vfs: canonicalize create mode in build_open_flags()

Userspace can pass weird create mode in open(2) that we canonicalize to 
"(mode & S_IALLUGO) | S_IFREG" in vfs_create().

The problem is that we use the uncanonicalized mode before calling vfs_create()
with unforseen consequences.

So do the canonicalization early in build_open_flags().

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
CC: stable@vger.kernel.org

fs/open.c

@@ -852,9 +852,10 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
 	int lookup_flags = 0;
 	int acc_mode;
 
-	if (!(flags & O_CREAT))
-		mode = 0;
-	op->mode = mode;
+	if (flags & O_CREAT)
+		op->mode = (mode & S_IALLUGO) | S_IFREG;
+	else
+		op->mode = 0;
 
 	/* Must never be set by userspace */
 	flags &= ~FMODE_NONOTIFY;