CRED: Fix load_flat_shared_library() to initialise bprm correctly

Fix binfmt_flag's load_flat_shared_library() to initialise bprm correctly.

Currently, prepare_binprm() is called with only .filename .file and .cred
fields set in bprm, but the .cred_prepared and .per_clear fields at least need
initialising.
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

fs/binfmt_flat.c

@@ -820,6 +820,8 @@ static int load_flat_shared_library(int id, struct lib_info *libs)
 	int res;
 	char buf[16];
 
+	memset(&bprm, 0, sizeof(bprm));
+
 	/* Create the file name */
 	sprintf(buf, "/lib/lib%d.so", id);
 
@@ -835,6 +837,12 @@ static int load_flat_shared_library(int id, struct lib_info *libs)
 	if (!bprm.cred)
 		goto out;
 
+	/* We don't really care about recalculating credentials at this point
+	 * as we're past the point of no return and are dealing with shared
+	 * libraries.
+	 */
+	bprm.cred_prepared = 1;
+
 	res = prepare_binprm(&bprm);
 
 	if (!IS_ERR_VALUE(res))