Fix undocumented free() in x509_string_to_names()

Now programs/x509/cert_write san="DN:CN=#0000;DN:CN=#0000" is no longer
crashing with use-after-free, instead it's now failing cleanly:

 failed
  !  mbedtls_x509_string_to_names returned -0x2800 - X509 - Input invalid

That's better of course but still not great, will be fixed by future
commits.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

ChangeLog.d/fix-string-to-names-memory-management.txt

@@ -0,0 +1,18 @@
+Security
+   * Fix possible use-after-free or double-free in code calling
+     mbedtls_x509_string_to_names(). This was caused by the function calling
+     mbedtls_asn1_free_named_data_list() on its head argument, while the
+     documentation did no suggest it did, making it likely for callers relying
+     on the documented behaviour to still hold pointers to memory blocks after
+     they were free()d, resulting in high risk of use-after-free or double-free,
+     with consequences ranging up to arbitrary code execution.
+     In particular, the two sample programs x509/cert_write and x509/cert_req
+     were affected (use-after-free if the san string contains more than one DN).
+     Code that does not call mbedtls_string_to_names() directly is not affected.
+     Found by Linh Le and Ngan Nguyen from Calif.
+
+Changes
+   * The function mbedtls_x509_string_to_names() now requires its head argument
+     to point to NULL on entry. This make it likely that existing risky uses of
+     this function (see the entry in the Security section) will be detected and
+     fixed.

include/mbedtls/x509.h

@@ -332,7 +332,8 @@ int mbedtls_x509_dn_gets(char *buf, size_t size, const mbedtls_x509_name *dn);
  *                   call to mbedtls_asn1_free_named_data_list().
  *
  * \param[out] head  Address in which to store the pointer to the head of the
- *                   allocated list of mbedtls_x509_name
+ *                   allocated list of mbedtls_x509_name. Must point to NULL on
+ *                   entry.
  * \param[in] name   The string representation of a DN to convert
  *
  * \return           0 on success, or a negative error code.

library/x509_create.c

@@ -467,8 +467,12 @@ int mbedtls_x509_string_to_names(mbedtls_asn1_named_data **head, const char *nam
     unsigned char data[MBEDTLS_X509_MAX_DN_NAME_SIZE];
     size_t data_len = 0;
 
-    /* Clear existing chain if present */
-    mbedtls_asn1_free_named_data_list(head);
+    /* Ensure the output parameter is not already populated.
+     * (If it were, overwriting it would likely cause a memory leak.)
+     */
+    if (*head != NULL) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
 
     while (c <= end) {
         if (in_attr_type && *c == '=') {