vfs: add NOFOLLOW flag to umount(2)

Add a new UMOUNT_NOFOLLOW flag to umount(2).  This is needed to prevent
symlink attacks in unprivileged unmounts (fuse, samba, ncpfs).

Additionally, return -EINVAL if an unknown flag is used (and specify
an explicitly unused flag: UMOUNT_UNUSED).  This makes it possible for
the caller to determine if a flag is supported or not.

CC: Eugene Teo <eugene@redhat.com>
CC: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1136,8 +1136,15 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
 {
 	struct path path;
 	int retval;
+	int lookup_flags = 0;
 
-	retval = user_path(name, &path);
+	if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+		return -EINVAL;
+
+	if (!(flags & UMOUNT_NOFOLLOW))
+		lookup_flags |= LOOKUP_FOLLOW;
+
+	retval = user_path_at(AT_FDCWD, name, lookup_flags, &path);
 	if (retval)
 		goto out;
 	retval = -EINVAL;

include/linux/fs.h

@@ -1305,6 +1305,8 @@ extern int send_sigurg(struct fown_struct *fown);
 #define MNT_FORCE	0x00000001	/* Attempt to forcibily umount */
 #define MNT_DETACH	0x00000002	/* Just detach from the tree */
 #define MNT_EXPIRE	0x00000004	/* Mark for expiry */
+#define UMOUNT_NOFOLLOW	0x00000008	/* Don't follow symlink on umount */
+#define UMOUNT_UNUSED	0x80000000	/* Flag guaranteed to be unused */
 
 extern struct list_head super_blocks;
 extern spinlock_t sb_lock;