ANDROID: inject correct HMAC digest into fips140.ko at build time

Inject the HMAC digest into the fips140.ko module once it has been
built. This involves a post-link step, so we need to use a special
Makefile that is wired up in the right way, and includes the special
handling only for fips140.ko in particular.

Bug: 153614920
Bug: 188620248
Change-Id: Ic66086a05fa997850ca15dedbbc14fc73f6f0da6
Signed-off-by: Ard Biesheuvel <ardb@google.com>

arch/arm64/Makefile.postlink

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: GPL-2.0
+
+#
+# This file is included by the generic Kbuild makefile to permit the
+# architecture to perform postlink actions on vmlinux and any .ko module file.
+# In this case, we only need it for fips140.ko, which needs a HMAC digest to be
+# injected into it. All other targets are NOPs.
+#
+
+PHONY := __archpost
+__archpost:
+
+-include include/config/auto.conf
+include scripts/Kbuild.include
+
+CMD_FIPS140_GEN_HMAC = crypto/fips140_gen_hmac
+quiet_cmd_gen_hmac = HMAC    $@
+      cmd_gen_hmac = $(CMD_FIPS140_GEN_HMAC) $@
+
+# `@true` prevents complaints when there is nothing to be done
+
+vmlinux: FORCE
+	@true
+
+$(objtree)/crypto/fips140.ko: FORCE
+	$(call cmd,gen_hmac)
+
+%.ko: FORCE
+	@true
+
+clean:
+	@true
+
+PHONY += FORCE clean
+
+FORCE:
+
+.PHONY: $(PHONY)

crypto/Makefile

@@ -233,4 +233,7 @@ obj-m			+= fips140.o
 
 CFLAGS_fips140-module.o += $(FIPS140_CFLAGS)
 
+hostprogs-always-y := fips140_gen_hmac
+HOSTLDLIBS_fips140_gen_hmac := -lcrypto -lelf
+
 endif

crypto/fips140_gen_hmac.c

@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021 - Google LLC
+ * Author: Ard Biesheuvel <ardb@google.com>
+ *
+ * This is a host tool that is intended to be used to take the HMAC digest of
+ * the .text and .rodata sections of the fips140.ko module, and store it inside
+ * the module. The module will perform an integrity selfcheck at module_init()
+ * time, by recalculating the digest and comparing it with the value calculated
+ * here.
+ *
+ * Note that the peculiar way an HMAC is being used as a digest with a public
+ * key rather than as a symmetric key signature is mandated by FIPS 140-2.
+ */
+
+#include <elf.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <openssl/hmac.h>
+
+static Elf64_Ehdr *ehdr;
+static Elf64_Shdr *shdr;
+static int num_shdr;
+static const char *strtab;
+static Elf64_Sym *syms;
+static int num_syms;
+
+static Elf64_Shdr *find_symtab_section(void)
+{
+	int i;
+
+	for (i = 0; i < num_shdr; i++)
+		if (shdr[i].sh_type == SHT_SYMTAB)
+			return &shdr[i];
+	return NULL;
+}
+
+static void *get_sym_addr(const char *sym_name)
+{
+	int i;
+
+	for (i = 0; i < num_syms; i++)
+		if (!strcmp(strtab + syms[i].st_name, sym_name))
+			return (void *)ehdr + shdr[syms[i].st_shndx].sh_offset +
+			       syms[i].st_value;
+	return NULL;
+}
+
+static void hmac_section(HMAC_CTX *hmac, const char *start, const char *end)
+{
+	void *start_addr = get_sym_addr(start);
+	void *end_addr = get_sym_addr(end);
+
+	HMAC_Update(hmac, start_addr, end_addr - start_addr);
+}
+
+int main(int argc, char **argv)
+{
+	Elf64_Shdr *symtab_shdr;
+	const char *hmac_key;
+	unsigned char *dg;
+	unsigned int dglen;
+	struct stat stat;
+	HMAC_CTX *hmac;
+	int fd, ret;
+
+	if (argc < 2) {
+		fprintf(stderr, "file argument missing\n");
+		exit(EXIT_FAILURE);
+	}
+
+	fd = open(argv[1], O_RDWR);
+	if (fd < 0) {
+		fprintf(stderr, "failed to open %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	ret = fstat(fd, &stat);
+	if (ret < 0) {
+		fprintf(stderr, "failed to stat() %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	ehdr = mmap(0, stat.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (ehdr == MAP_FAILED) {
+		fprintf(stderr, "failed to mmap() %s\n", argv[1]);
+		exit(EXIT_FAILURE);
+	}
+
+	shdr = (void *)ehdr + ehdr->e_shoff;
+	num_shdr = ehdr->e_shnum;
+
+	symtab_shdr = find_symtab_section();
+
+	syms = (void *)ehdr + symtab_shdr->sh_offset;
+	num_syms = symtab_shdr->sh_size / sizeof(Elf64_Sym);
+
+	strtab = (void *)ehdr + shdr[symtab_shdr->sh_link].sh_offset;
+
+	hmac_key = get_sym_addr("fips140_integ_hmac_key");
+	if (!hmac_key) {
+		fprintf(stderr, "failed to locate HMAC key in binary\n");
+		exit(EXIT_FAILURE);
+	}
+
+	dg = get_sym_addr("fips140_integ_hmac_digest");
+	if (!dg) {
+		fprintf(stderr, "failed to locate HMAC digest in binary\n");
+		exit(EXIT_FAILURE);
+	}
+
+	hmac = HMAC_CTX_new();
+	HMAC_Init_ex(hmac, hmac_key, strlen(hmac_key), EVP_sha256(), NULL);
+
+	hmac_section(hmac, "__fips140_text_start", "__fips140_text_end");
+	hmac_section(hmac, "__fips140_rodata_start", "__fips140_rodata_end");
+
+	HMAC_Final(hmac, dg, &dglen);
+
+	close(fd);
+	return 0;
+}