[Libguestfs] [PATCH nbdkit 1/2] ocaml: Initialize pread buffer with zeroes to avoid leaking heap memory.

[Richard W.M. Jones]

In the C part of the OCaml plugin we create a ‘bytes’ [byte array] and
pass it to the OCaml pread method.  The plugin should overwrite the
array with the returned data.

However if (eg. because of a bug) the plugin does not fill the array
then whatever was in the OCaml or possibly even the C heap before the
allocation is returned to the client, possibly resulting in a leak of
sensitive data.

We can avoid this by initializing the array with zeroes.

Credit: Eric Blake for finding the bug.
---
 plugins/ocaml/ocaml.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/plugins/ocaml/ocaml.c b/plugins/ocaml/ocaml.c
index d854f48..7193842 100644
--- a/plugins/ocaml/ocaml.c
+++ b/plugins/ocaml/ocaml.c
@@ -444,6 +444,10 @@ pread_wrapper (void *h, void *buf, uint32_t count, uint64_t offset,
   caml_leave_blocking_section ();
 
   strv = caml_alloc_string (count);
+  /* Initialize the buffer with zeroes in case the plugin does not
+   * fill it completely.
+   */
+  memset (String_val (strv), 0, count);
   offsetv = caml_copy_int64 (offset);
   flagsv = Val_flags (flags);
 
-- 
2.20.1