Reel in unsafety around InstanceHandle (#856)

* Reel in unsafety around `InstanceHandle`

This commit is an attempt, or at least is targeted at being a start, at
reeling in the unsafety around the `InstanceHandle` type. Currently this
type represents a sort of moral `Rc<Instance>` but is a bit more
specialized since the underlying memory is allocated through mmap.

Additionally, though, `InstanceHandle` exposes a fundamental flaw in its
safety by safetly allowing mutable access so long as you have `&mut
InstanceHandle`. This type, however, is trivially created by simply
cloning a `InstanceHandle` to get an owned reference. This means that
`&mut InstanceHandle` does not actually provide any guarantees about
uniqueness, so there's no more safety than `&InstanceHandle` itself.

This commit removes all `&mut self` APIs from `InstanceHandle`,
additionally removing some where `&self` was `unsafe` and `&mut self`
was safe (since it was trivial to subvert this "safety"). In doing so
interior mutability patterns are now used much more extensively through
structures such as `Table` and `Memory`. Additionally a number of
methods were refactored to be a bit clearer and use helper functions
where possible.

This is a relatively large commit unfortunately, but it snowballed very
quickly into touching quite a few places. My hope though is that this
will prevent developers working on wasmtime internals as well as
developers still yet to migrate to the `wasmtime` crate from falling
into trivial unsafe traps by accidentally using `&mut` when they can't.
All existing users relying on `&mut` will need to migrate to some form
of interior mutability, such as using `RefCell` or `Cell`.

This commit also additionally marks `InstanceHandle::new` as an `unsafe`
function. The rationale for this is that the `&mut`-safety is only the
beginning for the safety of `InstanceHandle`. In general the wasmtime
internals are extremely unsafe and haven't been audited for appropriate
usage of `unsafe`. Until that's done it's hoped that we can warn users
with this `unsafe` constructor and otherwise push users to the
`wasmtime` crate which we know is safe.

* Fix windows build

* Wrap up mutable memory state in one structure

Rather than having separate fields

* Use `Cell::set`, not `Cell::replace`, where possible

* Add a helper function for offsets from VMContext

* Fix a typo from merging

* rustfmt

* Use try_from, not as

* Tweak style of some setters

crates/api/src/externals.rs

@@ -420,12 +420,9 @@ fn set_table_item(
     item_index: u32,
     item: wasmtime_runtime::VMCallerCheckedAnyfunc,
 ) -> Result<()> {
-    if let Some(item_ref) = handle.table_get_mut(table_index, item_index) {
+    handle
-        *item_ref = item;
+        .table_set(table_index, item_index, item)
-        Ok(())
+        .map_err(|()| anyhow!("table element index out of bounds"))
-    } else {
-        bail!("table element index out of bounds")
-    }
 }
 
 impl Table {

crates/api/src/instance.rs

@@ -25,6 +25,7 @@ fn instantiate(
     imports: &[Extern],
 ) -> Result<InstanceHandle, Error> {
     let mut resolver = SimpleResolver { imports };
+    unsafe {
         let instance = compiled_module
             .instantiate(&mut resolver)
             .map_err(|e| -> Error {
@@ -38,6 +39,7 @@ fn instantiate(
             })?;
         Ok(instance)
     }
+}
 
 /// An instantiated WebAssembly module.
 ///
@@ -108,7 +110,7 @@ impl Instance {
     /// [issue]: https://github.com/bytecodealliance/wasmtime/issues/727
     pub fn new(module: &Module, imports: &[Extern]) -> Result<Instance, Error> {
         let store = module.store();
-        let mut instance_handle = instantiate(module.compiled_module(), imports)?;
+        let instance_handle = instantiate(module.compiled_module(), imports)?;
 
         let exports = {
             let mut exports = Vec::with_capacity(module.exports().len());
@@ -179,9 +181,8 @@ impl Instance {
     pub fn from_handle(store: &Store, instance_handle: InstanceHandle) -> Instance {
         let mut exports = Vec::new();
         let mut exports_types = Vec::new();
-        let mut mutable = instance_handle.clone();
+        for (name, _) in instance_handle.exports() {
-        for (name, _) in instance_handle.clone().exports() {
+            let export = instance_handle.lookup(name).expect("export");
-            let export = mutable.lookup(name).expect("export");
             if let wasmtime_runtime::Export::Function { signature, .. } = &export {
                 // HACK ensure all handles, instantiated outside Store, present in
                 // the store's SignatureRegistry, e.g. WASI instances that are
@@ -220,7 +221,6 @@ impl Instance {
 
