feat: make assembly block Solidity memory safe (#50)

privacy-scaling-explorations · Oct 30, 2023 · bd61f8d · bd61f8d
1 parent 37b99a3
commit bd61f8d
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Showing 3 changed files with 19 additions and 5 deletions.
diff --git a/snark-verifier/src/loader/evm/code.rs b/snark-verifier/src/loader/evm/code.rs
@@ -27,7 +27,13 @@ pragma solidity ^0.8.0;
 
 contract Halo2Verifier {{
     fallback(bytes calldata) external returns (bytes memory) {{
-        assembly {{
+        assembly (\"memory-safe\") {{
+            // Enforce that Solidity memory layout is respected
+            let data := mload(0x40)
+            if iszero(eq(data, 0x80)) {{
+                revert(0, 0)
+            }}
+
             let success := true
             let f_p := {base_modulus}
             let f_q := {scalar_modulus}

diff --git a/snark-verifier/src/loader/evm/loader.rs b/snark-verifier/src/loader/evm/loader.rs
@@ -22,6 +22,9 @@ use std::{
     rc::Rc,
 };
 
+/// Memory pointer starts at 0x80, which is the end of the Solidity memory layout scratch space.
+pub const MEM_PTR_START: usize = 0x80;
+
 #[derive(Clone, Debug)]
 pub enum Value<T> {
     Constant(T),
@@ -77,7 +80,7 @@ impl EvmLoader {
             base_modulus,
             scalar_modulus,
             code: RefCell::new(code),
-            ptr: Default::default(),
+            ptr: RefCell::new(MEM_PTR_START),
             cache: Default::default(),
         })
     }

diff --git a/snark-verifier/src/system/halo2/transcript/evm.rs b/snark-verifier/src/system/halo2/transcript/evm.rs
@@ -2,7 +2,12 @@
 
 use crate::{
     loader::{
-        evm::{loader::Value, u256_to_fe, util::MemoryChunk, EcPoint, EvmLoader, Scalar, U256},
+        evm::{
+            loader::{Value, MEM_PTR_START},
+            u256_to_fe,
+            util::MemoryChunk,
+            EcPoint, EvmLoader, Scalar, U256,
+        },
         native::{self, NativeLoader},
         Loader,
     },
@@ -40,7 +45,7 @@ where
     /// u256 for `transcript_initial_state`.
     pub fn new(loader: &Rc<EvmLoader>) -> Self {
         let ptr = loader.allocate(0x20);
-        assert_eq!(ptr, 0);
+        assert_eq!(ptr, MEM_PTR_START);
         let mut buf = MemoryChunk::new(ptr);
         buf.extend(0x20);
         Self {
@@ -118,7 +123,7 @@ where
 
     fn common_scalar(&mut self, scalar: &Scalar) -> Result<(), Error> {
         match scalar.value() {
-            Value::Constant(_) if self.buf.ptr() == 0 => {
+            Value::Constant(_) if self.buf.ptr() == MEM_PTR_START => {
                 self.loader.copy_scalar(scalar, self.buf.ptr());
             }
             Value::Memory(ptr) => {