cbfs: Enable CBFS verification Kconfigs

With the elimination of remaining non-verifying CBFS APIs in CB:59682,
CBFS verification is now ready to be used in its simplest form, so
enable the respective Kconfig options in menuconfig. Add a few more
restrictions to the TOCTOU_SAFETY option for problems that haven't been
solved yet, and transform a comment in cbfs.c into a die() to make sure
we don't accidentally forget implementing it once vboot integration gets
added.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Ifeba5c962c943856ab79bc6c4cb90a60c1de4a60
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59982
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>

2 files changed, 34 insertions, 20 deletions

diff --git a/src/lib/Kconfig.cbfs_verification b/src/lib/Kconfig.cbfs_verification
index fa90d9d9af0b..33e545865021 100644
--- a/src/lib/Kconfig.cbfs_verification
+++ b/src/lib/Kconfig.cbfs_verification
@@ -2,33 +2,41 @@
 #
 # This file is sourced from src/security/Kconfig for menuconfig convenience.
 
-#menu "CBFS verification"	# TODO: enable once it works
+menu "CBFS verification"
 
 config CBFS_VERIFICATION
-	bool	# TODO: make user selectable once it works
+	bool "Enable CBFS verification"
 	depends on !VBOOT_STARTS_BEFORE_BOOTBLOCK # this is gonna get tricky...
 	select VBOOT_LIB
 	help
-	  Work in progress. Do not use (yet).
+	  Say yes here to enable code that cryptographically verifies each CBFS
+	  file as it gets loaded by chaining it to a trust anchor that is
+	  embedded in the bootblock. This only makes sense if you use some
+	  out-of-band mechanism to guarantee the integrity of the bootblock
+	  itself, such as Intel BootGuard or flash write-protection.
+
+	  If a CBFS image was created with this option enabled, cbfstool will
+	  automatically update the hash embedded in the bootblock whenever it
+	  modifies the CBFS.
+
+if CBFS_VERIFICATION
 
 config TOCTOU_SAFETY
-	bool
-	depends on CBFS_VERIFICATION
+	bool "Protect against time-of-check vs. time-of-use vulnerabilities"
 	depends on !NO_FMAP_CACHE
 	depends on !NO_CBFS_MCACHE
 	depends on !USE_OPTION_TABLE && !FSP_CAR  # Known to access CBFS before CBMEM init
+	depends on !VBOOT # TODO: can only allow this once vboot fully integrated
+	depends on NO_XIP_EARLY_STAGES
 	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.
+	  Say yes here to eliminate time-of-check vs. time-of-use vulnerabilities
+	  for CBFS verification. This means that data from flash must be verified
+	  every time it is loaded (not just the first time), which requires a bit
+	  more overhead and is incompatible with certain configurations.
+
+	  Using this option only makes sense when the mechanism securing the
+	  bootblock is also safe against these vulnerabilities (i.e. there's no
+	  point in enabling this when you just rely on flash write-protection).
 
 config CBFS_HASH_ALGO
 	int
@@ -37,9 +45,13 @@ config CBFS_HASH_ALGO
 	default 3 if CBFS_HASH_SHA512
 
 choice
-	prompt "--> hash type"
-	depends on CBFS_VERIFICATION
+	prompt "Hash algorithm"
 	default CBFS_HASH_SHA256
+	help
+	  Select the hash algorithm used in CBFS verification. Note that SHA-1 is
+	  generally considered insecure today and should not be used without good
+	  reason. When using CBFS verification together with measured boot, using
+	  the same hash algorithm (usually SHA-256) for both is more efficient.
 
 config CBFS_HASH_SHA1
 	bool "SHA-1"
@@ -52,4 +64,6 @@ config CBFS_HASH_SHA512
 
 endchoice
 
-#endmenu
+endif
+
+endmenu
diff --git a/src/lib/cbfs.c b/src/lib/cbfs.c
index 991c9fe34244..3a044f742bdc 100644
--- a/src/lib/cbfs.c
+++ b/src/lib/cbfs.c
@@ -57,7 +57,7 @@ cb_err_t _cbfs_boot_lookup(const char *name, bool force_ro,
 			   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. */
+			die("TODO: set metadata_hash to RW metadata hash here.\n");
 		}
 		err = cbfs_lookup(&cbd->rdev, name, mdata, &data_offset, metadata_hash);
 	}