pvmfw: main: Introduce DICE helper functions

Refactor for supporting optionally skipping DICE.

Note: No functional change intended.

Test: m pvmfw
Bug: 393977894
Change-Id: I82287a6ef3ba6a665ef99afedd613949154e36c9
diff --git a/guest/pvmfw/src/main.rs b/guest/pvmfw/src/main.rs
index a4acf04..bef6fcb 100644
--- a/guest/pvmfw/src/main.rs
+++ b/guest/pvmfw/src/main.rs
@@ -37,8 +37,11 @@
 use crate::rollback::perform_rollback_protection;
 use alloc::borrow::Cow;
 use alloc::boxed::Box;
+use alloc::vec::Vec;
 use bssl_avf::Digester;
-use diced_open_dice::{bcc_handover_parse, DiceArtifacts, DiceContext, Hidden, VM_KEY_ALGORITHM};
+use diced_open_dice::{
+    bcc_handover_parse, DiceArtifacts, DiceContext, Hidden, HIDDEN_SIZE, VM_KEY_ALGORITHM,
+};
 use libfdt::Fdt;
 use log::{debug, error, info, trace, warn};
 use pvmfw_avb::verify_payload;
@@ -67,20 +70,12 @@
         debug!("Ramdisk: None");
     }
 
-    let dice_handover = bcc_handover_parse(current_dice_handover).map_err(|e| {
-        error!("Invalid DICE Handover: {e:?}");
-        RebootReason::InvalidDiceHandover
-    })?;
-    trace!("DICE handover: {dice_handover:x?}");
-
-    let dice_chain_info = DiceChainInfo::new(dice_handover.bcc()).map_err(|e| {
-        error!("{e}");
-        RebootReason::InvalidDiceHandover
-    })?;
+    let (dice_handover_bytes, dice_cdi_seal, dice_context, dice_debug_mode) =
+        parse_dice_handover(current_dice_handover)?;
 
     // The bootloader should never pass us a debug policy when the boot is secure (the bootloader
     // is locked). If it gets it wrong, disregard it & log it, to avoid it causing problems.
-    if debug_policy.is_some() && !dice_chain_info.is_debug_mode() {
+    if debug_policy.is_some() && !dice_debug_mode {
         warn!("Ignoring debug policy, DICE handover does not indicate Debug mode");
         debug_policy = None;
     }
@@ -101,67 +96,25 @@
         sanitize_device_tree(untrusted_fdt, vm_dtbo, vm_ref_dt, guest_page_size, hyp_page_size)?;
     let fdt = untrusted_fdt; // DT has now been sanitized.
 
-    let next_dice_handover_size = guest_page_size;
-    let next_dice_handover = heap::aligned_boxed_slice(next_dice_handover_size, guest_page_size)
-        .ok_or_else(|| {
-            error!("Failed to allocate the next-stage DICE handover");
-            RebootReason::InternalError
-        })?;
-    // By leaking the slice, its content will be left behind for the next stage.
-    let next_dice_handover = Box::leak(next_dice_handover);
-
     let instance_hash = salt_from_instance_id(fdt)?;
     let dice_inputs = PartialInputs::new(&verified_boot_data, instance_hash).map_err(|e| {
         error!("Failed to compute partial DICE inputs: {e:?}");
         RebootReason::InternalError
     })?;
 
-    let (new_instance, salt, defer_rollback_protection) = perform_rollback_protection(
-        fdt,
-        &verified_boot_data,
-        &dice_inputs,
-        dice_handover.cdi_seal(),
-    )?;
+    let (new_instance, salt, defer_rollback_protection) =
+        perform_rollback_protection(fdt, &verified_boot_data, &dice_inputs, &dice_cdi_seal)?;
     trace!("Got salt for instance: {salt:x?}");
 
