cmse: lint on unions crossing the secure boundary

compiler/rustc_lint/src/cmse_uninitialized_leak.rs

@@ -0,0 +1,189 @@
+use rustc_abi::ExternAbi;
+use rustc_hir::{self as hir, Expr, ExprKind};
+use rustc_middle::ty::{self, Ty, TypeVisitableExt};
+use rustc_session::{declare_lint, declare_lint_pass};
+
+use crate::{LateContext, LateLintPass, LintContext, lints};
+
+declare_lint! {
+    /// The `cmse_uninitialized_leak` lint detects types that contain a `union` that cross a
+    /// secure boundary. Such values may still contain secure data in their padding bytes.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (ABI is only supported on thumbv8)
+    /// extern "cmse-nonsecure-entry" fn foo() -> MaybeUninit<u64> {
+    ///     MaybeUninit::uninit()
+    /// }
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: value crossing a secure boundary contains a union which may leak secure information
+    ///   --> lint_example.rs:2:5
+    ///    |
+    ///  2 |     MaybeUninit::uninit()
+    ///    |     ^^^^^^^^^^^^^^^^^^^^^^
+    ///    |
+    ///    = note: union values can have variant-dependent padding that may contain stale secure data
+    ///    = note: `#[warn(cmse_uninitialized_leak)]` on by default
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// The cmse calling conventions clear unused registers, to prevent stale secure information
+    /// from leaking into the non-secure application. Padding in both `struct`s and `enum`s is
+    /// similarly cleared. But for `union` values the compiler does not know what byte ranges are
+    /// padding (and may hold stale secure data) and what bytes ranges are live data.
+    ///
+    /// This lint fires when a type that is or contains a `union` crosses the secure boundary:
+    ///
+    /// - when returned from a `cmse-nonsecure-entry` function
+    /// - when passed as an argument to a `cmse-nonsecure-call` function
+    pub CMSE_UNINITIALIZED_LEAK,
+    Warn,
+    "value containing a union may leak secure information"
+}
+
+declare_lint_pass!(CmseUninitializedLeak => [CMSE_UNINITIALIZED_LEAK]);
+
+impl<'tcx> LateLintPass<'tcx> for CmseUninitializedLeak {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        check_cmse_entry_return(cx, expr);
+        check_cmse_call_call(cx, expr);
+    }
+}
+
+fn check_cmse_call_call<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+    let ExprKind::Call(callee, arguments) = expr.kind else {
+        return;
+    };
+
+    // Determine the callee ABI.
+    let callee_ty = cx.typeck_results().expr_ty(callee);
+    let sig = match callee_ty.kind() {
+        ty::FnPtr(poly_sig, header) if header.abi() == ExternAbi::CmseNonSecureCall => {
+            poly_sig.skip_binder()
+        }
+        _ => return,
+    };
+
+    let fn_sig = cx.tcx.erase_and_anonymize_regions(sig);
+
+    for (arg, ty) in arguments.iter().zip(fn_sig.inputs()) {
+        // `impl Trait` is not allowed in the argument types.
+        if ty.has_opaque_types() {
+            continue;
+        }
+
+        if contains_union(cx, *ty) {
+            // Some part of the source type may be uninitialized.
+            cx.emit_span_lint(
+                CMSE_UNINITIALIZED_LEAK,
+                arg.span,
+                lints::CmseUninitializedMayLeakInformation,
+            );
+        }
+    }
+}
+
+fn check_cmse_entry_return<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+    let owner = cx.tcx.hir_enclosing_body_owner(expr.hir_id);
+
+    match cx.tcx.def_kind(owner) {
+        hir::def::DefKind::Fn | hir::def::DefKind::AssocFn => {}
+        _ => return,
+    }
+
+    // Only continue if the current expr is an (implicit) return.
+    let body = cx.tcx.hir_body_owned_by(owner);
+    let is_implicit_return = expr.hir_id == body.value.hir_id;
+    if !(matches!(expr.kind, ExprKind::Ret(_)) || is_implicit_return) {
+        return;
+    }
+
