krb5: avoid realloc(0)

If the requested size is zero, bail out with error instead of doing a
realloc() that would cause a double-free: realloc(0) acts as a free()
and then there's a second free in the cleanup path.

CVE-2016-8619

Bug: https://curl.haxx.se/docs/adv_20161102E.html
Reported-by: Cure53

diff --git a/lib/security.c b/lib/security.c
index a268d4a6252d5d50715bd241001ed04533f93d06..4cef8f89ffa085197c7280417f61347378168d6a 100644 (file)
--- a/lib/security.c
+++ b/lib/security.c
@@ -192,15 +192,18 @@ static CURLcode read_data(struct connectdata *conn,
                           struct krb5buffer *buf)
 {
   int len;
-  void* tmp;
+  void *tmp = NULL;
   CURLcode result;
 
   result = socket_read(fd, &len, sizeof(len));
   if(result)
     return result;
 
-  len = ntohl(len);
-  tmp = realloc(buf->data, len);
+  if(len) {
+    /* only realloc if there was a length */
+    len = ntohl(len);
+    tmp = realloc(buf->data, len);
+  }
   if(tmp == NULL)
     return CURLE_OUT_OF_MEMORY;