[PATCH v1 10/14] efi: loader: authenticate LoadImage() images when signing is forced

[Johannes Schneider]

efi_image_authenticate() accepts every image, so with signed images
forced, a verified payload can still load unsigned images through
LoadImage(). systemd-stub does exactly that for the addons it finds next
to a UKI, PE files carrying .cmdline, .dtb and .initrd sections, without
shim through plain LoadImage(). An unsigned addon could thus replace
the kernel command line or devicetree of a signed UKI.

With signed images forced, verify images in LoadImage() against the
"efi" keyring, and have StartImage() refuse images that failed: as the
UEFI specification has it, LoadImage() still creates the handle when it
returns EFI_SECURITY_VIOLATION.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Johannes Schneider <johannes.schneider at leica-geosystems.com>
---
 efi/loader/Kconfig                |  3 ++-
 efi/loader/boot.c                 |  3 +++
 efi/loader/pe.c                   |  7 ++++++-
 include/efi/loader/authenticode.h | 11 +++++++++++
 4 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/efi/loader/Kconfig b/efi/loader/Kconfig
index 8345fccd1a..156123212b 100644
--- a/efi/loader/Kconfig
+++ b/efi/loader/Kconfig
@@ -33,7 +33,8 @@ config EFI_LOADER_AUTHENTICODE
 	  image booted with bootm against the keys compiled into the "efi"
 	  keyring (CONFIG_CRYPTO_PUBLIC_KEYS, keyring=efi). With signed images
 	  forced, an EFI image then boots only if one of those keys verifies
-	  it, the same way a FIT image must carry a valid signature.
+	  it, the same way a FIT image must carry a valid signature, and so
+	  do the images an EFI payload loads through LoadImage().
 
 	  X.509 certificates in the signature are not evaluated: trust is
 	  anchored in the keyring. barebox does not report UEFI Secure Boot
diff --git a/efi/loader/boot.c b/efi/loader/boot.c
index 2d98c95b5c..3c9489e951 100644
--- a/efi/loader/boot.c
+++ b/efi/loader/boot.c
@@ -3098,6 +3098,9 @@ efi_status_t __efi_start_image(efi_handle_t image_handle,
 	if (image_obj->header.type != EFI_OBJECT_TYPE_LOADED_IMAGE)
 		return EFI_EXIT(EFI_INVALID_PARAMETER);
 
+	if (image_obj->auth_status != EFI_IMAGE_AUTH_PASSED)
+		return EFI_EXIT(EFI_SECURITY_VIOLATION);
+
 	ret = EFI_CALL(efi_open_protocol(image_handle, &efi_loaded_image_protocol_guid,
 					 (void **)&info, NULL, NULL,
 					 EFI_OPEN_PROTOCOL_GET_PROTOCOL));
diff --git a/efi/loader/pe.c b/efi/loader/pe.c
index 827b50378c..efec38b111 100644
--- a/efi/loader/pe.c
+++ b/efi/loader/pe.c
@@ -18,6 +18,8 @@
 #include <efi/memory.h>
 #include <efi/loader.h>
 #include <efi/loader/pe.h>
+#include <efi/loader/authenticode.h>
+#include <bootm.h>
 #include <efi/guid.h>
 #include <efi/error.h>
 #include <pe.h>
@@ -895,7 +897,10 @@ const void *efi_pe_find_section(void *efi, size_t len, const char *name,
 #ifdef CONFIG_EFI_LOADER
 static bool efi_image_authenticate(void *efi, size_t efi_size)
 {
-	return true;
+	if (!IS_ENABLED(CONFIG_BOOTM) || !bootm_signed_images_are_forced())
+		return true;
+
+	return !efi_authenticode_verify(efi, efi_size, EFI_AUTHENTICODE_KEYRING);
 }
 
 /**
diff --git a/include/efi/loader/authenticode.h b/include/efi/loader/authenticode.h
index 24c46ca7cf..df4fb7eab0 100644
--- a/include/efi/loader/authenticode.h
+++ b/include/efi/loader/authenticode.h
@@ -3,7 +3,18 @@
 #define __EFI_LOADER_AUTHENTICODE_H
 
 #include <linux/types.h>
+#include <linux/errno.h>
 
+#define EFI_AUTHENTICODE_KEYRING "efi"
+
+#ifdef CONFIG_EFI_AUTHENTICODE
 int efi_authenticode_verify(void *efi, size_t len, const char *keyring);
+#else
+static inline int efi_authenticode_verify(void *efi, size_t len,
+					  const char *keyring)
+{
+	return -ENOSYS;
+}
+#endif
 
 #endif
-- 
2.43.0