gpg,common: Make sure that all fd given are valid.

* common/sysutils.c (gnupg_fd_valid): New function.
* common/sysutils.h (gnupg_fd_valid): New declaration.
* common/logging.c (log_set_file): Use the new function.
* g10/cpr.c (set_status_fd): Likewise.
* g10/gpg.c (main): Likewise.
* g10/keylist.c (read_sessionkey_from_fd): Likewise.
* g10/passphrase.c (set_attrib_fd): Likewise.
* tests/openpgp/Makefile.am (XTESTS): Add the new test.
* tests/openpgp/issue2941.scm: New file.
--

Consider a situation where the user passes "--status-fd 3" but file
descriptor 3 is not open.

During the course of executing the rest of the commands, it's possible
that gpg itself will open some files, and file descriptor 3 will get
allocated.

In this situation, the status information will be appended directly to
whatever file happens to have landed on fd 3 (the trustdb? the
keyring?).

This is a potential data destruction issue for all writable file
descriptor options:

   --status-fd
   --attribute-fd
   --logger-fd

It's also a potential issue for readable file descriptor options, but
the risk is merely weird behavior, and not data corruption:

   --override-session-key-fd
   --passphrase-fd
   --command-fd

Fixes this by checking whether the fd is valid early on before using
it.

GnuPG-bug-id: 2941
Signed-off-by: Justus Winter <justus@g10code.com>

common/sysutils.c

@@ -1281,3 +1281,14 @@ gnupg_get_socket_name (int fd)
   return name;
 }
 #endif /*!HAVE_W32_SYSTEM*/
+
+/* Check whether FD is valid.  */
+int
+gnupg_fd_valid (int fd)
+{
+  int d = dup (fd);
+  if (d < 0)
+    return 0;
+  close (d);
+  return 1;
+}