Add a new X.509 API call for copy-less parsing of CRTs

Context:
The existing API `mbedtls_x509_parse_crt_der()` for parsing DER
encoded X.509 CRTs unconditionally makes creates a copy of the
input buffer in RAM. While this comes at the benefit of easy use,
-- specifically: allowing the user to free or re-use the input
buffer right after the call -- it creates a significant memory
overhead, as the CRT is duplicated in memory (at least temporarily).
This might not be tolerable a resource constrained device.

As a remedy, this commit adds a new X.509 API call

   `mbedtls_x509_parse_crt_der_nocopy()`

which has the same signature as `mbedtls_x509_parse_crt_der()`
and almost the same semantics, with one difference: The input
buffer must persist and be unmodified for the lifetime of the
established instance of `mbedtls_x509_crt`, that is, until
`mbedtls_x509_crt_free()` is called.

include/mbedtls/x509_crt.h

@@ -52,6 +52,8 @@ extern "C" {
  */
 typedef struct mbedtls_x509_crt
 {
+    int own_buffer;                     /**< Indicates if \c raw is owned
+                                         *   by the structure or not.        */
     mbedtls_x509_buf raw;               /**< The raw certificate data (DER). */
     mbedtls_x509_buf tbs;               /**< The raw certificate body (DER). The part that is To Be Signed. */
 
@@ -220,15 +222,57 @@ extern const mbedtls_x509_crt_profile mbedtls_x509_crt_profile_suiteb;
 
 /**
  * \brief          Parse a single DER formatted certificate and add it
- *                 to the chained list.
+ *                 to the end of the provided chained list.
  *
- * \param chain    points to the start of the chain
+ * \param chain    The pointer to the start of the CRT chain to attach to.
- * \param buf      buffer holding the certificate DER data
+ *                 When parsing the first CRT in a chain, this should point
- * \param buflen   size of the buffer
+ *                 to an instance of ::mbedtls_x509_crt initialized through
+ *                 mbedtls_x509_crt_init().
+ * \param buf      The buffer holding the DER encoded certificate.
+ * \param buflen   The size in Bytes of \p buf.
  *
- * \return         0 if successful, or a specific X509 or PEM error code
+ * \note           This function makes an internal copy of the CRT buffer
+ *                 \p buf. In particular, \p buf may be destroyed or reused
+ *                 after this call returns. To avoid duplicating the CRT
+ *                 buffer (at the cost of stricter lifetime constraints),
+ *                 use mbedtls_x509_crt_parse_der_nocopy() instead.
+ *
+ * \return         \c 0 if successful.
+ * \return         A negative error code on failure.
  */
-int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *buf,
+int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain,
+                                const unsigned char *buf,
+                                size_t buflen );
+
+/**
+ * \brief          Parse a single DER formatted certificate and add it
+ *                 to the end of the provided chained list. This is a
+ *                 variant of mbedtls_x509_crt_parse_der() which takes
+ *                 temporary ownership of the CRT buffer until the CRT
+ *                 is destroyed.
+ *
+ * \param chain    The pointer to the start of the CRT chain to attach to.
+ *                 When parsing the first CRT in a chain, this should point
+ *                 to an instance of ::mbedtls_x509_crt initialized through
+ *                 mbedtls_x509_crt_init().
+ * \param buf      The address of the readable buffer holding the DER encoded
+ *                 certificate to use. On success, this buffer must be
+ *                 retained and not be changed for the liftetime of the
+ *                 CRT chain \p chain, that is, until \p chain is destroyed
+ *                 through a call to mbedtls_x509_crt_free().
+ * \param buflen   The size in Bytes of \p buf.
+ *
+ * \note           This call is functionally equivalent to
+ *                 mbedtls_x509_crt_parse_der(), but it avoids creating a
+ *                 copy of the input buffer at the cost of stronger lifetime
+ *                 constraints. This is useful in constrained environments
+ *                 where duplication of the CRT cannot be tolerated.
+ *
+ * \return         \c 0 if successful.
+ * \return         A negative error code on failure.
+ */
+int mbedtls_x509_crt_parse_der_nocopy( mbedtls_x509_crt *chain,
+                                       const unsigned char *buf,
                                        size_t buflen );
 
