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>

1 files changed, 45 insertions, 8 deletions

diff --git a/src/lib/cbfs.c b/src/lib/cbfs.c
index beab74ec4d..5df1d8bd85 100644
--- a/src/lib/cbfs.c
+++ b/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;