@@ -28,6 +28,7 @@
*.gz
*.i
*.ko
+*.ko.hash
*.lex.c
*.ll
*.lst
@@ -79,7 +79,10 @@ generate a different temporary key for each build, resulting in the
modules being unreproducible. However, including a signing key with
your source would presumably defeat the purpose of signing modules.
-One approach to this is to divide up the build process so that the
+Instead ``CONFIG_MODULE_HASHES`` can be used to embed a static list
+of valid modules to load.
+
+Another approach to this is to divide up the build process so that the
unreproducible parts can be treated as sources:
1. Generate a persistent signing key. Add the certificate for the key
@@ -1535,8 +1535,10 @@ endif
# is an exception.
ifdef CONFIG_DEBUG_INFO_BTF_MODULES
KBUILD_BUILTIN := 1
+ifndef CONFIG_MODULE_HASHES
modules: vmlinux
endif
+endif
modules: modules_prepare
@@ -1916,7 +1918,11 @@ modules.order: $(build-dir)
# KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules.
# This is solely useful to speed up test compiles.
modules: modpost
-ifneq ($(KBUILD_MODPOST_NOFINAL),1)
+ifdef CONFIG_MODULE_HASHES
+ifeq ($(MODULE_HASHES_MODPOST_FINAL), 1)
+ $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modfinal
+endif
+else ifneq ($(KBUILD_MODPOST_NOFINAL),1)
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modfinal
endif
@@ -486,6 +486,8 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
\
PRINTK_INDEX \
\
+ MODULE_HASHES \
+ \
/* Kernel symbol table: Normal symbols */ \
__ksymtab : AT(ADDR(__ksymtab) - LOAD_OFFSET) { \
__start___ksymtab = .; \
@@ -895,6 +897,15 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
#define PRINTK_INDEX
#endif
+#ifdef CONFIG_MODULE_HASHES
+#define MODULE_HASHES \
+ .module_hashes : AT(ADDR(.module_hashes) - LOAD_OFFSET) { \
+ BOUNDED_SECTION_BY(.module_hashes, _module_hashes) \
+ }
+#else
+#define MODULE_HASHES
+#endif
+
/*
* Discard .note.GNU-stack, which is emitted as PROGBITS by the compiler.
* Otherwise, the type of .notes section would become PROGBITS instead of NOTES.
new file mode 100644
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef _LINUX_MODULE_HASHES_H
+#define _LINUX_MODULE_HASHES_H
+
+#include <linux/compiler_attributes.h>
+#include <linux/types.h>
+#include <crypto/sha2.h>
+
+#define __module_hashes_section __section(".module_hashes")
+#define MODULE_HASHES_HASH_SIZE SHA256_DIGEST_SIZE
+
+extern const u8 module_hashes[][MODULE_HASHES_HASH_SIZE];
+
+extern const typeof(module_hashes[0]) __start_module_hashes, __stop_module_hashes;
+
+#endif /* _LINUX_MODULE_HASHES_H */
@@ -212,7 +212,7 @@ config MODULE_SIG
config MODULE_SIG_POLICY
def_bool y
- depends on MODULE_SIG
+ depends on MODULE_SIG || MODULE_HASHES
config MODULE_SIG_FORCE
bool "Require modules to be validly signed"
@@ -348,6 +348,21 @@ config MODULE_DECOMPRESS
If unsure, say N.
+config MODULE_HASHES
+ bool "Module hash validation"
+ depends on $(success,cksum --algorithm sha256 --raw /dev/null)
+ select CRYPTO_LIB_SHA256
+ help
+ Validate modules by their hashes.
+ Only modules built together with the main kernel image can be
+ validated that way.
+
+ This is a reproducible-build compatible alternative to a build-time
+ generated module keyring, as enabled by
+ CONFIG_MODULE_SIG_KEY=certs/signing_key.pem.
+
+ Also see the warning in MODULE_SIG about stripping modules.
