[PATCH 4/5] common: Add OS anti-rollback grace period

[seanedmond@linux.microsoft.com]

From: Stephen Carlson <stcarlso@microsoft.com>

New config CONFIG_FIT_ARBVP_GRACE to add a one unit grace period to OS
anti-rollback protection, allowing images with anti-rollback counters
exactly one less than the platform value to still be loaded. No update to
the platform anti-rollback counter will be performed in this case.

Signed-off-by: Stephen Carlson <stcarlso@microsoft.com>
---
 boot/Kconfig         | 10 ++++++++++
 boot/image-fit-sig.c |  7 ++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/boot/Kconfig b/boot/Kconfig
index e08c274b7c..cd16bb8e53 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -112,6 +112,16 @@ config FIT_ARBP
 	  when a platform needs to retire previous versions of FIT images due to
 	  security flaws and prevent devices from being reverted to them.
 
+config FIT_ARBP_GRACE
+	bool "Enable FIT Anti rollback grace period"
+	depends on FIT_ARBP
+	default n
+	help
+	  Enables a one unit grace period for FIT image anti-rollback protection,
+	  where anti-rollback protection will still accept a FIT image with an
+	  anti-rollback version one less than the current number, but will not
+	  update the platform anti-rollback counter in that case.
+
 config FIT_VERBOSE
 	bool "Show verbose messages when FIT images fail"
 	depends on FIT
diff --git a/boot/image-fit-sig.c b/boot/image-fit-sig.c
index bf3b81a3a3..dc88a4b2cb 100644
--- a/boot/image-fit-sig.c
+++ b/boot/image-fit-sig.c
@@ -70,6 +70,7 @@ static int fit_image_verify_arbvn(const void *fit, int image_noffset)
 {
 	u64 image_arbvn;
 	u64 plat_arbvn = 0ULL;
+	u64 target_arbvn;
 	struct udevice *dev;
 	int ret;
 
@@ -85,7 +86,11 @@ static int fit_image_verify_arbvn(const void *fit, int image_noffset)
 	if (ret)
 		return -EIO;
 
-	if (image_arbvn < plat_arbvn) {
+	target_arbvn = plat_arbvn;
+	/* Calculate target ARBVN, including grace period if enabled */
+	if (CONFIG_IS_ENABLED(FIT_ARBP_GRACE) && plat_arbvn > 0ULL)
+		target_arbvn = plat_arbvn - 1ULL;
+	if (image_arbvn < target_arbvn) {
 		return -EPERM;
 	} else if (image_arbvn > plat_arbvn) {
 		ret = dm_security_arbvn_set(dev, image_arbvn);
-- 
2.40.0