cbfs: Add verification for RO CBFS metadata hash

This patch adds the first stage of the new CONFIG_CBFS_VERIFICATION
feature. It's not useful to end-users in this stage so it cannot be
selected in menuconfig (and should not be used other than for
development) yet. With this patch coreboot can verify the metadata hash
of the RO CBFS when it starts booting, but it does not verify individual
files yet. Likewise, verifying RW CBFSes with vboot is not yet
supported.

Verification is bootstrapped from a "metadata hash anchor" structure
that is embedded in the bootblock code and marked by a unique magic
number.  This anchor contains both the CBFS metadata hash and a separate
hash for the FMAP which is required to find the primary CBFS. Both are
verified on first use in the bootblock (and halt the system on failure).

The CONFIG_TOCTOU_SAFETY option is also added for illustrative purposes
to show some paths that need to be different when full protection
against TOCTOU (time-of-check vs. time-of-use) attacks is desired. For
normal verification it is sufficient to check the FMAP and the CBFS
metadata hash only once in the bootblock -- for TOCTOU verification we
do the same, but we need to be extra careful that we do not re-read the
FMAP or any CBFS metadata in later stages. This is mostly achieved by
depending on the CBFS metadata cache and FMAP cache features, but we
allow for one edge case in case the RW CBFS metadata cache overflows
(which may happen during an RW update and could otherwise no longer be
fixed because mcache size is defined by RO code). This code is added to
demonstrate design intent but won't really matter until RW CBFS
verification can be supported.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I8930434de55eb938b042fdada9aa90218c0b5a34
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41120
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/commonlib/bsd/include/commonlib/bsd/metadata_hash.h

@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0-only */
+
+#ifndef _COMMONLIB_BSD_METADATA_HASH_H_
+#define _COMMONLIB_BSD_METADATA_HASH_H_
+
+#include <stdint.h>
+#include <vb2_sha.h>
+
+/* This structure is embedded somewhere in the (uncompressed) bootblock. */
+struct metadata_hash_anchor {
+	uint8_t magic[8];
+	struct vb2_hash cbfs_hash;
+	/* NOTE: This is just reserving space. sizeof(struct vb2_hash) may change between
+	   configurations/versions and cannot be relied upon, so the FMAP hash must be placed
+	   right after the actual data for the particular CBFS hash algorithm used ends. */
+	uint8_t reserved_space_for_fmap_hash[VB2_MAX_DIGEST_SIZE];
+} __packed;
+
+/* Always use this function to figure out the actual location of the FMAP hash. It always uses
+   the same algorithm as the CBFS hash. */
+static inline uint8_t *metadata_hash_anchor_fmap_hash(struct metadata_hash_anchor *anchor)
+{
+	return anchor->cbfs_hash.raw + vb2_digest_size(anchor->cbfs_hash.algo);
+}
+
+/*
+ * Do not use this constant anywhere else in coreboot code to ensure the bit pattern really only
+ * appears once in the CBFS image. The only coreboot file allowed to use this is
+ * src/lib/metadata_anchor.c to define the actual anchor data structure. It is defined here so
+ * that it can be shared with cbfstool (which may use it freely).
+ */
+#define DO_NOT_USE_METADATA_HASH_ANCHOR_MAGIC_DO_NOT_USE	"\xadMdtHsh\x15"
+
+#endif /* _COMMONLIB_BSD_MASTER_HASH_H_ */

src/commonlib/include/commonlib/cbmem_id.h

@@ -25,6 +25,7 @@
 #define CBMEM_ID_IGD_OPREGION	0x4f444749
 #define CBMEM_ID_IMD_ROOT	0xff4017ff
 #define CBMEM_ID_IMD_SMALL	0x53a11439
+#define CBMEM_ID_MDATA_HASH	0x6873484D
 #define CBMEM_ID_MEMINFO	0x494D454D
 #define CBMEM_ID_MMA_DATA	0x4D4D4144
 #define CBMEM_ID_MMC_STATUS	0x4d4d4353

src/include/cbfs.h

@@ -7,6 +7,7 @@
 #include <commonlib/cbfs.h>
 #include <program_loading.h>
 #include <types.h>
+#include <vb2_sha.h>
 
 /***********************************************
  * Perform CBFS operations on the boot device. *
@@ -74,4 +75,13 @@ void cbfs_boot_device_find_mcache(struct cbfs_boot_device *cbd, uint32_t id);
  */
 const struct cbfs_boot_device *cbfs_get_boot_device(bool force_ro);
 
