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

seanedmond
2023-08-12T00:28:54Z

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