-    let new_dice_handover = if cfg!(dice_changes) {
-        Cow::Borrowed(current_dice_handover)
-    } else {
-        // It is possible that the DICE chain we were given is rooted in the UDS. We do not want to
-        // give such a chain to the payload, or even the associated CDIs. So remove the
-        // entire chain we were given and taint the CDIs. Note that the resulting CDIs are
-        // still deterministically derived from those we received, so will vary iff they do.
-        // TODO(b/280405545): Remove this post Android 14.
-        let truncated_dice_handover = dice::chain::truncate(dice_handover).map_err(|e| {
-            error!("{e}");
-            RebootReason::InternalError
-        })?;
-        Cow::Owned(truncated_dice_handover)
-    };
-
-    let cose_alg = dice_chain_info.leaf_subject_pubkey().cose_alg;
-    trace!("DICE chain leaf subject public key algorithm: {:?}", cose_alg);
-
-    let dice_context = DiceContext {
-        authority_algorithm: cose_alg.try_into().map_err(|e| {
-            error!("{e}");
-            RebootReason::InternalError
-        })?,
-        subject_algorithm: VM_KEY_ALGORITHM,
-    };
-    dice_inputs
-        .write_next_handover(
-            new_dice_handover.as_ref(),
-            &salt,
-            defer_rollback_protection,
-            next_dice_handover,
-            dice_context,
-        )
-        .map_err(|e| {
-            error!("Failed to derive next-stage DICE secrets: {e:?}");
-            RebootReason::SecretDerivationError
-        })?;
-    flush(next_dice_handover);
+    let next_dice_handover = perform_dice_derivation(
+        dice_handover_bytes.as_ref(),
+        dice_context,
+        dice_inputs,
+        &salt,
+        defer_rollback_protection,
+        guest_page_size,
+        guest_page_size,
+    )?;
 
     let kaslr_seed = u64::from_ne_bytes(rand::random_array().map_err(|e| {
         error!("Failed to generated guest KASLR seed: {e}");
@@ -186,6 +139,84 @@
     Ok((next_dice_handover, debuggable))
 }
 
+fn parse_dice_handover(
+    bytes: &[u8],
+) -> Result<(Cow<'_, [u8]>, Vec<u8>, DiceContext, bool), RebootReason> {
+    let dice_handover = bcc_handover_parse(bytes).map_err(|e| {
+        error!("Invalid DICE Handover: {e:?}");
+        RebootReason::InvalidDiceHandover
+    })?;
+    trace!("DICE handover: {dice_handover:x?}");
+
+    let dice_chain_info = DiceChainInfo::new(dice_handover.bcc()).map_err(|e| {
+        error!("{e}");
+        RebootReason::InvalidDiceHandover
+    })?;
+    let is_debug_mode = dice_chain_info.is_debug_mode();
+    let cose_alg = dice_chain_info.leaf_subject_pubkey().cose_alg;
+    trace!("DICE chain leaf subject public key algorithm: {:?}", cose_alg);
+
+    let dice_context = DiceContext {
+        authority_algorithm: cose_alg.try_into().map_err(|e| {
+            error!("{e}");
+            RebootReason::InternalError
+        })?,
+        subject_algorithm: VM_KEY_ALGORITHM,
+    };
+
+    let cdi_seal = dice_handover.cdi_seal().to_vec();
+
+    let bytes_for_next = if cfg!(dice_changes) {
+        Cow::Borrowed(bytes)
+    } else {
+        // It is possible that the DICE chain we were given is rooted in the UDS. We do not want to
+        // give such a chain to the payload, or even the associated CDIs. So remove the
+        // entire chain we were given and taint the CDIs. Note that the resulting CDIs are
+        // still deterministically derived from those we received, so will vary iff they do.
+        // TODO(b/280405545): Remove this post Android 14.
+        let truncated_bytes = dice::chain::truncate(dice_handover).map_err(|e| {
+            error!("{e}");
+            RebootReason::InternalError
+        })?;
+        Cow::Owned(truncated_bytes)
+    };
+
+    Ok((bytes_for_next, cdi_seal, dice_context, is_debug_mode))
+}
+
+fn perform_dice_derivation<'a>(
+    dice_handover_bytes: &[u8],
+    dice_context: DiceContext,
+    dice_inputs: PartialInputs,
+    salt: &[u8; HIDDEN_SIZE],
+    defer_rollback_protection: bool,
+    next_handover_size: usize,
+    next_handover_align: usize,
+) -> Result<&'a [u8], RebootReason> {
+    let next_dice_handover = heap::aligned_boxed_slice(next_handover_size, next_handover_align)
+        .ok_or_else(|| {
+            error!("Failed to allocate the next-stage DICE handover");
+            RebootReason::InternalError
+        })?;
+    // By leaking the slice, its content will be left behind for the next stage.
+    let next_dice_handover = Box::leak(next_dice_handover);
+
+    dice_inputs
+        .write_next_handover(
+            dice_handover_bytes.as_ref(),
+            salt,
+            defer_rollback_protection,
+            next_dice_handover,
+            dice_context,
+        )
+        .map_err(|e| {
+            error!("Failed to derive next-stage DICE secrets: {e:?}");
+            RebootReason::SecretDerivationError
+        })?;
+    flush(next_dice_handover);
+    Ok(next_dice_handover)
+}
+
 // Get the "salt" which is one of the input for DICE derivation.
 // This provides differentiation of secrets for different VM instances with same payloads.
 fn salt_from_instance_id(fdt: &Fdt) -> Result<Option<Hidden>, RebootReason> {