+/*
+ * Builds the mcache (if |cbd->mcache| is set) and verifies |metadata_hash| (if
+ * it is not NULL). If CB_CBFS_CACHE_FULL is returned, the mcache is incomplete
+ * but still valid and the metadata hash was still verified. Should be called
+ * once per *boot* (not once per stage) before the first CBFS access.
+ */
+cb_err_t cbfs_init_boot_device(const struct cbfs_boot_device *cbd,
+			       struct vb2_hash *metadata_hash);
+
 #endif

src/include/cbfs_glue.h

@@ -5,8 +5,19 @@
 
 #include <commonlib/region.h>
 #include <console/console.h>
+#include <rules.h>
 
-#define CBFS_ENABLE_HASHING 0
+/*
+ * This flag prevents linking hashing functions into stages where they're not required. We don't
+ * need them at all if verification is disabled. If verification is enabled without TOCTOU
+ * safety, we only need to verify the metadata hash in the initial stage and can assume it stays
+ * valid in later stages. If TOCTOU safety is required, we may need them in every stage to
+ * reverify metadata that had to be reloaded from flash (e.g. because it didn't fit the mcache).
+ * Note that this only concerns metadata hashing -- file access functions may still link hashing
+ * routines independently for file data hashing.
+ */
+#define CBFS_ENABLE_HASHING (CONFIG(CBFS_VERIFICATION) && \
+			     (CONFIG(TOCTOU_SAFETY) || ENV_INITIAL_STAGE))
 
 #define ERROR(...) printk(BIOS_ERR, "CBFS ERROR: " __VA_ARGS__)
 #define LOG(...) printk(BIOS_ERR, "CBFS: " __VA_ARGS__)

src/include/metadata_hash.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* This file is part of the coreboot project. */
+
+#ifndef _METADATA_HASH_H_
+#define _METADATA_HASH_H_
+
+#include <commonlib/bsd/metadata_hash.h>
+
+/* Verify the an FMAP data structure with the FMAP hash that is stored together with the CBFS
+   metadata hash in the bootblock's metadata hash anchor (when CBFS verification is enabled). */
+vb2_error_t metadata_hash_verify_fmap(const void *fmap_base, size_t fmap_size);
+
+#if CONFIG(CBFS_VERIFICATION)
+/* Get the (RO) CBFS metadata hash for this CBFS image, which forms the root of trust for CBFS
+   verification. This function is only available in the bootblock. */
+struct vb2_hash *metadata_hash_get(void);
+#else
+static inline struct vb2_hash *metadata_hash_get(void) { return NULL; }
+#endif
+
+#endif

src/lib/Kconfig.cbfs_verification

@@ -0,0 +1,57 @@
+# SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0-or-later
+#
+# This file is part of the coreboot project.
+#
+# This file is sourced from src/security/Kconfig for menuconfig convenience.
+
+#menu "CBFS verification"	# TODO: enable once it works
+
+config CBFS_VERIFICATION
+	bool	# TODO: make user selectable once it works
+	depends on !COMPRESS_BOOTBLOCK	# TODO: figure out decompressor anchor
+	depends on !VBOOT_STARTS_BEFORE_BOOTBLOCK # this is gonna get tricky...
+	select VBOOT_LIB
+	help
+	  Work in progress. Do not use (yet).
+
+config TOCTOU_SAFETY
+	bool
+	depends on CBFS_VERIFICATION
+	depends on !NO_FMAP_CACHE
+	depends on !NO_CBFS_MCACHE
+	help
+	  Work in progress. Not actually TOCTOU safe yet. Do not use.
+
+	  Design idea here is that mcache overflows in this mode are only legal
+	  for the RW CBFS, because it's relatively easy to retrieve the RW
+	  metadata hash from persistent vboot context at any time, but the RO
+	  metadata hash is lost after the bootblock is unloaded. This avoids the
+	  need to carry yet another piece forward through the stages. Mcache
+	  overflows are mostly a concern for RW updates (if an update adds more
+	  files than originally planned for), for the RO section it should
+	  always be possible to dimension the mcache correctly beforehand, so
+	  this should be an acceptable limitation.
+
+config CBFS_HASH_ALGO
+	int
+	default 1 if CBFS_HASH_SHA1
+	default 2 if CBFS_HASH_SHA256
+	default 3 if CBFS_HASH_SHA512
+
+choice
+	prompt "--> hash type"
+	depends on CBFS_VERIFICATION
+	default CBFS_HASH_SHA256
+
+config CBFS_HASH_SHA1
+	bool "SHA-1"
+
+config CBFS_HASH_SHA256
+	bool "SHA-256"
+
+config CBFS_HASH_SHA512
+	bool "SHA-512"
+
+endchoice
+
+#endmenu

