Rollup merge of #118013 - sivadeilra:user/ardavis/ehcont, r=wesleywiser

Enable Rust to use the EHCont security feature of Windows

In the future Windows will enable Control-flow Enforcement Technology (CET aka Shadow Stacks). To protect the path where the context is updated during exception handling, the binary is required to enumerate valid unwind entrypoints in a dedicated section which is validated when the context is being set during exception handling.

The required support for EHCONT Guard has already been merged into LLVM, long ago. This change simply adds the Rust codegen option to enable it.

Relevant LLVM change: https://reviews.llvm.org/D40223

This also adds a new `ehcont-guard` option to the bootstrap config which enables EHCont Guard when building std.

We at Microsoft have been using this feature for a significant period of time; we are confident that the LLVM feature, when enabled, generates well-formed code.

We currently enable EHCONT using a codegen feature, but I'm certainly open to refactoring this to be a target feature instead, or to use any appropriate mechanism to enable it.

compiler/rustc_codegen_llvm/src/context.rs

@@ -351,6 +351,16 @@ pub unsafe fn create_module<'ll>(
         );
     }
 
+    // Set module flag to enable Windows EHCont Guard (/guard:ehcont).
+    if sess.opts.unstable_opts.ehcont_guard {
+        llvm::LLVMRustAddModuleFlag(
+            llmod,
+            llvm::LLVMModFlagBehavior::Warning,
+            "ehcontguard\0".as_ptr() as *const _,
+            1,
+        )
+    }
+
     // Insert `llvm.ident` metadata.
     //
     // On the wasm targets it will get hooked up to the "producer" sections

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2378,6 +2378,11 @@ fn add_order_independent_options(
         cmd.control_flow_guard();
     }
 
+    // OBJECT-FILES-NO, AUDIT-ORDER
+    if sess.opts.unstable_opts.ehcont_guard {
+        cmd.ehcont_guard();
+    }
+
     add_rpath_args(cmd, sess, codegen_results, out_filename);
 }
 

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -185,6 +185,7 @@ pub trait Linker {
     fn optimize(&mut self);
     fn pgo_gen(&mut self);
     fn control_flow_guard(&mut self);
+    fn ehcont_guard(&mut self);
     fn debuginfo(&mut self, strip: Strip, natvis_debugger_visualizers: &[PathBuf]);
     fn no_crt_objects(&mut self);
     fn no_default_libraries(&mut self);
@@ -605,6 +606,8 @@ impl<'a> Linker for GccLinker<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn debuginfo(&mut self, strip: Strip, _: &[PathBuf]) {
         // MacOS linker doesn't support stripping symbols directly anymore.
         if self.sess.target.is_like_osx {
@@ -914,6 +917,12 @@ impl<'a> Linker for MsvcLinker<'a> {
         self.cmd.arg("/guard:cf");
     }
 
+    fn ehcont_guard(&mut self) {
+        if self.sess.target.pointer_width == 64 {
+            self.cmd.arg("/guard:ehcont");
+        }
+    }
+
     fn debuginfo(&mut self, strip: Strip, natvis_debugger_visualizers: &[PathBuf]) {
         match strip {
             Strip::None => {
@@ -1127,6 +1136,8 @@ impl<'a> Linker for EmLinker<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn debuginfo(&mut self, _strip: Strip, _: &[PathBuf]) {
         // Preserve names or generate source maps depending on debug info
         // For more information see https://emscripten.org/docs/tools_reference/emcc.html#emcc-g
@@ -1319,6 +1330,8 @@ impl<'a> Linker for WasmLd<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn no_crt_objects(&mut self) {}
 
     fn no_default_libraries(&mut self) {}
@@ -1472,6 +1485,8 @@ impl<'a> Linker for L4Bender<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn no_crt_objects(&mut self) {}
 }
 
@@ -1613,6 +1628,8 @@ impl<'a> Linker for AixLinker<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn debuginfo(&mut self, strip: Strip, _: &[PathBuf]) {
         match strip {
             Strip::None => {}
@@ -1835,6 +1852,8 @@ impl<'a> Linker for PtxLinker<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, _symbols: &[String]) {}
 
     fn subsystem(&mut self, _subsystem: &str) {}
@@ -1931,6 +1950,8 @@ impl<'a> Linker for BpfLinker<'a> {
 
     fn control_flow_guard(&mut self) {}
 
+    fn ehcont_guard(&mut self) {}
+
     fn export_symbols(&mut self, tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
         let path = tmpdir.join("symbols");
         let res: io::Result<()> = try {

