[otbn] Patch an error in the OTBN boot services program

There is an error in the OTBN boot services program loaded by the ROM
that can affect the random share values.  This bug is caused by a
simple transposition of operands in the `p256_base` source code.

Since the OTBN program is stored in ROM and loaded into OTBN by the ROM,
we patch it in the immutable section before we use that portion of
the program.

DO NOT cherrypick this change to the `master` branch.

Signed-off-by: Chris Frantz <cfrantz@google.com>

Author: cfrantz

sw/device/silicon_creator/lib/drivers/otbn.c

@@ -264,3 +264,20 @@ rom_error_t sc_otbn_load_app(const sc_otbn_app_t app) {
   }
   return kErrorOk;
 }
+
+void sc_otbn_patch(void) {
+  enum {
+    // Offset in the code of the bad instruction:
+    kBugOffset = 0x958,
+    // The bad instruction is:
+    // bn.rshi   w16, w20, w31 >> 192
+    kBadInstr = 0xc1fa387b,
+    // The good instruction is:
+    // bn.rshi   w16, w31, w20 >> 192
+    kGoodInstr = 0xc14fb87b,
+  };
+  uint32_t inst = abs_mmio_read32(kBase + OTBN_IMEM_REG_OFFSET + kBugOffset);
+  if (inst == kBadInstr) {
+    abs_mmio_write32(kBase + OTBN_IMEM_REG_OFFSET + kBugOffset, kGoodInstr);
+  }
+}

sw/device/silicon_creator/lib/drivers/otbn.h

@@ -322,6 +322,18 @@ rom_error_t sc_otbn_imem_sec_wipe(void);
 OT_WARN_UNUSED_RESULT
 rom_error_t sc_otbn_dmem_sec_wipe(void);
 
+/**
+ * Patch a bad instruction in OTBN imem.
+ *
+ * There is a bug in the OTBN boot services program loaded by ROM.  There is a
+ * single instruction with mistakenly transposed operands which can affect the
+ * random share values.  This bug DOES NOT affect the correctness of the OTBN
+ * calculations.
+ *
+ * This function patches the bad instruction with the correctly instruction.
+ */
+void sc_otbn_patch(void);
+
 #ifdef __cplusplus
 }
 #endif

sw/device/silicon_creator/rom_ext/imm_section/imm_section.c

@@ -11,6 +11,7 @@
 #include "sw/device/silicon_creator/lib/base/sec_mmio.h"
 #include "sw/device/silicon_creator/lib/cert/dice_chain.h"
 #include "sw/device/silicon_creator/lib/drivers/flash_ctrl.h"
+#include "sw/device/silicon_creator/lib/drivers/otbn.h"
 #include "sw/device/silicon_creator/lib/drivers/rnd.h"
 #include "sw/device/silicon_creator/lib/epmp_state.h"
 #include "sw/device/silicon_creator/lib/error.h"
@@ -21,6 +22,13 @@
 
 OT_WARN_UNUSED_RESULT
 static rom_error_t imm_section_start(void) {
+  // Patch a bug in the OTBN ECDSA-P256 program.
+  //
+  // There is a single instruction with mistakenly transposed operands which
+  // can affect the random share values.  This bug DOES NOT affect the
+  // correctness of the OTBN calculations.
+  sc_otbn_patch();
+
   // Check the ePMP state.
   HARDENED_RETURN_IF_ERROR(epmp_state_check());
   // Check sec_mmio expectations.