src/lib/Makefile.inc

@@ -47,6 +47,7 @@ bootblock-y += prog_ops.c
 bootblock-y += cbfs.c
 bootblock-$(CONFIG_GENERIC_GPIO_LIB) += gpio.c
 bootblock-y += libgcc.c
+bootblock-$(CONFIG_CBFS_VERIFICATION) += metadata_hash.c
 bootblock-$(CONFIG_GENERIC_UDELAY) += timer.c
 
 bootblock-$(CONFIG_COLLECT_TIMESTAMPS) += timestamp.c

src/lib/cbfs.c

@@ -10,6 +10,7 @@
 #include <console/console.h>
 #include <fmap.h>
 #include <lib.h>
+#include <metadata_hash.h>
 #include <security/tpm/tspi/crtm.h>
 #include <security/vboot/vboot_common.h>
 #include <stdlib.h>
@@ -29,8 +30,21 @@ cb_err_t cbfs_boot_lookup(const char *name, bool force_ro,
 	if (!CONFIG(NO_CBFS_MCACHE) && !ENV_SMM)
 		err = cbfs_mcache_lookup(cbd->mcache, cbd->mcache_size,
 					  name, mdata, &data_offset);
-	if (err == CB_CBFS_CACHE_FULL)
-		err = cbfs_lookup(&cbd->rdev, name, mdata, &data_offset, NULL);
+	if (err == CB_CBFS_CACHE_FULL) {
+		struct vb2_hash *metadata_hash = NULL;
+		if (CONFIG(TOCTOU_SAFETY)) {
+			if (ENV_SMM)  /* Cannot provide TOCTOU safety for SMM */
+				dead_code();
+			/* We can only reach this for the RW CBFS -- an mcache
+			   overflow in the RO CBFS would have been caught when
+			   building the mcache in cbfs_get_boot_device().
+			   (Note that TOCTOU_SAFETY implies !NO_CBFS_MCACHE.) */
+			assert(cbd == vboot_get_cbfs_boot_device());
+			/* TODO: set metadata_hash to RW metadata hash here. */
+		}
+		err = cbfs_lookup(&cbd->rdev, name, mdata, &data_offset,
+				  metadata_hash);
+	}
 
 	if (CONFIG(VBOOT_ENABLE_CBFS_FALLBACK) && !force_ro &&
 	    err == CB_CBFS_NOT_FOUND) {
@@ -405,6 +419,26 @@ void cbfs_boot_device_find_mcache(struct cbfs_boot_device *cbd, uint32_t id)
 	}
 }
 
+cb_err_t cbfs_init_boot_device(const struct cbfs_boot_device *cbd,
+			       struct vb2_hash *metadata_hash)
+{
+	/* If we have an mcache, mcache_build() will also check mdata hash. */
+	if (!CONFIG(NO_CBFS_MCACHE) && !ENV_SMM && cbd->mcache_size > 0)
+		return cbfs_mcache_build(&cbd->rdev, cbd->mcache,
+					 cbd->mcache_size, metadata_hash);
+
+	/* No mcache and no verification means we have nothing special to do. */
+	if (!CONFIG(CBFS_VERIFICATION) || !metadata_hash)
+		return CB_SUCCESS;
+
+	/* Verification only: use cbfs_walk() without a walker() function to
+	   just run through the CBFS once, will return NOT_FOUND by default. */
+	cb_err_t err = cbfs_walk(&cbd->rdev, NULL, NULL, metadata_hash, 0);
+	if (err == CB_CBFS_NOT_FOUND)
+		err = CB_SUCCESS;
+	return err;
+}
+
 const struct cbfs_boot_device *cbfs_get_boot_device(bool force_ro)
 {
 	static struct cbfs_boot_device ro;
@@ -426,15 +460,18 @@ const struct cbfs_boot_device *cbfs_get_boot_device(bool force_ro)
 		return &ro;
 
 	if (fmap_locate_area_as_rdev("COREBOOT", &ro.rdev))
-		return NULL;
+		die("Cannot locate primary CBFS");
 
 	cbfs_boot_device_find_mcache(&ro, CBMEM_ID_CBFS_RO_MCACHE);
 