     #[doc(hidden)]
     pub fn get_wasmtime_memory(&self) -> Option<wasmtime_runtime::Export> {
-        let mut instance_handle = self.instance_handle.clone();
+        self.instance_handle.lookup("memory")
-        instance_handle.lookup("memory")
     }
 }

crates/api/src/trampoline/create_handle.rs

@@ -36,6 +36,7 @@ pub(crate) fn create_handle(
         })
         .unwrap_or_else(PrimaryMap::new);
 
+    unsafe {
         Ok(InstanceHandle::new(
             Arc::new(module),
             finished_functions.into_boxed_slice(),
@@ -44,6 +45,6 @@ pub(crate) fn create_handle(
             signatures.into_boxed_slice(),
             None,
             state,
-    )
+        )?)
-    .expect("instance"))
+    }
 }

crates/api/src/trampoline/func.rs

@@ -70,7 +70,7 @@ unsafe extern "C" fn stub_fn(
     call_id: u32,
     values_vec: *mut i128,
 ) -> u32 {
-    let mut instance = InstanceHandle::from_vmctx(vmctx);
+    let instance = InstanceHandle::from_vmctx(vmctx);
 
     let (args, returns_len) = {
         let module = instance.module_ref();
@@ -89,7 +89,7 @@ unsafe extern "C" fn stub_fn(
     let mut returns = vec![Val::null(); returns_len];
     let func = &instance
         .host_state()
-        .downcast_mut::<TrampolineState>()
+        .downcast_ref::<TrampolineState>()
         .expect("state")
         .func;
 

crates/api/src/trampoline/global.rs

@@ -34,12 +34,12 @@ pub fn create_global(gt: &GlobalType, val: Val) -> Result<(wasmtime_runtime::Exp
         },
         initializer: wasm::GlobalInit::Import, // TODO is it right?
     };
-    let mut handle =
+    let handle =
         create_handle(Module::new(), None, PrimaryMap::new(), Box::new(())).expect("handle");
     Ok((
         wasmtime_runtime::Export::Global {
             definition: definition.as_mut(),
-            vmctx: handle.vmctx_mut_ptr(),
+            vmctx: handle.vmctx_ptr(),
             global,
         },
         GlobalState { definition, handle },

crates/api/src/trampoline/mod.rs

@@ -23,7 +23,7 @@ pub fn generate_func_export(
     func: &Rc<dyn Callable + 'static>,
     store: &Store,
 ) -> Result<(wasmtime_runtime::InstanceHandle, wasmtime_runtime::Export)> {
-    let mut instance = create_handle_with_function(ft, func, store)?;
+    let instance = create_handle_with_function(ft, func, store)?;
     let export = instance.lookup("trampoline").expect("trampoline export");
     Ok((instance, export))
 }
@@ -38,7 +38,7 @@ pub fn generate_global_export(
 pub fn generate_memory_export(
     m: &MemoryType,
 ) -> Result<(wasmtime_runtime::InstanceHandle, wasmtime_runtime::Export)> {
-    let mut instance = create_handle_with_memory(m)?;
+    let instance = create_handle_with_memory(m)?;
     let export = instance.lookup("memory").expect("memory export");
     Ok((instance, export))
 }
@@ -46,7 +46,7 @@ pub fn generate_memory_export(
 pub fn generate_table_export(
     t: &TableType,
 ) -> Result<(wasmtime_runtime::InstanceHandle, wasmtime_runtime::Export)> {
-    let mut instance = create_handle_with_table(t)?;
+    let instance = create_handle_with_table(t)?;
     let export = instance.lookup("table").expect("table export");
     Ok((instance, export))
 }

crates/api/src/values.rs

@@ -217,7 +217,7 @@ pub(crate) fn into_checked_anyfunc(
 }
 
 pub(crate) fn from_checked_anyfunc(
-    item: &wasmtime_runtime::VMCallerCheckedAnyfunc,
+    item: wasmtime_runtime::VMCallerCheckedAnyfunc,
     store: &Store,
 ) -> Val {
     if item.type_index == wasmtime_runtime::VMSharedSignatureIndex::default() {

crates/jit/src/action.rs

@@ -231,7 +231,7 @@ pub fn inspect_memory<'instance>(
     start: usize,
     len: usize,
 ) -> Result<&'instance [u8], ActionError> {
-    let definition = match unsafe { instance.lookup_immutable(memory_name) } {
+    let definition = match instance.lookup(memory_name) {
         Some(Export::Memory {
             definition,
             memory: _memory,
@@ -259,7 +259,7 @@ pub fn inspect_memory<'instance>(
 
