sign stack usage: Re-use y/h buffer

mldsa/src/sign.c

@@ -432,6 +432,24 @@ __contract__(
 /* @[FIPS204, Appendix C].                                        */
 #define MLD_NONCE_UB ((UINT16_MAX - MLDSA_L) / MLDSA_L)
 
+#ifdef CBMC
+static void mld_cbmc_workaround_8813(mld_polyvecl **p1, mld_polyveck **p2)
+__contract__(
+  requires(memory_no_alias(p1, sizeof(mld_polyvecl *)))
+  requires(memory_no_alias(p2, sizeof(mld_polyveck *)))
+  requires(memory_no_alias(*p2, sizeof(mld_polyveck)))
+  requires(*p1 == *p2)
+  assigns(*p1)
+  assigns(*p2)
+  ensures(memory_no_alias(*p2, sizeof(mld_polyveck)))
+  ensures(*p1 == *p2)
+)
+{
+  (void)p1;
+  (void)p2;
+}
+#endif /* CBMC */
+
 /*************************************************
  * Name:        attempt_signature_generation
  *
@@ -486,20 +504,33 @@ __contract__(
   uint32_t z_invalid, w0_invalid, h_invalid;
   int ret;
   MLD_ALLOC(challenge_bytes, uint8_t, MLDSA_CTILDEBYTES);
-  MLD_ALLOC(y, mld_polyvecl, 1);
+  typedef union
+  {
+    mld_polyvecl y;
+    mld_polyveck h;
+  } u_yh;
+  MLD_ALLOC(yh, u_yh, 1);
   MLD_ALLOC(z, mld_polyvecl, 1);
   MLD_ALLOC(w1, mld_polyveck, 1);
   MLD_ALLOC(w0, mld_polyveck, 1);
-  MLD_ALLOC(h, mld_polyveck, 1);
   MLD_ALLOC(cp, mld_poly, 1);
+  mld_polyvecl *y = &yh->y;
+  mld_polyveck *h = &yh->h;
+
+#ifdef CBMC
+  /* TODO: Remove once
+   * https://github.com/diffblue/cbmc/issues/8813 is resolved */
+  mld_cbmc_workaround_8813(&y, &h);
+#endif
 
-  if (challenge_bytes == NULL || y == NULL || z == NULL || w1 == NULL ||
-      w0 == NULL || h == NULL || cp == NULL)
+  if (challenge_bytes == NULL || yh == NULL || z == NULL || w1 == NULL ||
+      w0 == NULL || cp == NULL)
   {
     ret = MLD_ERR_OUT_OF_MEMORY;
     goto cleanup;
   }
 
+
   /* Sample intermediate vector y */
   mld_polyvecl_uniform_gamma1(y, rhoprime, nonce);
 
@@ -611,11 +642,10 @@ __contract__(
 cleanup:
   /* @[FIPS204, Section 3.6.3] Destruction of intermediate values. */
   MLD_FREE(challenge_bytes, uint8_t, MLDSA_CTILDEBYTES);
-  MLD_FREE(y, mld_polyvecl, 1);
+  MLD_FREE(yh, u_yh, 1);
   MLD_FREE(z, mld_polyvecl, 1);
   MLD_FREE(w1, mld_polyveck, 1);
   MLD_FREE(w0, mld_polyveck, 1);
-  MLD_FREE(h, mld_polyveck, 1);
   MLD_FREE(cp, mld_poly, 1);
 
   return ret;

proofs/cbmc/attempt_signature_generation/Makefile

@@ -43,7 +43,7 @@ USE_FUNCTION_CONTRACTS=$(MLD_NAMESPACE)polyvecl_uniform_gamma1 \
                        $(MLD_NAMESPACE)polyveck_add \
                        $(MLD_NAMESPACE)polyveck_make_hint \
                        $(MLD_NAMESPACE)pack_sig
-USE_FUNCTION_CONTRACTS+=mld_zeroize
+USE_FUNCTION_CONTRACTS+=mld_zeroize mld_cbmc_workaround_8813
 
 APPLY_LOOP_CONTRACTS=on
 USE_DYNAMIC_FRAMES=1

proofs/cbmc/cbmc_workaround_8813/Makefile

@@ -0,0 +1,58 @@
+# Copyright (c) The mldsa-native project authors
+# SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+include ../Makefile_params.common
+
+HARNESS_ENTRY = harness
+HARNESS_FILE = cbmc_workaround_8813_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 = mld_cbmc_workaround_8813
+
+DEFINES +=
+INCLUDES +=
+
+REMOVE_FUNCTION_BODY +=
+UNWINDSET +=
+
+PROOF_SOURCES += $(PROOFDIR)/$(HARNESS_FILE).c
+PROJECT_SOURCES += $(SRCDIR)/mldsa/src/sign.c
+
+CHECK_FUNCTION_CONTRACTS=mld_cbmc_workaround_8813
+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
+CBMCFLAGS += --slice-formula
+CBMCFLAGS += --no-array-field-sensitivity
+
+FUNCTION_NAME = mld_cbmc_workaround_8813
+
+# 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 = 12
+
+# 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
+# ("mldsa/poly.c") in PROJECT_SOURCES).
+# REWRITTEN_SOURCES = $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i
+# include ../Makefile.common
+# $(PROOFDIR)/<__SOURCE_FILE_BASENAME__>.i_SOURCE = $(SRCDIR)/mldsa/src/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

proofs/cbmc/cbmc_workaround_8813/cbmc_workaround_8813_harness.c

@@ -0,0 +1,14 @@
+// Copyright (c) The mldsa-native project authors
+// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT
+
+#include "sign.h"
+
+void mld_cbmc_workaround_8813(mld_polyvecl **p1, mld_polyveck **p2);
+
+void harness(void)
+{
+  mld_polyvecl **p1;
+  mld_polyveck **p2;
+
+  mld_cbmc_workaround_8813(p1, p2);
+}