compiler/rustc_session/src/options.rs

@@ -1582,6 +1582,8 @@ options! {
         "version of DWARF debug information to emit (default: 2 or 4, depending on platform)"),
     dylib_lto: bool = (false, parse_bool, [UNTRACKED],
         "enables LTO for dylib crate type"),
+    ehcont_guard: bool = (false, parse_bool, [TRACKED],
+        "generate Windows EHCont Guard tables"),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
         "emit a section containing stack size metadata (default: no)"),
     emit_thin_lto: bool = (true, parse_bool, [TRACKED],

config.example.toml

@@ -686,6 +686,10 @@ change-id = 116881
 # This only applies from stage 1 onwards, and only for Windows targets.
 #control-flow-guard = false
 
+# Enable Windows EHCont Guard checks in the standard library.
+# This only applies from stage 1 onwards, and only for Windows targets.
+#ehcont-guard = false
+
 # Enable symbol-mangling-version v0. This can be helpful when profiling rustc,
 # as generics will be preserved in symbols (rather than erased into opaque T).
 # When no setting is given, the new scheme will be used when compiling the

src/bootstrap/src/core/builder.rs

@@ -1964,6 +1964,16 @@ impl<'a> Builder<'a> {
             rustflags.arg("-Ccontrol-flow-guard");
         }
 
+        // If EHCont Guard is enabled, pass the `-Zehcont-guard` flag to rustc when compiling the
+        // standard library, since this might be linked into the final outputs produced by rustc.
+        // Since this mitigation is only available on Windows, only enable it for the standard
+        // library in case the compiler is run on a non-Windows platform.
+        // This is not needed for stage 0 artifacts because these will only be used for building
+        // the stage 1 compiler.
+        if cfg!(windows) && mode == Mode::Std && self.config.ehcont_guard && compiler.stage >= 1 {
+            rustflags.arg("-Zehcont-guard");
+        }
+
         // For `cargo doc` invocations, make rustdoc print the Rust version into the docs
         // This replaces spaces with tabs because RUSTDOCFLAGS does not
         // support arguments with regular spaces. Hopefully someday Cargo will

src/bootstrap/src/core/config/config.rs

@@ -248,6 +248,7 @@ pub struct Config {
     pub local_rebuild: bool,
     pub jemalloc: bool,
     pub control_flow_guard: bool,
+    pub ehcont_guard: bool,
 
     // dist misc
     pub dist_sign_folder: Option<PathBuf>,
@@ -1019,6 +1020,7 @@ define_config! {
         test_compare_mode: Option<bool> = "test-compare-mode",
         llvm_libunwind: Option<String> = "llvm-libunwind",
         control_flow_guard: Option<bool> = "control-flow-guard",
+        ehcont_guard: Option<bool> = "ehcont-guard",
         new_symbol_mangling: Option<bool> = "new-symbol-mangling",
         profile_generate: Option<String> = "profile-generate",
         profile_use: Option<String> = "profile-use",
@@ -1452,6 +1454,7 @@ impl Config {
             config.rust_thin_lto_import_instr_limit = rust.thin_lto_import_instr_limit;
             set(&mut config.rust_remap_debuginfo, rust.remap_debuginfo);
             set(&mut config.control_flow_guard, rust.control_flow_guard);
+            set(&mut config.ehcont_guard, rust.ehcont_guard);
             config.llvm_libunwind_default = rust
                 .llvm_libunwind
                 .map(|v| v.parse().expect("failed to parse rust.llvm-libunwind"));

src/bootstrap/src/tests/builder.rs

@@ -1,6 +1,6 @@
 use super::*;
-use crate::core::config::{Config, DryRun, TargetSelection};
 use crate::core::build_steps::doc::DocumentationFormat;
+use crate::core::config::{Config, DryRun, TargetSelection};
 use std::thread;
 
 fn configure(cmd: &str, host: &[&str], target: &[&str]) -> Config {

tests/codegen/ehcontguard_disabled.rs

@@ -0,0 +1,10 @@
+// compile-flags:
+
+#![crate_type = "lib"]
+
+// A basic test function.
+pub fn test() {
+}
+
+// Ensure the module flag ehcontguard is not present
+// CHECK-NOT: !"ehcontguard"

tests/codegen/ehcontguard_enabled.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Z ehcont-guard
+
+#![crate_type = "lib"]
+
+// A basic test function.
+pub fn test() {
+}
+
+// Ensure the module flag ehcontguard=1 is present
+// CHECK: !"ehcontguard", i32 1