CBMC: Add proofs for verify and cmov

There is some bitfiddling in cmov and verify that CBMC didn't like:

- Replace `b = -b` by `b = (-b) & 0xFF` to avoid truncation error
- Disable signed-to-unsigned integer conversion check which by default
  rejects conversion of negative integers to unsigned integers.

Signed-off-by: Hanno Becker <[email protected]>

cbmc/proofs/cmov/Makefile

@@ -0,0 +1,54 @@
+# SPDX-License-Identifier: Apache-2.0
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = cmov_harness
+
+# This should be a unique identifier for this proof, and will appear on the
+# Litani dashboard. It can be human-readable and contain spaces if you wish.
+PROOF_UID = cmov
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mlkem/verify.c
+
+CHECK_FUNCTION_CONTRACTS=$(MLKEM_NAMESPACE)cmov
+USE_FUNCTION_CONTRACTS=
+APPLY_LOOP_CONTRACTS=on
+USE_DYNAMIC_FRAMES=1
+
+# Disable any setting of EXTERNAL_SAT_SOLVER, and choose SMT backend instead
+EXTERNAL_SAT_SOLVER=
+CBMCFLAGS=--smt2
+
+FUNCTION_NAME = $(MLKEM_NAMESPACE)cmov
+
+# If this proof is found to consume huge amounts of RAM, you can set the
+# EXPENSIVE variable. With new enough versions of the proof tools, this will
+# restrict the number of EXPENSIVE CBMC jobs running at once. See the
+# documentation in Makefile.common under the "Job Pools" heading for details.
+# EXPENSIVE = true
+
+# This function is large enough to need...
+CBMC_OBJECT_BITS = 8
+
+# If you require access to a file-local ("static") function or object to conduct
+# your proof, set the following (and do not include the original source file
+# ("mlkem/poly.c") in PROJECT_SOURCES).
+# REWRITTEN_SOURCES = $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i
+# include ../Makefile.common
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_SOURCE = $(SRCDIR)/mlkem/poly.c
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_FUNCTIONS = foo bar
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_OBJECTS = baz
+# Care is required with variables on the left-hand side: REWRITTEN_SOURCES must
+# be set before including Makefile.common, but any use of variables on the
+# left-hand side requires those variables to be defined. Hence, _SOURCE,
+# _FUNCTIONS, _OBJECTS is set after including Makefile.common.
+
+include ../Makefile.common

cbmc/proofs/cmov/cbmc-proof.txt

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: Apache-2.0
+
+# This file marks this directory as containing a CBMC proof.

cbmc/proofs/cmov/cmov_harness.c

@@ -0,0 +1,29 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: MIT-0 AND Apache-2.0
+
+/*
+ * Insert copyright notice
+ */
+
+/**
+ * @file cmov_harness.c
+ * @brief Implements the proof harness for cmov function.
+ */
+#include "verify.h"
+
+/*
+ * Insert project header files that
+ *   - include the declaration of the function
+ *   - include the types needed to declare function arguments
+ */
+
+/**
+ * @brief Starting point for formal analysis
+ *
+ */
+void harness(void) {
+  uint8_t *x, *y;
+  size_t len;
+  uint8_t b;
+  cmov(x, y, len, b);
+}

cbmc/proofs/verify/Makefile

@@ -25,9 +25,9 @@ USE_DYNAMIC_FRAMES=1
 
 # Disable any setting of EXTERNAL_SAT_SOLVER, and choose SMT backend instead
 EXTERNAL_SAT_SOLVER=
-CBMCFLAGS=--bitwuzla
+CBMCFLAGS=--smt2
 
-FUNCTION_NAME = verify
+FUNCTION_NAME = $(MLKEM_NAMESPACE)verify
 
 # If this proof is found to consume huge amounts of RAM, you can set the
 # EXPENSIVE variable. With new enough versions of the proof tools, this will

cbmc/proofs/verify/verify_harness.c

@@ -7,9 +7,9 @@
 
 /**
  * @file verify_harness.c
- * @brief Implements the proof harness for basemul_cached function.
+ * @brief Implements the proof harness for verify function.
  */
-#include "poly.h"
+#include "verify.h"
 
 /*
  * Insert project header files that

mlkem/verify.c

@@ -35,10 +35,14 @@ int verify(const uint8_t *a, const uint8_t *b, const size_t len) {
 void cmov(uint8_t *r, const uint8_t *x, size_t len, uint8_t b) {
   size_t i;
 
-  b = -b;
-  for (i = 0; i < len; i++) {
-    r[i] ^= b & (r[i] ^ x[i]);
-  }
+  b = (-b) & 0xFF;
+  for (i = 0; i < len; i++)  // clang-format off
+    ASSIGNS(i, OBJECT_UPTO(r, len))
+    INVARIANT(i <= len)
+    // clang-format on
+    {
+      r[i] ^= b & (r[i] ^ x[i]);
+    }
 }
 
 /*************************************************

mlkem/verify.h

@@ -43,10 +43,12 @@ ENSURES(RETURN_VALUE == (1 - FORALL(int, i, 0, ((int)len - 1), (a[i] == b[i]))))
  *              uint8_t b:        Condition bit; has to be in {0,1}
  **************************************************/
 void cmov(uint8_t *r, const uint8_t *x, size_t len,
-          uint8_t b)  // clang-format on
-    REQUIRES(IS_FRESH(r, len)) REQUIRES(IS_FRESH(x, len))
-        REQUIRES(b == 0 || b == 1) ASSIGNS(OBJECT_UPTO(r, len));
-// clang-format off
+          uint8_t b)  // clang-format off
+  REQUIRES(IS_FRESH(r, len))
+  REQUIRES(IS_FRESH(x, len))
+  REQUIRES(b == 0 || b == 1)
+  ASSIGNS(OBJECT_UPTO(r, len));
+// clang-format on
 
 #define cmov_int16 MLKEM_NAMESPACE(cmov_int16)
 /*************************************************