mike.malyshev@gmail.com
Signed-off-by: Nicolas Schier <nsc@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/scripts/Makefile.modinst b/scripts/Makefile.modinst
index 10df89b9ef67..dc947e7872b9 100644
--- a/scripts/Makefile.modinst
+++ b/scripts/Makefile.modinst
@@ -67,7 +67,7 @@ endif
 #
 ifeq ($(CONFIG_MODULE_SIG_ALL),y)
 ifeq ($(filter pkcs11:%, $(CONFIG_MODULE_SIG_KEY)),)
-sig-key := $(if $(wildcard $(CONFIG_MODULE_SIG_KEY)),,$(srctree)/)$(CONFIG_MODULE_SIG_KEY)
+sig-key := $(if $(wildcard $(CONFIG_MODULE_SIG_KEY)),,$(objtree)/)$(CONFIG_MODULE_SIG_KEY)
 else
 sig-key := $(CONFIG_MODULE_SIG_KEY)
 endif