 /// Read a global in the given instance identified by an export name.
 pub fn get(instance: &InstanceHandle, global_name: &str) -> Result<RuntimeValue, ActionError> {
-    let (definition, global) = match unsafe { instance.lookup_immutable(global_name) } {
+    let (definition, global) = match instance.lookup(global_name) {
         Some(Export::Global {
             definition,
             vmctx: _,

crates/jit/src/context.rs

@@ -105,7 +105,7 @@ impl Context {
             .map_err(|e| format!("module did not validate: {}", e.to_string()))
     }
 
-    fn instantiate(&mut self, data: &[u8]) -> Result<InstanceHandle, SetupError> {
+    unsafe fn instantiate(&mut self, data: &[u8]) -> Result<InstanceHandle, SetupError> {
         self.validate(&data).map_err(SetupError::Validate)?;
         let debug_info = self.debug_info();
 
@@ -125,7 +125,11 @@ impl Context {
     }
 
     /// Instantiate a module instance and register the instance.
-    pub fn instantiate_module(
+    ///
+    /// # Unsafety
+    ///
+    /// See `InstanceHandle::new`
+    pub unsafe fn instantiate_module(
         &mut self,
         instance_name: Option<String>,
         data: &[u8],

crates/jit/src/instantiate.rs

@@ -184,7 +184,11 @@ impl CompiledModule {
     /// Note that if only one instance of this module is needed, it may be more
     /// efficient to call the top-level `instantiate`, since that avoids copying
     /// the data initializers.
-    pub fn instantiate(
+    ///
+    /// # Unsafety
+    ///
+    /// See `InstanceHandle::new`
+    pub unsafe fn instantiate(
         &self,
         resolver: &mut dyn Resolver,
     ) -> Result<InstanceHandle, InstantiationError> {
@@ -242,8 +246,12 @@ impl OwnedDataInitializer {
 ///
 /// This is equivalent to createing a `CompiledModule` and calling `instantiate()` on it,
 /// but avoids creating an intermediate copy of the data initializers.
+///
+/// # Unsafety
+///
+/// See `InstanceHandle::new`
 #[allow(clippy::implicit_hasher)]
-pub fn instantiate(
+pub unsafe fn instantiate(
     compiler: &mut Compiler,
     data: &[u8],
     resolver: &mut dyn Resolver,

crates/runtime/src/memory.rs

@@ -5,6 +5,7 @@
 use crate::mmap::Mmap;
 use crate::vmcontext::VMMemoryDefinition;
 use more_asserts::{assert_ge, assert_le};
+use std::cell::RefCell;
 use std::convert::TryFrom;
 use wasmtime_environ::{MemoryPlan, MemoryStyle, WASM_MAX_PAGES, WASM_PAGE_SIZE};
 
@@ -12,10 +13,7 @@ use wasmtime_environ::{MemoryPlan, MemoryStyle, WASM_MAX_PAGES, WASM_PAGE_SIZE};
 #[derive(Debug)]
 pub struct LinearMemory {
     // The underlying allocation.
-    mmap: Mmap,
+    mmap: RefCell<WasmMmap>,
-
-    // The current logical size in wasm pages of this linear memory.
-    current: u32,
 
     // The optional maximum size in wasm pages of this linear memory.
     maximum: Option<u32>,
@@ -29,6 +27,14 @@ pub struct LinearMemory {
     pub(crate) needs_signal_handlers: bool,
 }
 
+#[derive(Debug)]
+struct WasmMmap {
+    // Our OS allocation of mmap'd memory.
+    alloc: Mmap,
+    // The current logical size in wasm pages of this linear memory.
+    size: u32,
+}
+
 impl LinearMemory {
     /// Create a new linear memory instance with specified minimum and maximum number of wasm pages.
     pub fn new(plan: &MemoryPlan) -> Result<Self, String> {
@@ -59,11 +65,13 @@ impl LinearMemory {
         let mapped_pages = plan.memory.minimum as usize;
         let mapped_bytes = mapped_pages * WASM_PAGE_SIZE as usize;
 
-        let mmap = Mmap::accessible_reserved(mapped_bytes, request_bytes)?;
+        let mmap = WasmMmap {
+            alloc: Mmap::accessible_reserved(mapped_bytes, request_bytes)?,
+            size: plan.memory.minimum,
+        };
 
