Make salt in Microdroid's instance img obsolete

Salt has been used to provide differentiation of secrets of 2
non-protected VMs (as hidden input in DICE). Have hidden input be
derived from instance_id of the (non protected) VM. In all other cases,
it will be all 0s.

Test: Microdroid tests use this code path.
Bug: 291306122
Change-Id: I686c7a5ee2755947d97df8d98ef2a0205525fb5a

microdroid_manager/src/dice.rs

diff --git a/microdroid_manager/src/dice.rs b/microdroid_manager/src/dice.rs
index a8b88aa..2469325 100644
--- a/microdroid_manager/src/dice.rs
+++ b/microdroid_manager/src/dice.rs
@@ -14,11 +14,11 @@
 
 use crate::dice_driver::DiceDriver;
 use crate::instance::{ApexData, ApkData};
-use crate::{is_debuggable, MicrodroidData};
+use crate::{is_debuggable, is_strict_boot, MicrodroidData};
 use anyhow::{bail, Context, Result};
 use ciborium::{cbor, Value};
 use coset::CborSerializable;
-use diced_open_dice::OwnedDiceArtifacts;
+use diced_open_dice::{Hidden, OwnedDiceArtifacts, HIDDEN_SIZE};
 use microdroid_metadata::PayloadMetadata;
 use openssl::sha::{sha512, Sha512};
 use std::iter::once;
@@ -53,10 +53,37 @@
     let debuggable = is_debuggable()?;
 
     // Send the details to diced
-    let hidden = instance_data.salt.clone().try_into().unwrap();
+    let hidden = if cfg!(llpvm_changes) {
+        hidden_input_from_instance_id()?
+    } else {
+        instance_data.salt.clone().try_into().unwrap()
+    };
     dice.derive(code_hash, &config_descriptor, authority_hash, debuggable, hidden)
 }
 
+// Get the "Hidden input" for DICE derivation.
+// This provides differentiation of secrets for different VM instances with same payload.
+fn hidden_input_from_instance_id() -> Result<Hidden> {
+    // For protected VM: this is all 0s, pvmfw ensures differentiation is added early in secrets.
+    // For non-protected VM: this is derived from instance_id of the VM instance.
+    let hidden_input = if !is_strict_boot() {
+        if let Some(id) = super::get_instance_id()? {
+            sha512(&id)
+        } else {
+            // TODO(b/325094712): Absence of instance_id occurs due to missing DT in some
+            // x86_64 test devices (such as Cuttlefish). From security perspective, this is
+            // acceptable for non-protected VM.
+            log::warn!(
+                "Instance Id missing, this may lead to 2 non protected VMs having same secrets"
+            );
+            [0u8; HIDDEN_SIZE]
+        }
+    } else {
+        [0u8; HIDDEN_SIZE]
+    };
+    Ok(hidden_input)
+}
+
 struct Subcomponent {
     name: String,
     version: u64,

microdroid_manager/src/instance.rs

diff --git a/microdroid_manager/src/instance.rs b/microdroid_manager/src/instance.rs
index 7a9f0e0..f42b86d 100644
--- a/microdroid_manager/src/instance.rs
+++ b/microdroid_manager/src/instance.rs
@@ -273,6 +273,8 @@
 
 #[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
 pub struct MicrodroidData {
+    // `salt` is obsolete, it was used as a differentiator for non-protected VM instances running
+    // same payload. Instance-id (present in DT) is used for that now.
     pub salt: Vec<u8>, // Should be [u8; 64] but that isn't serializable.
     pub apk_data: ApkData,
     pub extra_apks_data: Vec<ApkData>,

microdroid_manager/src/verify.rs

diff --git a/microdroid_manager/src/verify.rs b/microdroid_manager/src/verify.rs
index 445c1ae..65c32b0 100644
--- a/microdroid_manager/src/verify.rs
+++ b/microdroid_manager/src/verify.rs
@@ -169,13 +169,14 @@
     // verified is consistent with the root hash) or because we have the saved APK data which will
     // be checked as identical to the data we have verified.
 
-    // Use the salt from a verified instance, or generate a salt for a new instance.
-    let salt = if let Some(saved_data) = saved_data {
-        saved_data.salt.clone()
-    } else if is_strict_boot() {
-        // No need to add more entropy as a previous stage must have used a new, random salt.
+    let salt = if cfg!(llpvm_changes) || is_strict_boot() {
+        // Salt is obsolete with llpvm_changes.
         vec![0u8; 64]
+    } else if let Some(saved_data) = saved_data {
+        // Use the salt from a verified instance.
+        saved_data.salt.clone()
     } else {
+        // Generate a salt for a new instance.
         let mut salt = vec![0u8; 64];
         salt.as_mut_slice().try_fill(&mut rand::thread_rng())?;
         salt