cfi: Simpler launder implementation for common types

cfi/derive/src/cfi_asm_test.rs

@@ -0,0 +1,78 @@
+// Licensed under the Apache-2.0 license
+
+// These tests are here so that they are excluded in FPGA tests.
+
+// These tests don't directly import the CFI code. If they fail,
+// this likely indicates that the CFI laundering code may not
+// be doing what we want, and we need to investigate.
+
+#[cfg(test)]
+mod test {
+
+    const START: &str = "
+#![no_std]
+
+pub fn add(mut a: u32, mut b: u32) -> u32 {
+    launder(a) + launder(a) + launder(b) + launder(b)
+}
+";
+
+    const LAUNDER: &str = "
+#[inline(always)]
+fn launder(mut val: u32) -> u32 {
+    // Safety: this is a no-op, since we don't modify the input.
+    unsafe {
+        core::arch::asm!(
+            \"/* {t} */\",
+            t = inout(reg) val,
+        );
+    }
+    val
+}";
+
+    const NO_LAUNDER: &str = "
+#[inline(always)]
+fn launder(mut val: u32) -> u32 {
+    val
+}
+";
+
+    fn compile_to_riscv32_asm(src: String) -> String {
+        let dir = std::env::temp_dir();
+        let src_path = dir.join("asm.rs");
+        let dst_path = dir.join("asm.s");
+
+        std::fs::write(src_path.clone(), src).expect("could not write asm file");
+
+        let p = std::process::Command::new("rustc")
+            .args([
+                "--crate-type=lib",
+                "--target",
+                "riscv32imc-unknown-none-elf",
+                "-C",
+                "opt-level=s",
+                "--emit",
+                "asm",
+                src_path.to_str().expect("could not convert path"),
+                "-o",
+                dst_path.to_str().expect("could not convert path"),
+            ])
+            .output()
+            .expect("failed to compile");
+        assert!(p.status.success());
+        std::fs::read_to_string(dst_path).expect("could not read asm file")
+    }
+
+    #[test]
+    fn test_launder() {
+        // With no laundering, LLVM can simplify the double add to a shift left.
+        let src = format!("{}{}", START, NO_LAUNDER);
+        let asm = compile_to_riscv32_asm(src);
+        assert!(asm.contains("sll"));
+
+        // With laundering, LLVM cannot simplify the double add and has to use the register twice.
+        let src = format!("{}{}", START, LAUNDER);
+        let asm = compile_to_riscv32_asm(src);
+        assert!(!asm.contains("sll"));
+    }
+}

cfi/derive/src/lib.rs

@@ -16,11 +16,14 @@ References:
 
 --*/
 
+mod cfi_asm_test;
+
 use proc_macro::TokenStream;
 use quote::{format_ident, quote, ToTokens};
 use syn::__private::TokenStream2;
 use syn::parse_macro_input;
 use syn::parse_quote;
+use syn::DeriveInput;
 use syn::FnArg;
 use syn::ItemFn;
 
@@ -94,3 +97,15 @@ fn cfi_fn(mod_fn: bool, input: TokenStream) -> TokenStream {
 
     code.into()
 }
+
+#[proc_macro_derive(Launder)]
+pub fn derive_launder_trait(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    let name = input.ident;
+    let (impl_generics, ty_generics, _) = input.generics.split_for_impl();
+    let expanded = quote! {
+        impl #impl_generics caliptra_cfi_lib::LaunderTrait<#name #ty_generics> for caliptra_cfi_lib::Launder<#name #ty_generics> {}
+        impl #impl_generics caliptra_cfi_lib::LaunderTrait<&#name #ty_generics> for caliptra_cfi_lib::Launder<&#name #ty_generics> {}
+    };
+    TokenStream::from(expanded)
+}

cfi/lib/src/cfi.rs

@@ -20,7 +20,7 @@ use caliptra_error::CaliptraError;
 use crate::CfiCounter;
 use core::cfg;
 use core::cmp::{Eq, Ord, PartialEq, PartialOrd};
-use core::marker::Copy;
+use core::marker::{Copy, PhantomData};
 
