url: fix alignment of ssl_backend_data struct

- Align the array of ssl_backend_data on a max 32 byte boundary.

8 is likely to be ok but I went with 32 for posterity should one of
the ssl_backend_data structs change to contain a larger sized variable
in the future.

Prior to this change (since dev 70f1db3, release 7.56) the connectdata
structure was undersized by 4 bytes in 32-bit builds with ssl enabled
because long long * was mistakenly used for alignment instead of
long long, with the intention being an 8 byte boundary. Also long long
may not be an available type.

The undersized connectdata could lead to oob read/write past the end in
what was expected to be the last 4 bytes of the connection's secondary
socket https proxy ssl_backend_data struct (the secondary socket in a
connection is used by ftp, others?).

Closes https://github.com/curl/curl/issues/2093

CVE-2017-8818

Bug: https://curl.haxx.se/docs/adv_2017-af0a.html

lib/url.c

@@ -1793,15 +1793,41 @@ static void llist_dtor(void *user, void *element)
  */
 static struct connectdata *allocate_conn(struct Curl_easy *data)
 {
+  struct connectdata *conn;
+  size_t connsize = sizeof(struct connectdata);
+
 #ifdef USE_SSL
-#define SSL_EXTRA + 4 * Curl_ssl->sizeof_ssl_backend_data - sizeof(long long)
+/* SSLBK_MAX_ALIGN: The max byte alignment a CPU would use */
-#else
+#define SSLBK_MAX_ALIGN 32
-#define SSL_EXTRA 0
+  /* The SSL backend-specific data (ssl_backend_data) objects are allocated as
+     part of connectdata at the end. To ensure suitable alignment we will
+     assume a maximum of SSLBK_MAX_ALIGN for alignment. Since calloc returns a
+     pointer suitably aligned for any variable this will ensure the
+     ssl_backend_data array has proper alignment, even if that alignment turns
+     out to be less than SSLBK_MAX_ALIGN. */
+  size_t paddingsize = sizeof(struct connectdata) % SSLBK_MAX_ALIGN;
+  size_t alignsize = paddingsize ? (SSLBK_MAX_ALIGN - paddingsize) : 0;
+  size_t sslbksize = Curl_ssl->sizeof_ssl_backend_data;
+  connsize += alignsize + (4 * sslbksize);
 #endif
-  struct connectdata *conn = calloc(1, sizeof(struct connectdata) + SSL_EXTRA);
+
+  conn = calloc(1, connsize);
   if(!conn)
     return NULL;
 
+#ifdef USE_SSL
+  /* Point to the ssl_backend_data objects at the end of connectdata.
+     Note that these backend pointers can be swapped by vtls (eg ssl backend
+     data becomes proxy backend data). */
+  {
+    char *end = (char *)conn + connsize;
+    conn->ssl[0].backend = ((void *)(end - (4 * sslbksize)));
+    conn->ssl[1].backend = ((void *)(end - (3 * sslbksize)));
+    conn->proxy_ssl[0].backend = ((void *)(end - (2 * sslbksize)));
+    conn->proxy_ssl[1].backend = ((void *)(end - (1 * sslbksize)));
+  }
+#endif
+
   conn->handler = &Curl_handler_dummy;  /* Be sure we have a handler defined
                                            already from start to avoid NULL
                                            situations and checks */
@@ -1881,23 +1907,6 @@ static struct connectdata *allocate_conn(struct Curl_easy *data)
 
   conn->ip_version = data->set.ipver;
 
-#ifdef USE_SSL
-  /*
-   * To save on malloc()s, the SSL backend-specific data has been allocated
-   * at the end of the connectdata struct.
-   */
-  {
-    char *p = (char *)&conn->align_data__do_not_use;
-    conn->ssl[0].backend = (struct ssl_backend_data *)p;
-    conn->ssl[1].backend =
-      (struct ssl_backend_data *)(p + Curl_ssl->sizeof_ssl_backend_data);
-    conn->proxy_ssl[0].backend =
-      (struct ssl_backend_data *)(p + Curl_ssl->sizeof_ssl_backend_data * 2);
-    conn->proxy_ssl[1].backend =
-      (struct ssl_backend_data *)(p + Curl_ssl->sizeof_ssl_backend_data * 3);
-  }
-#endif
-
 #if !defined(CURL_DISABLE_HTTP) && defined(USE_NTLM) && \
     defined(NTLM_WB_ENABLED)
   conn->ntlm_auth_hlpr_socket = CURL_SOCKET_BAD;

lib/urldata.h

@@ -1004,16 +1004,6 @@ struct connectdata {
   char *unix_domain_socket;
   bool abstract_unix_socket;
 #endif
-
-#ifdef USE_SSL
-  /*
-   * To avoid multiple malloc() calls, the ssl_connect_data structures
-   * associated with a connectdata struct are allocated in the same block
-   * as the latter. This field forces alignment to an 8-byte boundary so
-   * that this all works.
-   */
-  long long *align_data__do_not_use;
-#endif
 };
 
 /* The end of connectdata. */