+    let sig = cx.tcx.fn_sig(owner).skip_binder();
+    if sig.abi() != ExternAbi::CmseNonSecureEntry {
+        return;
+    }
+
+    let fn_sig = cx.tcx.instantiate_bound_regions_with_erased(sig);
+    let fn_sig = cx.tcx.erase_and_anonymize_regions(fn_sig);
+    let return_type = fn_sig.output();
+
+    // `impl Trait` is not allowed in the return type.
+    if return_type.has_opaque_types() {
+        return;
+    }
+
+    if contains_union(cx, return_type) {
+        let return_expr_span = if is_implicit_return {
+            match expr.kind {
+                ExprKind::Block(block, _) => match block.expr {
+                    Some(tail) => tail.span,
+                    None => expr.span,
+                },
+                _ => expr.span,
+            }
+        } else {
+            expr.span
+        };
+
+        // Some part of the source type may be uninitialized.
+        cx.emit_span_lint(
+            CMSE_UNINITIALIZED_LEAK,
+            return_expr_span,
+            lints::CmseUninitializedMayLeakInformation,
+        );
+    }
+}
+
+/// Traverse `T` for any `union` or `enum`, and check whether it contains any padding that is
+/// variant-dependent or unstable.
+fn contains_union<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    let tcx = cx.tcx;
+
+    // Types cross the secure boundary fully monomorphized.
+    let typing_env = ty::TypingEnv::fully_monomorphized();
+
+    match ty.kind() {
+        ty::Adt(adt_def, args) => {
+            if adt_def.is_union() {
+                // It is still unclear whether a union value where all fields are equally
+                // large and allow the same bit patterns can be considered initialized
+                // (see rust-lang/unsafe-code-guidelines#438), so for now we just warn on
+                // any union.
+                return true;
+            }
+
+            // For enums and structs we recurse into the fields.
+            // A non-repr(C) struct already triggers `improper_ctypes`.
+            adt_def.all_fields().any(|field| {
+                let field_ty = tcx.normalize_erasing_regions(typing_env, field.ty(tcx, args));
+                contains_union(cx, field_ty)
+            })
+        }
+
+        ty::Tuple(elems) => {
+            // Element types might contain unions.
+            //
+            // Passing a tuple already triggers `improper_ctypes`.
+            elems.iter().any(|elem| contains_union(cx, elem))
+        }
+        ty::Array(elem, _) => {
+            // The element type might contain unions.
+            //
+            // Passing an array already triggers `improper_ctypes`.
+            contains_union(cx, *elem)
+        }
+
+        _ => {
+            // Other types are either scalar (hence no padding), or behind some kind of indirection.
+            //
+            // Note that the system traps when dereferencing a pointer to secure memory while in
+            // non-secure mode, so passing a value with indirection is useless in practice.
+            false
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -34,6 +34,7 @@ mod async_fn_in_trait;
 mod autorefs;
 pub mod builtin;
 mod c_void_returns;
+mod cmse_uninitialized_leak;
 mod context;
 mod dangling;
 mod default_could_be_derived;
@@ -89,6 +90,7 @@ use async_fn_in_trait::AsyncFnInTrait;
 use autorefs::*;
 use builtin::*;
 use c_void_returns::*;
+use cmse_uninitialized_leak::CmseUninitializedLeak;
 use dangling::*;
 use default_could_be_derived::DefaultCouldBeDerived;
 use deref_into_dyn_supertrait::*;
@@ -271,6 +273,7 @@ late_lint_methods!(
             CheckTransmutes: CheckTransmutes,
             LifetimeSyntax: LifetimeSyntax,
             InternalEqTraitMethodImpls: InternalEqTraitMethodImpls,
+            CmseUninitializedLeak: CmseUninitializedLeak,
             ImplicitProvenanceCasts: ImplicitProvenanceCasts,
             CVoidReturns: CVoidReturns,
         ]

