core: Do not leak the override session key to ps(1).

* src/engine-gpg.c (struct engine_gpg): New field
override_session_key.
(gpg_release): Free that field.
(gpg_decrypt): With gnupg 2.1.16 use --override-session-key-fd.

* tests/run-decrypt.c (main): Fix setting over the override key.
--

Note that this works only with gnupg 2.1.16 and later.

Signed-off-by: Werner Koch <wk@gnupg.org>

doc/gpgme.texi

@@ -2910,7 +2910,9 @@ not exported.
 The string given in @var{value} is passed to the GnuPG engine to override
 the session key for decryption.  The format of that session key is
 specific to GnuPG and can be retrieved during a decrypt operation when
-the context flag "export-session-key" is enabled.
+the context flag "export-session-key" is enabled.  Please be aware that
+using this feature with GnuPG < 2.1.16 will leak the session key on
+many platforms via ps(1).
 
 @end table
 

src/engine-gpg.c

@@ -139,6 +139,9 @@ struct engine_gpg
 
   struct gpgme_io_cbs io_cbs;
   gpgme_pinentry_mode_t pinentry_mode;
+
+  /* NULL or the data object fed to --override_session_key-fd.  */
+  gpgme_data_t override_session_key;
 };
 
 typedef struct engine_gpg *engine_gpg_t;
@@ -441,6 +444,8 @@ gpg_release (void *engine)
   if (gpg->cmd.keyword)
     free (gpg->cmd.keyword);
 
+  gpgme_data_release (gpg->override_session_key);
+
   free (gpg);
 }
 
@@ -1563,9 +1568,30 @@ gpg_decrypt (void *engine, gpgme_data_t ciph, gpgme_data_t plain,
 
   if (!err && override_session_key && *override_session_key)
     {
-      err = add_arg (gpg, "--override-session-key");
+      if (have_gpg_version (gpg, "2.1.16"))
-      if (!err)
+        {
-        err = add_arg (gpg, override_session_key);
+          gpgme_data_release (gpg->override_session_key);
+          TRACE2 (DEBUG_ENGINE, "override", gpg, "seskey='%s' len=%zu\n",
+                  override_session_key,
+                  strlen (override_session_key));
+
+          err = gpgme_data_new_from_mem (&gpg->override_session_key,
+                                         override_session_key,
+                                         strlen (override_session_key), 1);
+          if (!err)
+            {
+              err = add_arg (gpg, "--override-session-key-fd");
+              if (!err)
+                err = add_data (gpg, gpg->override_session_key, -2, 0);
+            }
+        }
+      else
+        {
+          /* Using that option may leak the session key via ps(1).  */
+          err = add_arg (gpg, "--override-session-key");
+          if (!err)
+            err = add_arg (gpg, override_session_key);
+        }
     }
 
   /* Tell the gpg object about the data.  */

tests/run-decrypt.c

@@ -185,7 +185,8 @@ main (int argc, char **argv)
     }
   if (override_session_key)
     {
-      err = gpgme_set_ctx_flag (ctx, "overrride-session-key", "1");
+      err = gpgme_set_ctx_flag (ctx, "override-session-key",
+                                override_session_key);
       if (err)
         {
           fprintf (stderr, PGM ": error overriding session key: %s\n",