 /// CFI Panic Information
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
@@ -94,16 +94,107 @@ impl From<CfiPanicInfo> for CaliptraError {
 /// # Returns
 ///
 /// `T` - Same value
-pub fn cfi_launder<T>(val: T) -> T {
+pub fn cfi_launder<T>(val: T) -> T
+where
+    Launder<T>: LaunderTrait<T>,
+{
     if cfg!(feature = "cfi") {
-        // Note: The black box seems to be disabling more optimization
-        // than necessary and results in larger binary size
-        core::hint::black_box(val)
+        Launder { _val: PhantomData }.launder(val)
     } else {
         val
     }
 }
 
+pub trait LaunderTrait<T> {
+    fn launder(&self, val: T) -> T {
+        core::hint::black_box(val)
+    }
+}
+
+pub struct Launder<T> {
+    _val: PhantomData<T>,
+}
+
+// Inline-assembly laundering trick is adapted from OpenTitan:
+// https://github.com/lowRISC/opentitan/blob/master/sw/device/lib/base/hardened.h#L193
+//
+// NOTE: This implementation is LLVM-specific, and should be considered to be
+// a no-op in every other compiler. For example, GCC has in the past peered
+// into the insides of assembly blocks.
+//
+// At the time of writing, it seems preferable to have something we know is
+// correct rather than being overly clever; this is recorded here in case
+// the current implementation is unsuitable and we need something more
+// carefully tuned.
+//
+// Unlike in C, we don't have volatile assembly blocks, so this doesn't
+// necessarily prevent reordering by LLVM.
+//
+// When we're building for static analysis, reduce false positives by
+// short-circuiting the inline assembly block.
+impl LaunderTrait<u32> for Launder<u32> {
+    #[allow(asm_sub_register)]
+    fn launder(&self, val: u32) -> u32 {
+        let mut val = val;
+        // Safety: this is a no-op, since we don't modify the input.
+        unsafe {
+            // We use inout so that LLVM thinks the value might
+            // be mutated by the assembly and can't eliminate it.
+            core::arch::asm!(
+                "/* {t} */",
+                t = inout(reg) val,
+            );
+        }
+        val
+    }
+}
+
+impl LaunderTrait<bool> for Launder<bool> {
+    #[allow(asm_sub_register)]
+    fn launder(&self, val: bool) -> bool {
+        let mut val = val as u32;
+        // Safety: this is a no-op, since we don't modify the input.
+        unsafe {
+            core::arch::asm!(
+                "/* {t} */",
+                t = inout(reg) val,
+            );
+        }
+        val != 0
+    }
+}
+
+impl LaunderTrait<usize> for Launder<usize> {
+    #[allow(asm_sub_register)]
+    fn launder(&self, mut val: usize) -> usize {
+        // Safety: this is a no-op, since we don't modify the input.
+        unsafe {
+            core::arch::asm!(
+                "/* {t} */",
+                t = inout(reg) val,
+            );
+        }
+        val
+    }
+}
+
+impl<const N: usize, T> LaunderTrait<[T; N]> for Launder<[T; N]> {}
+impl<'a, const N: usize, T> LaunderTrait<&'a [T; N]> for Launder<&'a [T; N]> {
+    fn launder(&self, val: &'a [T; N]) -> &'a [T; N] {
+        let mut valp = val.as_ptr() as *const [T; N];
+        // Safety: this is a no-op, since we don't modify the input.
+        unsafe {
+            core::arch::asm!(
+                "/* {t} */",
+                t = inout(reg) valp,
+            );
+            &*valp
+        }
+    }
+}
+impl LaunderTrait<Option<u32>> for Launder<Option<u32>> {}
+impl LaunderTrait<CfiPanicInfo> for Launder<CfiPanicInfo> {}
+
 /// Control flow integrity panic
 ///
 /// This panic is raised when the control flow integrity error is detected
@@ -157,6 +248,7 @@ macro_rules! cfi_assert_macro {
         pub fn $name<T>(lhs: T, rhs: T)
         where
             T: $trait1 + $trait2,
+            Launder<T>: LaunderTrait<T>,
         {
             if cfg!(feature = "cfi") {
                 CfiCounter::delay();
@@ -184,10 +276,28 @@ cfi_assert_macro!(cfi_assert_lt, <, Ord, PartialOrd, AssertLtFail);
 cfi_assert_macro!(cfi_assert_ge, >=, Ord, PartialOrd, AssertGeFail);
 cfi_assert_macro!(cfi_assert_le, <=, Ord, PartialOrd, AssertLeFail);
 