-	if (ENV_INITIAL_STAGE && !CONFIG(NO_CBFS_MCACHE)) {
-		cb_err_t err = cbfs_mcache_build(&ro.rdev, ro.mcache,
-						 ro.mcache_size, NULL);
-		if (err && err != CB_CBFS_CACHE_FULL)
-			die("Failed to build RO mcache");
+	if (ENV_INITIAL_STAGE) {
+		cb_err_t err = cbfs_init_boot_device(&ro, metadata_hash_get());
+		if (err == CB_CBFS_HASH_MISMATCH)
+			die("RO CBFS metadata hash verification failure");
+		else if (CONFIG(TOCTOU_SAFETY) && err == CB_CBFS_CACHE_FULL)
+			die("RO mcache overflow breaks TOCTOU safety!\n");
+		else if (err && err != CB_CBFS_CACHE_FULL)
+			die("RO CBFS initialization error: %d", err);
 	}
 
 	return &ro;

src/lib/fmap.c

@@ -4,6 +4,7 @@
 #include <cbmem.h>
 #include <console/console.h>
 #include <fmap.h>
+#include <metadata_hash.h>
 #include <stddef.h>
 #include <string.h>
 #include <symbols.h>
@@ -27,9 +28,20 @@ uint64_t get_fmap_flash_offset(void)
 	return FMAP_OFFSET;
 }
 
-static int check_signature(const struct fmap *fmap)
+static int verify_fmap(const struct fmap *fmap)
 {
-	return memcmp(fmap->signature, FMAP_SIGNATURE, sizeof(fmap->signature));
+	if (memcmp(fmap->signature, FMAP_SIGNATURE, sizeof(fmap->signature)))
+		return -1;
+
+	static bool done = false;
+	if (!CONFIG(CBFS_VERIFICATION) || !ENV_INITIAL_STAGE || done)
+		return 0;	/* Only need to check hash in first stage. */
+
+	if (metadata_hash_verify_fmap(fmap, FMAP_SIZE) != VB2_SUCCESS)
+		return -1;
+
+	done = true;
+	return 0;
 }
 
 static void report(const struct fmap *fmap)
@@ -63,10 +75,12 @@ static void setup_preram_cache(struct mem_region_device *cache_mrdev)
 	if (!(ENV_INITIAL_STAGE)) {
 		/* NOTE: This assumes that the first stage will make
 		   at least one FMAP access (usually from finding CBFS). */
-		if (!check_signature(fmap))
+		if (!verify_fmap(fmap))
 			goto register_cache;
 
 		printk(BIOS_ERR, "ERROR: FMAP cache corrupted?!\n");
+		if (CONFIG(TOCTOU_SAFETY))
+			die("TOCTOU safety relies on FMAP cache");
 	}
 
 	/* In case we fail below, make sure the cache is invalid. */
@@ -80,7 +94,7 @@ static void setup_preram_cache(struct mem_region_device *cache_mrdev)
 	/* memlayout statically guarantees that the FMAP_CACHE is big enough. */
 	if (rdev_readat(boot_rdev, fmap, FMAP_OFFSET, FMAP_SIZE) != FMAP_SIZE)
 		return;
-	if (check_signature(fmap))
+	if (verify_fmap(fmap))
 		return;
 	report(fmap);
 
@@ -111,8 +125,9 @@ static int find_fmap_directory(struct region_device *fmrd)
 	if (fmap == NULL)
 		return -1;
 
