commit 91dc67d31ca12b1ac2bbfdbfd4a55059713497f9
author Janos Follath <janos.follath@arm.com> Fri Jul 22 14:24:58 2022 +0100
committer Janos Follath <janos.follath@arm.com> Fri Aug 05 17:08:52 2022 +0100
tree af158787a54c862ed58d409127a6ccd0a4daf741
parent 4670f8899135837b17cbaf2255b7f8614b9cd9ba

Allow (NULL, 0) as a representation of 0

- We don't check for NULL pointers this deep in the library
- Accessing a NULL pointer when the limb number is 0 as a mistake is the
  very similar to any other out of bounds access
- We could potentially mandate at least 1 limb representation for 0 but
  we either would need to enforce it or the implementation would be less
  robust.
- Allowing zero limb representation - (NULL, 0) in particular - for zero
  is present in the legacy interface, if we disallow it, the
  compatibility code will need to deal with this (more code size and
  opportunities for mistakes)

In summary, interpreting (NULL, 0) as the number zero in the core
interface is the least of the two evils.

Signed-off-by: Janos Follath <janos.follath@arm.com>

library/bignum_new.c

diff --git a/library/bignum_new.c b/library/bignum_new.c
index 6cbc867..04f6049 100644
--- a/library/bignum_new.c
+++ b/library/bignum_new.c
@@ -190,17 +190,13 @@
                            size_t nx,
                            size_t limbs )
 {
-    if( X == NULL )
-        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
-
-    else if( nx < limbs )
+    if( nx < limbs )
         return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
 
-    else
-    {
+    if( X != NULL )
         memset( X, 0, nx * ciL );
-        return( 0 );
-    }
+
+    return( 0 );
 }
 
 /* Convert a big-endian byte array aligned to the size of mbedtls_mpi_uint

tests/suites/test_suite_mpi.data

diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index 30447a6..1974e3f 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -82,6 +82,9 @@
 Test mpi_write_string #10 (Negative hex with odd number of digits)
 mpi_read_write_string:16:"-1":16:"":3:0:MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL
 
+Test mbedtls_mpi_core_io functions with null pointers
+mbedtls_mpi_core_io_null
+
 Test mbedtls_mpi_core_io_be #1 (Buffer and limbs just fit, input limb-aligned)
 mbedtls_mpi_core_io_be:"0941379d00fed1491fe15df284dfde4a142f68aa8d412023195cee66883e6290ffe703f4ea5963bf212713cee46b107c09182b5edcd955adac418bf4918e2889af48e1099d513830cec85c26ac1e158b52620e33ba8692f893efbb2f958b4424":96:12:0:0
 

tests/suites/test_suite_mpi.function

diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index 34710c4..b0d947c 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -198,6 +198,37 @@
 /* END_CASE */
 
 /* BEGIN_CASE */
+void mbedtls_mpi_core_io_null()
+{
+    mbedtls_mpi_uint X = 0;
+    int ret;
+
+    ret = mbedtls_mpi_core_read_be( &X, 1, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+    ret = mbedtls_mpi_core_write_be( &X, 1, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+
+    ret = mbedtls_mpi_core_read_be( NULL, 0, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+    ret = mbedtls_mpi_core_write_be( NULL, 0, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+
+    ret = mbedtls_mpi_core_read_le( &X, 1, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+    ret = mbedtls_mpi_core_write_le( &X, 1, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+
+    ret = mbedtls_mpi_core_read_le( NULL, 0, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+    ret = mbedtls_mpi_core_write_le( NULL, 0, NULL, 0 );
+    TEST_ASSERT( ret == 0 );
+
+exit:
+    ;
+}
+/* END_CASE */
+
+/* BEGIN_CASE */
 void mbedtls_mpi_core_io_be( data_t *input, int nb_int, int nx_64_int, int iret,
                              int oret )
 {