+
config MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS
bool "Allow loading of modules with missing namespace imports"
help
@@ -23,3 +23,4 @@ obj-$(CONFIG_KGDB_KDB) += kdb.o
obj-$(CONFIG_MODVERSIONS) += version.o
obj-$(CONFIG_MODULE_UNLOAD_TAINT_TRACKING) += tracking.o
obj-$(CONFIG_MODULE_STATS) += stats.o
+obj-$(CONFIG_MODULE_HASHES) += hashes.o
new file mode 100644
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#define pr_fmt(fmt) "module/hash: " fmt
+
+#include <linux/int_log.h>
+#include <linux/module_hashes.h>
+#include <linux/module.h>
+#include <crypto/sha2.h>
+#include "internal.h"
+
+static inline size_t module_hashes_count(void)
+{
+ return (__stop_module_hashes - __start_module_hashes) / MODULE_HASHES_HASH_SIZE;
+}
+
+static __init __maybe_unused int module_hashes_init(void)
+{
+ size_t num_hashes = module_hashes_count();
+ int num_width = (intlog10(num_hashes) >> 24) + 1;
+ size_t i;
+
+ pr_debug("Known hashes (%zu):\n", num_hashes);
+
+ for (i = 0; i < num_hashes; i++)
+ pr_debug("%*zu %*phN\n", num_width, i,
+ (int)sizeof(module_hashes[i]), module_hashes[i]);
+
+ return 0;
+}
+
+#if IS_ENABLED(CONFIG_MODULE_DEBUG)
+early_initcall(module_hashes_init);
+#endif
+
+int module_hash_check(struct load_info *info, int flags)
+{
+ u8 digest[MODULE_HASHES_HASH_SIZE];
+ size_t i;
+
+ sha256((const u8 *)info->hdr, info->len, digest);
+
+ for (i = 0; i < module_hashes_count(); i++) {
+ if (memcmp(module_hashes[i], digest, MODULE_HASHES_HASH_SIZE) == 0) {
+ pr_debug("allow %*phN\n", (int)sizeof(digest), digest);
+ info->sig_ok = true;
+ return 0;
+ }
+ }
+
+ pr_debug("block %*phN\n", (int)sizeof(digest), digest);
+ return -ENOKEY;
+}
@@ -334,6 +334,7 @@ int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
char *secstrings, struct module *mod);
int module_sig_check(struct load_info *info, int flags);
+int module_hash_check(struct load_info *info, int flags);
#ifdef CONFIG_DEBUG_KMEMLEAK
void kmemleak_load_module(const struct module *mod, const struct load_info *info);
@@ -3218,6 +3218,12 @@ static int module_integrity_check(struct load_info *info, int flags)
{
int err = 0;
+ if (IS_ENABLED(CONFIG_MODULE_HASHES)) {
+ err = module_hash_check(info, flags);
+ if (!err)
+ return 0;
+ }
+
if (IS_ENABLED(CONFIG_MODULE_SIG))
err = module_sig_check(info, flags);
@@ -43,6 +43,9 @@ quiet_cmd_btf_ko = BTF [M] $@
$(RESOLVE_BTFIDS) -b $(objtree)/vmlinux $@; \
fi;
+quiet_cmd_cksum_ko =
+ cmd_cksum_ko = cksum --algorithm sha256 --raw $@ > $@.hash
+
# Same as newer-prereqs, but allows to exclude specified extra dependencies
newer_prereqs_except = $(filter-out $(PHONY) $(1),$?)
@@ -57,6 +60,9 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check), \
ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+$(if $(newer-prereqs),$(call cmd,btf_ko))
endif
+ifdef CONFIG_MODULE_HASHES
+ $(call cmd,cksum_ko)
+endif
targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) .module-common.o
@@ -79,6 +79,11 @@ ifdef CONFIG_DEBUG_INFO_BTF
vmlinux: $(RESOLVE_BTFIDS)
endif
+ifdef CONFIG_MODULE_HASHES
+vmlinux: $(srctree)/scripts/module-hashes.sh
+vmlinux: modules.order
+endif
+
# module.builtin.ranges
# ---------------------------------------------------------------------------
ifdef CONFIG_BUILTIN_MODULE_RANGES
@@ -104,7 +104,7 @@ vmlinux_link()
${ld} ${ldflags} -o ${output} \
${wl}--whole-archive ${objs} ${wl}--no-whole-archive \
${wl}--start-group ${libs} ${wl}--end-group \
- ${kallsymso} ${btf_vmlinux_bin_o} ${arch_vmlinux_o} ${ldlibs}
+ ${kallsymso} ${btf_vmlinux_bin_o} ${module_hashes_o} ${arch_vmlinux_o} ${ldlibs}
}
# generate .BTF typeinfo from DWARF debuginfo
@@ -215,6 +215,7 @@ fi
btf_vmlinux_bin_o=
kallsymso=
+module_hashes_o=
strip_debug=
if is_enabled CONFIG_KALLSYMS; then
@@ -222,6 +223,17 @@ if is_enabled CONFIG_KALLSYMS; then
kallsyms .tmp_vmlinux0.syms .tmp_vmlinux0.kallsyms
fi
+if is_enabled CONFIG_MODULE_HASHES; then
+ # At this point the hashes are still wrong.
