commit 5047f032c93fd8957be6529f8f1ad7a3fe626057
author Michael Grand <m.grand@trustngo.tech> Thu Nov 24 16:49:56 2022 +0100
committer David Brown <davidb@davidb.org> Mon Jan 30 09:34:34 2023 -0700
tree 35e61d7a9a991a8d8b2b9aed336f2188a49d0b2a
parent 78d50b2f07ca545b26224775ea0f19a65b21f44c

fih: Hardening of fault injection countermeasures

Returned values are now hardcoded. Indeed, while it is not
strictly needed (few return values different from SUCCESS
or FAILURE) complexity added by encoding return values might
cause the software to be vulnerable to fault attacks.

Return type changed from fih_int to fih_ret to make
the whole thing much simpler and therefore more robust
to fault attacks. In addition, its easier to predict
compiler behavior.

Affectation of sentive variables has been hardened using macro
FIH_SET (affectation + check wether write access has been properly
done). FIH_DECLARE() is added to ease the declaration of sentive
variables.

Equality tests fih_eq() and fih_not_eq() are now macros because
inlining produce more complex code (and weaker) than macros.
In addition fih_not_eq is modified to be the negation of fih_eq
which was not the case until now.

when FIH_NOT_EQ is used , FIH_SET(fih_rc, FIH_FAILURE) has been added
in some part of the code.

variable image_mask (bootutil_priv.h) is now volatile because a
double IF test is made on it.

some others parts of the code have been hardenned (eg. loop on images)

Signed-off-by: Michael Grand <m.grand@trustngo.tech>

boot/bootutil/src/image_rsa.c

diff --git a/boot/bootutil/src/image_rsa.c b/boot/bootutil/src/image_rsa.c
index 42d2db7..0e0782b 100644
--- a/boot/bootutil/src/image_rsa.c
+++ b/boot/bootutil/src/image_rsa.c
@@ -161,7 +161,7 @@
  * v2.2, section 9.1.2, with many parameters required to have fixed
  * values.
  */
-static fih_int
+static fih_ret
 bootutil_cmp_rsasig(mbedtls_rsa_context *ctx, uint8_t *hash, uint32_t hlen,
   uint8_t *sig)
 {
@@ -170,22 +170,18 @@
     uint8_t db_mask[PSS_MASK_LEN];
     uint8_t h2[PSS_HLEN];
     int i;
-    int rc = 0;
-    fih_int fih_rc = FIH_FAILURE;
+    FIH_DECLARE(fih_rc, FIH_FAILURE);
 
     if (ctx->MBEDTLS_CONTEXT_MEMBER(len) != PSS_EMLEN ||
         PSS_EMLEN > MBEDTLS_MPI_MAX_SIZE) {
-        rc = -1;
         goto out;
     }
 
     if (hlen != PSS_HLEN) {
-        rc = -1;
         goto out;
     }
 
     if (mbedtls_rsa_public(ctx, sig, em)) {
-        rc = -1;
         goto out;
     }
 
@@ -214,7 +210,6 @@
      * 0xbc, output inconsistent and stop.
      */
     if (em[PSS_EMLEN - 1] != 0xbc) {
-        rc = -1;
         goto out;
     }
 
@@ -255,13 +250,11 @@
      * hexadecimal value 0x01, output "inconsistent" and stop. */
     for (i = 0; i < PSS_MASK_ZERO_COUNT; i++) {
         if (db_mask[i] != 0) {
-            rc = -1;
             goto out;
         }
     }
 
     if (db_mask[PSS_MASK_ONE_POS] != 1) {
-        rc = -1;
         goto out;
     }
 
@@ -282,20 +275,16 @@
     FIH_CALL(boot_fih_memequal, fih_rc, h2, &em[PSS_HASH_OFFSET], PSS_HLEN);
 
 out:
-    if (rc) {
-        fih_rc = fih_int_encode(rc);
-    }
-
     FIH_RET(fih_rc);
 }
 
-fih_int
+fih_ret
 bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
   uint8_t key_id)
 {
     mbedtls_rsa_context ctx;
     int rc;
-    fih_int fih_rc = FIH_FAILURE;
+    FIH_DECLARE(fih_rc, FIH_FAILURE);
     uint8_t *cp;
     uint8_t *end;