         Ok(Self {
-            mmap,
+            mmap: mmap.into(),
-            current: plan.memory.minimum,
             maximum: plan.memory.maximum,
             offset_guard_size: offset_guard_bytes,
             needs_signal_handlers,
@@ -72,25 +80,26 @@ impl LinearMemory {
 
     /// Returns the number of allocated wasm pages.
     pub fn size(&self) -> u32 {
-        self.current
+        self.mmap.borrow().size
     }
 
     /// Grow memory by the specified amount of wasm pages.
     ///
     /// Returns `None` if memory can't be grown by the specified amount
     /// of wasm pages.
-    pub fn grow(&mut self, delta: u32) -> Option<u32> {
+    pub fn grow(&self, delta: u32) -> Option<u32> {
         // Optimization of memory.grow 0 calls.
+        let mut mmap = self.mmap.borrow_mut();
         if delta == 0 {
-            return Some(self.current);
+            return Some(mmap.size);
         }
 
-        let new_pages = match self.current.checked_add(delta) {
+        let new_pages = match mmap.size.checked_add(delta) {
             Some(new_pages) => new_pages,
             // Linear memory size overflow.
             None => return None,
         };
-        let prev_pages = self.current;
+        let prev_pages = mmap.size;
 
         if let Some(maximum) = self.maximum {
             if new_pages > maximum {
@@ -111,7 +120,7 @@ impl LinearMemory {
         let prev_bytes = usize::try_from(prev_pages).unwrap() * WASM_PAGE_SIZE as usize;
         let new_bytes = usize::try_from(new_pages).unwrap() * WASM_PAGE_SIZE as usize;
 
-        if new_bytes > self.mmap.len() - self.offset_guard_size {
+        if new_bytes > mmap.alloc.len() - self.offset_guard_size {
             // If the new size is within the declared maximum, but needs more memory than we
             // have on hand, it's a dynamic heap and it can move.
             let guard_bytes = self.offset_guard_size;
@@ -119,25 +128,26 @@ impl LinearMemory {
 
             let mut new_mmap = Mmap::accessible_reserved(new_bytes, request_bytes).ok()?;
 
-            let copy_len = self.mmap.len() - self.offset_guard_size;
+            let copy_len = mmap.alloc.len() - self.offset_guard_size;
-            new_mmap.as_mut_slice()[..copy_len].copy_from_slice(&self.mmap.as_slice()[..copy_len]);
+            new_mmap.as_mut_slice()[..copy_len].copy_from_slice(&mmap.alloc.as_slice()[..copy_len]);
 
-            self.mmap = new_mmap;
+            mmap.alloc = new_mmap;
         } else if delta_bytes > 0 {
             // Make the newly allocated pages accessible.
-            self.mmap.make_accessible(prev_bytes, delta_bytes).ok()?;
+            mmap.alloc.make_accessible(prev_bytes, delta_bytes).ok()?;
         }
 
-        self.current = new_pages;
+        mmap.size = new_pages;
 
         Some(prev_pages)
     }
 
     /// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm code.
-    pub fn vmmemory(&mut self) -> VMMemoryDefinition {
+    pub fn vmmemory(&self) -> VMMemoryDefinition {
+        let mut mmap = self.mmap.borrow_mut();
         VMMemoryDefinition {
-            base: self.mmap.as_mut_ptr(),
+            base: mmap.alloc.as_mut_ptr(),
-            current_length: self.current as usize * WASM_PAGE_SIZE as usize,
+            current_length: mmap.size as usize * WASM_PAGE_SIZE as usize,
         }
     }
 }

crates/runtime/src/table.rs

@@ -3,6 +3,7 @@
 //! `Table` is to WebAssembly tables what `LinearMemory` is to WebAssembly linear memories.
 
 use crate::vmcontext::{VMCallerCheckedAnyfunc, VMTableDefinition};
+use std::cell::RefCell;
 use std::convert::{TryFrom, TryInto};
 use wasmtime_environ::wasm::TableElementType;
 use wasmtime_environ::{TablePlan, TableStyle};
@@ -10,7 +11,7 @@ use wasmtime_environ::{TablePlan, TableStyle};
 /// A table instance.
 #[derive(Debug)]
 pub struct Table {
-    vec: Vec<VMCallerCheckedAnyfunc>,
+    vec: RefCell<Vec<VMCallerCheckedAnyfunc>>,
     maximum: Option<u32>,
 }
 
