fix: memory limit (#1123)

zkvm/entrypoint/src/syscalls/halt.rs

@@ -26,33 +26,22 @@ pub extern "C" fn syscall_halt(exit_code: u8) -> ! {
             .unwrap()
             .finalize();
 
-        // Convert the digest bytes into words, since we will be calling COMMIT ecall with
-        // the words as a parameter.
-        let pv_digest_words: [u32; PV_DIGEST_NUM_WORDS] = pv_digest_bytes
-            .as_slice()
-            .chunks_exact(4)
-            .map(|chunk| u32::from_le_bytes(chunk.try_into().unwrap()))
-            .collect::<Vec<_>>()
-            .try_into()
-            .unwrap();
-
         // For each digest word, call COMMIT ecall.  In the runtime, this will store the digest words
         // into the runtime's execution record's public values digest.  In the AIR, it will be used
         // to verify that the provided public values digest matches the one computed by the program.
         for i in 0..PV_DIGEST_NUM_WORDS {
-            asm!("ecall", in("t0") crate::syscalls::COMMIT, in("a0") i, in("a1") pv_digest_words[i]);
+            // Convert the digest bytes into words, since we will call COMMIT one word at a time.
+            let word = u32::from_le_bytes(pv_digest_bytes[i * 4..(i + 1) * 4].try_into().unwrap());
+            asm!("ecall", in("t0") crate::syscalls::COMMIT, in("a0") i, in("a1") word);
         }
 
         cfg_if! {
             if #[cfg(feature = "verify")] {
                 let deferred_proofs_digest = zkvm::DEFERRED_PROOFS_DIGEST.as_mut().unwrap();
-                let deferred_proofs_digest_words = deferred_proofs_digest
-                    .iter()
-                    .map(|baby_bear| baby_bear.as_canonical_u32())
-                    .collect::<Vec<_>>();
 
                 for i in 0..POSEIDON_NUM_WORDS {
-                    asm!("ecall", in("t0") crate::syscalls::COMMIT_DEFERRED_PROOFS, in("a0") i, in("a1") deferred_proofs_digest_words[i]);
+                    let word = deferred_proofs_digest[i].as_canonical_u32();
+                    asm!("ecall", in("t0") crate::syscalls::COMMIT_DEFERRED_PROOFS, in("a0") i, in("a1") word);
                 }
             } else {
                 for i in 0..POSEIDON_NUM_WORDS {

zkvm/entrypoint/src/syscalls/memory.rs

@@ -12,7 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-const SYSTEM_START: usize = 0x0C00_0000;
+// Memory addresses must be lower than BabyBear prime.
+const MAX_MEMORY: usize = 0x78000000;
 
 #[allow(clippy::missing_safety_doc)]
 #[no_mangle]
@@ -39,11 +40,10 @@ pub unsafe extern "C" fn sys_alloc_aligned(bytes: usize, align: usize) -> *mut u
     }
 
     let ptr = heap_pos as *mut u8;
-    heap_pos += bytes;
+    let (heap_pos, overflowed) = heap_pos.overflowing_add(bytes);
 
-    // Check to make sure heap doesn't collide with SYSTEM memory.
-    if SYSTEM_START < heap_pos {
-        panic!();
+    if overflowed || MAX_MEMORY < heap_pos {
+        panic!("Memory limit exceeded (0x78000000)");
     }
 
     unsafe { HEAP_POS = heap_pos };