 /**

library/x509_crt.c

@@ -829,8 +829,10 @@ static int x509_get_crt_ext( unsigned char **p,
 /*
  * Parse and fill a single X.509 certificate in DER format
  */
-static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *buf,
+static int x509_crt_parse_der_core( mbedtls_x509_crt *crt,
-                                    size_t buflen )
+                                    const unsigned char *buf,
+                                    size_t buflen,
+                                    int make_copy )
 {
     int ret;
     size_t len;
@@ -847,7 +849,7 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
     if( crt == NULL || buf == NULL )
         return( MBEDTLS_ERR_X509_BAD_INPUT_DATA );
 
-    // Use the original buffer until we figure out actual length
+    /* Use the original buffer until we figure out actual length. */
     p = (unsigned char*) buf;
     len = buflen;
     end = p + len;
@@ -865,25 +867,26 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
         return( MBEDTLS_ERR_X509_INVALID_FORMAT );
     }
 
-    if( len > (size_t) ( end - p ) )
+    end = crt_end = p + len;
-    {
-        mbedtls_x509_crt_free( crt );
-        return( MBEDTLS_ERR_X509_INVALID_FORMAT +
-                MBEDTLS_ERR_ASN1_LENGTH_MISMATCH );
-    }
-    crt_end = p + len;
-
-    // Create and populate a new buffer for the raw field
     crt->raw.len = crt_end - buf;
+    if( make_copy != 0 )
+    {
+        /* Create and populate a new buffer for the raw field. */
         crt->raw.p = p = mbedtls_calloc( 1, crt->raw.len );
-    if( p == NULL )
+        if( crt->raw.p == NULL )
             return( MBEDTLS_ERR_X509_ALLOC_FAILED );
 
-    memcpy( p, buf, crt->raw.len );
+        memcpy( crt->raw.p, buf, crt->raw.len );
+        crt->own_buffer = 1;
 
-    // Direct pointers to the new buffer
         p += crt->raw.len - len;
         end = crt_end = p + len;
+    }
+    else
+    {
+        crt->raw.p = (unsigned char*) buf;
+        crt->own_buffer = 0;
+    }
 
     /*
      * TBSCertificate  ::=  SEQUENCE  {
@@ -1086,8 +1089,10 @@ static int x509_crt_parse_der_core( mbedtls_x509_crt *crt, const unsigned char *
  * Parse one X.509 certificate in DER format from a buffer and add them to a
  * chained list
  */
-int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *buf,
+static int mbedtls_x509_crt_parse_der_internal( mbedtls_x509_crt *chain,
-                        size_t buflen )
+                                                const unsigned char *buf,
+                                                size_t buflen,
+                                                int make_copy )
 {
     int ret;
     mbedtls_x509_crt *crt = chain, *prev = NULL;
@@ -1119,7 +1124,7 @@ int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *bu
         crt = crt->next;
     }
 
-    if( ( ret = x509_crt_parse_der_core( crt, buf, buflen ) ) != 0 )
+    if( ( ret = x509_crt_parse_der_core( crt, buf, buflen, make_copy ) ) != 0 )
     {
         if( prev )
             prev->next = NULL;
@@ -1133,11 +1138,27 @@ int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain, const unsigned char *bu
     return( 0 );
 }
 
+int mbedtls_x509_crt_parse_der_nocopy( mbedtls_x509_crt *chain,
+                                       const unsigned char *buf,
+                                       size_t buflen )
+{
+    return( mbedtls_x509_crt_parse_der_internal( chain, buf, buflen, 0 ) );
+}
+
+int mbedtls_x509_crt_parse_der( mbedtls_x509_crt *chain,
+                                const unsigned char *buf,
+                                size_t buflen )
+{
+    return( mbedtls_x509_crt_parse_der_internal( chain, buf, buflen, 1 ) );
+}
+
 /*
  * Parse one or more PEM certificates from a buffer and add them to the chained
  * list
  */
-int mbedtls_x509_crt_parse( mbedtls_x509_crt *chain, const unsigned char *buf, size_t buflen )
+int mbedtls_x509_crt_parse( mbedtls_x509_crt *chain,
+                            const unsigned char *buf,
+                            size_t buflen )
 {
 #if defined(MBEDTLS_PEM_PARSE_C)
     int success = 0, first_error = 0, total_failed = 0;
@@ -2675,7 +2696,7 @@ void mbedtls_x509_crt_free( mbedtls_x509_crt *crt )
             mbedtls_free( seq_prv );
         }
 
-        if( cert_cur->raw.p != NULL )
+        if( cert_cur->raw.p != NULL && cert_cur->own_buffer )
         {
             mbedtls_platform_zeroize( cert_cur->raw.p, cert_cur->raw.len );
             mbedtls_free( cert_cur->raw.p );