-	if (check_signature(fmap)) {
-		printk(BIOS_DEBUG, "No FMAP found at %zx offset.\n", offset);
+	if (verify_fmap(fmap)) {
+		printk(BIOS_ERR, "FMAP missing or corrupted at offset 0x%zx!\n",
+		       offset);
 		rdev_munmap(boot, fmap);
 		return -1;
 	}

src/lib/metadata_hash.c

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/* This file is part of the coreboot project. */
+
+#include <assert.h>
+#include <cbmem.h>
+#include <metadata_hash.h>
+#include <symbols.h>
+
+__attribute__((used, section(".metadata_hash_anchor")))
+static struct metadata_hash_anchor metadata_hash_anchor = {
+	/* This is the only place in all of coreboot where we actually need to use this. */
+	.magic = DO_NOT_USE_METADATA_HASH_ANCHOR_MAGIC_DO_NOT_USE,
+	.cbfs_hash = { .algo = CONFIG_CBFS_HASH_ALGO }
+};
+
+struct vb2_hash *metadata_hash_get(void)
+{
+	return &metadata_hash_anchor.cbfs_hash;
+}
+
+vb2_error_t metadata_hash_verify_fmap(const void *fmap_buffer, size_t fmap_size)
+{
+	struct vb2_hash hash = { .algo = metadata_hash_anchor.cbfs_hash.algo };
+	memcpy(hash.raw, metadata_hash_anchor_fmap_hash(&metadata_hash_anchor),
+	       vb2_digest_size(hash.algo));
+	return vb2_hash_verify(fmap_buffer, fmap_size, &hash);
+}

src/lib/program.ld

@@ -18,6 +18,7 @@
 #if !ENV_X86 && (ENV_DECOMPRESSOR || ENV_BOOTBLOCK && !CONFIG(COMPRESS_BOOTBLOCK))
 	KEEP(*(.id));
 #endif
+	KEEP(*(.metadata_hash_anchor));
 	*(.text);
 	*(.text.*);
 

src/security/Kconfig

@@ -1,5 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-only
 
+# These features are implemented in src/lib/cbfs.c, but they are security
+# features so sort them in here for menuconfig.
+source "src/lib/Kconfig.cbfs_verification"
+
 source "src/security/vboot/Kconfig"
 source "src/security/tpm/Kconfig"
 source "src/security/memory/Kconfig"

src/security/vboot/vboot_loader.c

@@ -25,18 +25,17 @@ _Static_assert(!CONFIG(VBOOT_RETURN_FROM_VERSTAGE) ||
 
 int vboot_executed;
 
-static void build_rw_mcache(void)
+static void after_verstage(void)
 {
-	if (CONFIG(NO_CBFS_MCACHE))
-		return;
+	vboot_executed = 1;	/* Mark verstage execution complete. */
 
 	const struct cbfs_boot_device *cbd = vboot_get_cbfs_boot_device();
-	if (!cbd)	/* Don't build RW mcache in recovery mode. */
+	if (!cbd)	/* Can't initialize RW CBFS in recovery mode. */
 		return;
-	cb_err_t err = cbfs_mcache_build(&cbd->rdev, cbd->mcache,
-					 cbd->mcache_size, NULL);
-	if (err && err != CB_CBFS_CACHE_FULL)
-		die("Failed to build RW mcache.");	/* TODO: -> recovery? */
+
+	cb_err_t err = cbfs_init_boot_device(cbd, NULL); /* TODO: RW hash */
+	if (err && err != CB_CBFS_CACHE_FULL)	/* TODO: -> recovery? */
+		die("RW CBFS initialization failure: %d", err);
 }
 
 void vboot_run_logic(void)
@@ -44,8 +43,7 @@ void vboot_run_logic(void)
 	if (verification_should_run()) {
 		/* Note: this path is not used for VBOOT_RETURN_FROM_VERSTAGE */
 		verstage_main();
-		vboot_executed = 1;
-		build_rw_mcache();
+		after_verstage();
 	} else if (verstage_should_load()) {
 		struct cbfsf file;
 		struct prog verstage =
@@ -72,8 +70,7 @@ void vboot_run_logic(void)
 		if (!CONFIG(VBOOT_RETURN_FROM_VERSTAGE))
 			return;
 
-		vboot_executed = 1;
-		build_rw_mcache();
+		after_verstage();
 	}
 }
 

util/cbfstool/cbfs.h

@@ -11,6 +11,11 @@
 
 #define CBFS_HEADPTR_ADDR_X86 0xFFFFFFFC
 
+/* cbfstool is allowed to use this constant freely since it's not part of the
+   CBFS image, so make an alias for the name that's a little less aggressive. */
+#define METADATA_HASH_ANCHOR_MAGIC \
+	DO_NOT_USE_METADATA_HASH_ANCHOR_MAGIC_DO_NOT_USE
+
 struct typedesc_t {
 	uint32_t type;
 	const char *name;