@@ -25,10 +26,10 @@ impl Table {
         };
         match plan.style {
             TableStyle::CallerChecksSignature => Self {
-                vec: vec![
+                vec: RefCell::new(vec![
                     VMCallerCheckedAnyfunc::default();
                     usize::try_from(plan.table.minimum).unwrap()
-                ],
+                ]),
                 maximum: plan.table.maximum,
             },
         }
@@ -36,14 +37,14 @@ impl Table {
 
     /// Returns the number of allocated elements.
     pub fn size(&self) -> u32 {
-        self.vec.len().try_into().unwrap()
+        self.vec.borrow().len().try_into().unwrap()
     }
 
     /// Grow table by the specified amount of elements.
     ///
     /// Returns `None` if table can't be grown by the specified amount
     /// of elements.
-    pub fn grow(&mut self, delta: u32) -> Option<u32> {
+    pub fn grow(&self, delta: u32) -> Option<u32> {
         let new_len = match self.size().checked_add(delta) {
             Some(len) => {
                 if let Some(max) = self.maximum {
@@ -57,7 +58,7 @@ impl Table {
                 return None;
             }
         };
-        self.vec.resize(
+        self.vec.borrow_mut().resize(
             usize::try_from(new_len).unwrap(),
             VMCallerCheckedAnyfunc::default(),
         );
@@ -67,34 +68,31 @@ impl Table {
     /// Get reference to the specified element.
     ///
     /// Returns `None` if the index is out of bounds.
-    pub fn get(&self, index: u32) -> Option<&VMCallerCheckedAnyfunc> {
+    pub fn get(&self, index: u32) -> Option<VMCallerCheckedAnyfunc> {
-        self.vec.get(index as usize)
+        self.vec.borrow().get(index as usize).cloned()
     }
 
-    /// Get mutable reference to the specified element.
+    /// Set reference to the specified element.
     ///
-    /// Returns `None` if the index is out of bounds.
+    /// # Panics
-    pub fn get_mut(&mut self, index: u32) -> Option<&mut VMCallerCheckedAnyfunc> {
+    ///
-        self.vec.get_mut(index as usize)
+    /// Panics if `index` is out of bounds.
+    pub fn set(&self, index: u32, func: VMCallerCheckedAnyfunc) -> Result<(), ()> {
+        match self.vec.borrow_mut().get_mut(index as usize) {
+            Some(slot) => {
+                *slot = func;
+                Ok(())
+            }
+            None => Err(()),
+        }
     }
 
     /// Return a `VMTableDefinition` for exposing the table to compiled wasm code.
-    pub fn vmtable(&mut self) -> VMTableDefinition {
+    pub fn vmtable(&self) -> VMTableDefinition {
+        let mut vec = self.vec.borrow_mut();
         VMTableDefinition {
-            base: self.vec.as_mut_ptr() as *mut u8,
+            base: vec.as_mut_ptr() as *mut u8,
-            current_elements: self.vec.len().try_into().unwrap(),
+            current_elements: vec.len().try_into().unwrap(),
         }
     }
 }
-
-impl AsRef<[VMCallerCheckedAnyfunc]> for Table {
-    fn as_ref(&self) -> &[VMCallerCheckedAnyfunc] {
-        self.vec.as_slice()
-    }
-}
-
-impl AsMut<[VMCallerCheckedAnyfunc]> for Table {
-    fn as_mut(&mut self) -> &mut [VMCallerCheckedAnyfunc] {
-        self.vec.as_mut_slice()
-    }
-}

crates/runtime/src/vmcontext.rs

@@ -605,16 +605,16 @@ impl VMContext {
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
     #[allow(clippy::cast_ptr_alignment)]
-    pub(crate) unsafe fn instance(&mut self) -> &mut Instance {
+    pub(crate) unsafe fn instance(&self) -> &Instance {
-        &mut *((self as *mut Self as *mut u8).offset(-Instance::vmctx_offset()) as *mut Instance)
+        &*((self as *const Self as *mut u8).offset(-Instance::vmctx_offset()) as *const Instance)
     }
 