+ # This step reserves the exact amount of space for the objcopy step
+ # after BTF generation.
+ ${srctree}/scripts/module-hashes.sh prealloc > .tmp_module_hashes.c
+ module_hashes_o=.tmp_module_hashes.o
+ info CC ${module_hashes_o}
+ ${CC} ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} \
+ ${KBUILD_CFLAGS_KERNEL} -c -o "${module_hashes_o}" ".tmp_module_hashes.c"
+fi
+
if is_enabled CONFIG_KALLSYMS || is_enabled CONFIG_DEBUG_INFO_BTF; then
# The kallsyms linking does not need debug symbols, but the BTF does.
@@ -302,6 +314,17 @@ if is_enabled CONFIG_BUILDTIME_TABLE_SORT; then
fi
fi
+if is_enabled CONFIG_MODULE_HASHES; then
+ info MAKE modules
+ ${MAKE} -f Makefile MODULE_HASHES_MODPOST_FINAL=1 modules
+ ${srctree}/scripts/module-hashes.sh > .tmp_module_hashes.c
+ info CC ${module_hashes_o}
+ ${CC} ${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS} ${KBUILD_CFLAGS} \
+ ${KBUILD_CFLAGS_KERNEL} -c -o "${module_hashes_o}" ".tmp_module_hashes.c"
+ ${OBJCOPY} --dump-section .module_hashes=.tmp_module_hashes.bin ${module_hashes_o}
+ ${OBJCOPY} --update-section .module_hashes=.tmp_module_hashes.bin vmlinux
+fi
+
# step a (see comment above)
if is_enabled CONFIG_KALLSYMS; then
if ! cmp -s System.map "${kallsyms_sysmap}"; then
new file mode 100755
@@ -0,0 +1,26 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+set -e
+set -u
+set -o pipefail
+
+prealloc="${1:-}"
+
+echo "#include <linux/module_hashes.h>"
+echo
+echo "const u8 module_hashes[][MODULE_HASHES_HASH_SIZE] __module_hashes_section = {"
+
+for mod in $(< modules.order); do
+ mod="${mod/%.o/.ko}"
+ if [ "$prealloc" = "prealloc" ]; then
+ modhash=""
+ else
+ modhash="$(cat "$mod".hash | hexdump -v -e '"0x" 1/1 "%02x, "')"
+ fi
+ echo -e "\t/* $mod */"
+ echo -e "\t{ $modhash},"
+ echo
+done
+
+echo "};"
@@ -1,7 +1,7 @@
config SECURITY_LOCKDOWN_LSM
bool "Basic module for enforcing kernel lockdown"
depends on SECURITY
- depends on !MODULES || MODULE_SIG
+ depends on !MODULES || MODULE_SIG || MODULE_HASHES
help
Build support for an LSM that enforces a coarse kernel lockdown
behaviour.
The current signature-based module integrity checking has some drawbacks in combination with reproducible builds: Either the module signing key is generated at build time, which makes the build unreproducible, or a static key is used, which precludes rebuilds by third parties and makes the whole build and packaging process much more complicated. Introduce a new mechanism to ensure only well-known modules are loaded by embedding a list of hashes of all modules built as part of the full kernel build into vmlinux. Non-builtin modules can be validated as before through signatures. Signed-off-by: Thomas Weißschuh <linux@weissschuh.net> --- .gitignore | 1 + Documentation/kbuild/reproducible-builds.rst | 5 ++- Makefile | 8 ++++- include/asm-generic/vmlinux.lds.h | 11 ++++++ include/linux/module_hashes.h | 17 +++++++++ kernel/module/Kconfig | 17 ++++++++- kernel/module/Makefile | 1 + kernel/module/hashes.c | 52 ++++++++++++++++++++++++++++ kernel/module/internal.h | 1 + kernel/module/main.c | 6 ++++ scripts/Makefile.modfinal | 6 ++++ scripts/Makefile.vmlinux | 5 +++ scripts/link-vmlinux.sh | 25 ++++++++++++- scripts/module-hashes.sh | 26 ++++++++++++++ security/lockdown/Kconfig | 2 +- 15 files changed, 178 insertions(+), 5 deletions(-)