flat: fix uninitialized ptr with shared libs

The new credentials code broke load_flat_shared_library() as it now uses
an uninitialized cred pointer.
Reported-by: Bernd Schmidt <bernds_cb1@t-online.de>
Tested-by: Bernd Schmidt <bernds_cb1@t-online.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_flat.c

@@ -828,15 +828,22 @@ static int load_flat_shared_library(int id, struct lib_info *libs)
 	if (IS_ERR(bprm.file))
 		return res;
 
+	bprm.cred = prepare_exec_creds();
+	res = -ENOMEM;
+	if (!bprm.cred)
+		goto out;
+
 	res = prepare_binprm(&bprm);
 
 	if (res <= (unsigned long)-4096)
 		res = load_flat_file(&bprm, libs, id, NULL);
-	if (bprm.file) {
-		allow_write_access(bprm.file);
-		fput(bprm.file);
-		bprm.file = NULL;
-	}
+
+	abort_creds(bprm.cred);
+
+out:
+	allow_write_access(bprm.file);
+	fput(bprm.file);
+
 	return(res);
 }