-    /// Return a mutable reference to the host state associated with this `Instance`.
+    /// Return a reference to the host state associated with this `Instance`.
     ///
     /// # Safety
     /// This is unsafe because it doesn't work on just any `VMContext`, it must
     /// be a `VMContext` allocated as part of an `Instance`.
-    pub unsafe fn host_state(&mut self) -> &mut dyn Any {
+    pub unsafe fn host_state(&self) -> &dyn Any {
         self.instance().host_state()
     }
 }

crates/wasi-common/wig/src/wasi.rs

@@ -201,8 +201,8 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
                         #format_str,
                         #(#format_args),*
                     );
-                    let wasi_ctx = match get_wasi_ctx(&mut *ctx) {
+                    let mut wasi_ctx = match get_wasi_ctx(&mut *ctx) {
-                        Ok(e) => e,
+                        Ok(e) => e.borrow_mut(),
                         Err(e) => #handle_early_error,
                     };
                     let memory = match get_memory(&mut *caller_ctx) {
@@ -210,7 +210,7 @@ pub fn add_wrappers_to_module(args: TokenStream) -> TokenStream {
                         Err(e) => #handle_early_error,
                     };
                     hostcalls::#name_ident(
-                        wasi_ctx,
+                        &mut *wasi_ctx,
                         memory,
                         #(#hostcall_args),*
                     ) #cvt_ret

crates/wasi/src/instantiate.rs

@@ -2,6 +2,7 @@ use cranelift_codegen::ir::types;
 use cranelift_codegen::{ir, isa};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::DefinedFuncIndex;
+use std::cell::RefCell;
 use std::fs::File;
 use std::sync::Arc;
 use target_lexicon::HOST;
@@ -78,6 +79,7 @@ pub fn instantiate_wasi_with_context(
     let data_initializers = Vec::new();
     let signatures = PrimaryMap::new();
 
+    unsafe {
         InstanceHandle::new(
             Arc::new(module),
             finished_functions.into_boxed_slice(),
@@ -85,20 +87,21 @@ pub fn instantiate_wasi_with_context(
             &data_initializers,
             signatures.into_boxed_slice(),
             None,
-        Box::new(wasi_ctx),
+            Box::new(RefCell::new(wasi_ctx)),
         )
     }
+}
 
 wig::define_add_wrappers_to_module!(
     "snapshot" "wasi_snapshot_preview1"
 );
 
 // Used by `add_wrappers_to_module` defined in the macro above
-fn get_wasi_ctx(vmctx: &mut VMContext) -> Result<&mut WasiCtx, wasi::__wasi_errno_t> {
+fn get_wasi_ctx(vmctx: &mut VMContext) -> Result<&RefCell<WasiCtx>, wasi::__wasi_errno_t> {
     unsafe {
         vmctx
             .host_state()
-            .downcast_mut::<WasiCtx>()
+            .downcast_ref()
             .ok_or_else(|| panic!("no host state named WasiCtx available"))
     }
 }

crates/wasi/src/old/snapshot_0/instantiate.rs

@@ -2,6 +2,7 @@ use cranelift_codegen::ir::types;
 use cranelift_codegen::{ir, isa};
 use cranelift_entity::PrimaryMap;
 use cranelift_wasm::DefinedFuncIndex;
+use std::cell::RefCell;
 use std::fs::File;
 use std::sync::Arc;
 use target_lexicon::HOST;
@@ -78,6 +79,7 @@ pub fn instantiate_wasi_with_context(
     let data_initializers = Vec::new();
     let signatures = PrimaryMap::new();
 
+    unsafe {
         InstanceHandle::new(
             Arc::new(module),
             finished_functions.into_boxed_slice(),
@@ -85,16 +87,17 @@ pub fn instantiate_wasi_with_context(
             &data_initializers,
             signatures.into_boxed_slice(),
             None,
-        Box::new(wasi_ctx),
+            Box::new(RefCell::new(wasi_ctx)),
         )
     }
+}
 
 // Used by `add_wrappers_to_module` defined in the macro above
-fn get_wasi_ctx(vmctx: &mut VMContext) -> Result<&mut WasiCtx, wasi::__wasi_errno_t> {
+fn get_wasi_ctx(vmctx: &mut VMContext) -> Result<&RefCell<WasiCtx>, wasi::__wasi_errno_t> {
     unsafe {
         vmctx
             .host_state()
-            .downcast_mut::<WasiCtx>()
+            .downcast_ref()
             .ok_or_else(|| panic!("no host state named WasiCtx available"))
     }
 }

tests/instantiate.rs

@@ -21,6 +21,8 @@ fn test_environ_translate() {
 
     let mut resolver = NullResolver {};
     let mut compiler = Compiler::new(isa, CompilationStrategy::Auto);
+    unsafe {
         let instance = instantiate(&mut compiler, &data, &mut resolver, false);
         assert!(instance.is_ok());
     }
+}