+// special case for bool assert
+#[inline(always)]
+#[allow(unused)]
+pub fn cfi_assert_bool(cond: bool) {
+    if cfg!(feature = "cfi") {
+        CfiCounter::delay();
+        if !cond {
+            cfi_panic(CfiPanicInfo::AssertEqFail);
+        }
+
+        // Second check for glitch protection
+        CfiCounter::delay();
+        if !cfi_launder(cond) {
+            cfi_panic(CfiPanicInfo::AssertEqFail);
+        }
+    }
+}
+
 #[macro_export]
 macro_rules! cfi_assert {
     ($cond: expr) => {
-        cfi_assert_eq($cond, true)
+        cfi_assert_bool($cond)
     };
 }
 

drivers/src/array.rs

@@ -13,6 +13,7 @@ Abstract:
 
 --*/
 
+use caliptra_cfi_derive::Launder;
 use core::mem::MaybeUninit;
 use zerocopy::{AsBytes, FromBytes};
 use zeroize::Zeroize;
@@ -26,7 +27,7 @@ macro_rules! static_assert {
 /// The `Array4xN` type represents large arrays in the native format of the Caliptra
 /// cryptographic hardware, and provides From traits for converting to/from byte arrays.
 #[repr(transparent)]
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Zeroize)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Launder, Zeroize)]
 pub struct Array4xN<const W: usize, const B: usize>(pub [u32; W]);
 impl<const W: usize, const B: usize> Array4xN<W, B> {
     pub const fn new(val: [u32; W]) -> Self {

drivers/src/fuse_bank.rs

@@ -13,6 +13,7 @@ Abstract:
 --*/
 
 use crate::Array4x12;
+use caliptra_cfi_derive::Launder;
 use caliptra_registers::soc_ifc::SocIfcReg;
 use zerocopy::AsBytes;
 
@@ -33,7 +34,7 @@ pub enum X509KeyIdAlgo {
 }
 
 bitflags::bitflags! {
-    #[derive(Default, Copy, Clone, Debug)]
+    #[derive(Default, Copy, Clone, Debug, Launder)]
     pub struct VendorPubKeyRevocation : u32 {
         const KEY0 = 0b0001;
         const KEY1 = 0b0010;
@@ -76,7 +77,7 @@ impl FuseBank<'_> {
     /// * None
     ///
     /// # Returns
-    ///     key id crypto algorithm  
+    ///     key id crypto algorithm
     ///
     pub fn idev_id_x509_key_id_algo(&self) -> X509KeyIdAlgo {
         let soc_ifc_regs = self.soc_ifc.regs();
@@ -101,7 +102,7 @@ impl FuseBank<'_> {
     /// * None
     ///
     /// # Returns
-    ///     manufacturer serial number  
+    ///     manufacturer serial number
     ///
     pub fn ueid(&self) -> [u8; 17] {
         let soc_ifc_regs = self.soc_ifc.regs();

drivers/src/soc_ifc.rs

@@ -12,6 +12,7 @@ Abstract:
 
 --*/
 
+use caliptra_cfi_derive::Launder;
 use caliptra_error::{CaliptraError, CaliptraResult};
 use caliptra_registers::soc_ifc::enums::DeviceLifecycleE;
 use caliptra_registers::soc_ifc::{self, SocIfcReg};
@@ -382,7 +383,7 @@ impl From<u32> for MfgFlags {
 }
 
 /// Reset Reason
-#[derive(Debug, Eq, PartialEq, Copy, Clone)]
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Launder)]
 pub enum ResetReason {
     /// Cold Reset
     ColdReset,

fmc/src/flow/rt_alias.rs

@@ -12,8 +12,7 @@ Abstract:
 
 --*/
 use caliptra_cfi_derive::cfi_impl_fn;
-use caliptra_cfi_lib::cfi_assert_eq;
-use caliptra_cfi_lib::{cfi_assert, cfi_launder};
+use caliptra_cfi_lib::{cfi_assert, cfi_assert_bool, cfi_assert_eq, cfi_launder};
 
 use crate::flow::crypto::Crypto;
 use crate::flow::dice::{DiceInput, DiceOutput};

image/types/Cargo.toml

@@ -10,6 +10,8 @@ doctest = false
 
 [dependencies]
 arbitrary = { workspace = true, optional = true }
+caliptra-cfi-derive.workspace = true
+caliptra-cfi-lib.workspace = true
 caliptra-lms-types.workspace = true
 memoffset.workspace = true
 zerocopy.workspace = true