Dispose llvm::TargetMachines prior to llvm::Context being disposed

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -25,6 +25,7 @@ use std::ffi::{CStr, CString};
 use std::fs::File;
 use std::io;
 use std::iter;
+use std::mem::ManuallyDrop;
 use std::path::Path;
 use std::slice;
 use std::sync::Arc;
@@ -734,7 +735,7 @@ pub unsafe fn optimize_thin_module(
     let llcx = llvm::LLVMRustContextCreate(cgcx.fewer_names);
     let llmod_raw = parse_module(llcx, module_name, thin_module.data(), &diag_handler)? as *const _;
     let mut module = ModuleCodegen {
-        module_llvm: ModuleLlvm { llmod_raw, llcx, tm },
+        module_llvm: ModuleLlvm { llmod_raw, llcx, tm: ManuallyDrop::new(tm) },
         name: thin_module.name().to_string(),
         kind: ModuleKind::Regular,
     };

compiler/rustc_codegen_llvm/src/lib.rs

@@ -52,6 +52,7 @@ use rustc_span::symbol::Symbol;
 use std::any::Any;
 use std::ffi::CStr;
 use std::io::Write;
+use std::mem::ManuallyDrop;
 
 mod back {
     pub mod archive;
@@ -407,8 +408,9 @@ pub struct ModuleLlvm {
     llcx: &'static mut llvm::Context,
     llmod_raw: *const llvm::Module,
 
-    // independent from llcx and llmod_raw, resources get disposed by drop impl
-    tm: OwnedTargetMachine,
+    // This field is `ManuallyDrop` because it is important that the `TargetMachine`
+    // is disposed prior to the `Context` being disposed otherwise UAFs can occur.
+    tm: ManuallyDrop<OwnedTargetMachine>,
 }
 
 unsafe impl Send for ModuleLlvm {}
@@ -419,15 +421,23 @@ impl ModuleLlvm {
         unsafe {
             let llcx = llvm::LLVMRustContextCreate(tcx.sess.fewer_names());
             let llmod_raw = context::create_module(tcx, llcx, mod_name) as *const _;
-            ModuleLlvm { llmod_raw, llcx, tm: create_target_machine(tcx, mod_name) }
+            ModuleLlvm {
+                llmod_raw,
+                llcx,
+                tm: ManuallyDrop::new(create_target_machine(tcx, mod_name)),
+            }
         }
     }
 
     fn new_metadata(tcx: TyCtxt<'_>, mod_name: &str) -> Self {
         unsafe {
             let llcx = llvm::LLVMRustContextCreate(tcx.sess.fewer_names());
             let llmod_raw = context::create_module(tcx, llcx, mod_name) as *const _;
-            ModuleLlvm { llmod_raw, llcx, tm: create_informational_target_machine(tcx.sess) }
+            ModuleLlvm {
+                llmod_raw,
+                llcx,
+                tm: ManuallyDrop::new(create_informational_target_machine(tcx.sess)),
+            }
         }
     }
 
@@ -448,7 +458,7 @@ impl ModuleLlvm {
                 }
             };
 
-            Ok(ModuleLlvm { llmod_raw, llcx, tm })
+            Ok(ModuleLlvm { llmod_raw, llcx, tm: ManuallyDrop::new(tm) })
         }
     }
 
@@ -460,6 +470,7 @@ impl ModuleLlvm {
 impl Drop for ModuleLlvm {
     fn drop(&mut self) {
         unsafe {
+            ManuallyDrop::drop(&mut self.tm);
             llvm::LLVMContextDispose(&mut *(self.llcx as *mut _));
         }
     }