fix(core/bootloader): fix unlocking bootloader granting HDP access on U5

[no changelog]

core/embed/trezorhal/secret.h

@@ -56,6 +56,9 @@ secbool secret_optiga_get(uint8_t dest[SECRET_OPTIGA_KEY_LEN]);
 // Checks if the optiga pairing secret is present in the secret storage
 secbool secret_optiga_present(void);
 
+// Checks if the optiga pairing secret can be written to the secret storage
+secbool secret_optiga_writable(void);
+
 // Erases optiga pairing secret from the secret storage
 void secret_optiga_erase(void);
 

core/embed/trezorhal/stm32f4/secret.c

@@ -102,6 +102,8 @@ secbool secret_optiga_present(void) {
   return (sectrue != secret_wiped()) * sectrue;
 }
 
+secbool secret_optiga_writable(void) { return secret_wiped(); }
+
 void secret_optiga_erase(void) { secret_erase(); }
 
 void secret_prepare_fw(secbool allow_run_with_secret, secbool _trust_all) {

core/embed/trezorhal/stm32u5/secret.c

@@ -176,6 +176,29 @@ secbool secret_optiga_present(void) {
   return secret_present(SECRET_OPTIGA_KEY_OFFSET, SECRET_OPTIGA_KEY_LEN);
 }
 
+secbool secret_optiga_writable(void) {
+  const uint32_t offset = SECRET_OPTIGA_KEY_OFFSET;
+  const uint32_t len = SECRET_OPTIGA_KEY_LEN;
+
+  const uint8_t *const secret =
+      (uint8_t *)flash_area_get_address(&SECRET_AREA, offset, len);
+
+  if (secret == NULL) {
+    return secfalse;
+  }
+
+  int secret_empty_bytes = 0;
+
+  for (int i = 0; i < len; i++) {
+    // 0xFF being the default value of the flash memory (before any write)
+    // 0x00 being the value of the flash memory after manual erase
+    if (secret[i] == 0xFF) {
+      secret_empty_bytes++;
+    }
+  }
+  return sectrue * (secret_empty_bytes == len);
+}
+
 // Backs up the optiga pairing secret from the secret storage to the backup
 // register
 static void secret_optiga_cache(void) {
@@ -270,16 +293,25 @@ void secret_prepare_fw(secbool allow_run_with_secret, secbool trust_all) {
   secret_bhk_lock();
 #ifdef USE_OPTIGA
   secret_optiga_uncache();
-  if (sectrue == allow_run_with_secret) {
-    if (secfalse != secret_optiga_present()) {
-      secret_optiga_cache();
-      secret_disable_access();
-    }
-  } else {
-    if (secfalse != secret_optiga_present()) {
-      show_install_restricted_screen();
-    }
-    secret_disable_access();
+  secbool optiga_secret_present = secret_optiga_present();
+  secbool optiga_secret_writable = secret_optiga_writable();
+  if (sectrue == trust_all && sectrue == allow_run_with_secret &&
+      sectrue == optiga_secret_writable && secfalse == optiga_secret_present) {
+    // Secret is not present and the secret sector is writable.
+    // This means the U5 chip is unprovisioned.
+    // Allow trusted firmware (prodtest presumably) to access the secret sector,
+    // early return here.
+    return;
+  }
+  if (sectrue == allow_run_with_secret && sectrue == optiga_secret_present) {
+    // Firmware is trusted and the Optiga secret is present, make it available.
+    secret_optiga_cache();
+  }
+  // Disable access unconditionally.
+  secret_disable_access();
+  if (sectrue != trust_all && sectrue == optiga_secret_present) {
+    // Untrusted firmware, locked bootloader. Show the restricted screen.
+    show_install_restricted_screen();
   }
 #else
   secret_disable_access();