compiler/rustc_lint/src/lints.rs

@@ -1813,6 +1813,12 @@ pub(crate) struct NonLocalDefinitionsCargoUpdateNote {
     pub crate_name: Symbol,
 }
 
+// cmse_uninitialized_leak.rs
+#[derive(Diagnostic)]
+#[diag("value crossing a secure boundary contains a union which may leak secure information")]
+#[note("union values can have variant-dependent padding that may contain stale secure data")]
+pub(crate) struct CmseUninitializedMayLeakInformation;
+
 // precedence.rs
 #[derive(Diagnostic)]
 #[diag("`-` has lower precedence than method calls, which might be unexpected")]

tests/auxiliary/minicore.rs

@@ -118,12 +118,14 @@ pub enum Option<T> {
     Some(T),
 }
 impl<T: Copy> Copy for Option<T> {}
+pub use Option::*;
 
 pub enum Result<T, E> {
     Ok(T),
     Err(E),
 }
 impl<T: Copy, E: Copy> Copy for Result<T, E> {}
+pub use Result::*;
 
 #[lang = "manually_drop"]
 #[repr(transparent)]

tests/ui/cmse-nonsecure/cmse-nonsecure-call/params-uninitialized.rs

@@ -0,0 +1,102 @@
+//@ add-minicore
+//@ compile-flags: --target thumbv8m.main-none-eabi --crate-type lib
+//@ needs-llvm-components: arm
+//@ check-pass
+//@ ignore-backends: gcc
+#![feature(abi_cmse_nonsecure_call, no_core, lang_items)]
+#![no_core]
+#![allow(improper_ctypes_definitions)]
+
+extern crate minicore;
+use minicore::*;
+
+#[repr(Rust)]
+union ReprRustUnionU64 {
+    _unused: u64,
+}
+
+#[repr(C)]
+union ReprCUnionU64 {
+    _unused: u64,
+    _unused1: u32,
+}
+
+#[no_mangle]
+fn basic(
+    f1: extern "cmse-nonsecure-call" fn(ReprRustUnionU64),
+    f2: extern "cmse-nonsecure-call" fn(ReprCUnionU64),
+    f3: extern "cmse-nonsecure-call" fn(MaybeUninit<u32>),
+    f4: extern "cmse-nonsecure-call" fn(MaybeUninit<u64>),
+) {
+    f1(ReprRustUnionU64 { _unused: 1 });
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f2(ReprCUnionU64 { _unused: 1 });
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f3(MaybeUninit::uninit());
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f4(MaybeUninit::uninit());
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+}
+
+#[repr(C)]
+struct ReprCAggregate {
+    a: usize,
+    b: ReprCUnionU64,
+}
+
+#[repr(Rust)]
+union ReprRustUnionPartiallyUninit {
+    _unused1: u32,
+    _unused2: u16,
+}
+
+#[no_mangle]
+fn composite(
+    f5: extern "cmse-nonsecure-call" fn((usize, MaybeUninit<u64>)),
+    f6: extern "cmse-nonsecure-call" fn(ReprCAggregate),
+    f7: extern "cmse-nonsecure-call" fn(ReprRustUnionPartiallyUninit),
+) {
+    f5((0, MaybeUninit::uninit()));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f6(ReprCAggregate { a: 0, b: ReprCUnionU64 { _unused: 1 } });
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f7(ReprRustUnionPartiallyUninit { _unused1: 0 });
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+}
+
+#[no_mangle]
+fn nested(
+    f12: extern "cmse-nonsecure-call" fn(Option<MaybeUninit<u32>>),
+    f13: extern "cmse-nonsecure-call" fn(Result<MaybeUninit<u32>, ReprCUnionU64>),
+) {
+    f12(Some(MaybeUninit::uninit()));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f13(Ok(MaybeUninit::new(42)));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f13(Err(ReprCUnionU64 { _unused1: 0 }));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+}
+
+struct ArrayVec<T, const N: usize>([MaybeUninit<T>; N]);
+
+#[no_mangle]
+fn array_vec(
+    f0: extern "cmse-nonsecure-call" fn(ArrayVec<u8, 8>),
+    f1: extern "cmse-nonsecure-call" fn(ArrayVec<u8, 0>),
+) {
+    f0(ArrayVec([MaybeUninit::uninit(); 8]));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f0(ArrayVec([MaybeUninit::new(0xAA); 8]));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+
+    f1(ArrayVec([MaybeUninit::uninit(); 0]));
+    //~^ WARN value crossing a secure boundary contains a union which may leak secure information
+}