do_move_mount(): fix an unsafe use of is_anon_ns()

What triggers it is a race between mount --move and umount -l
of the source; we should reject it (the source is parentless *and*
not the root of anon namespace at that), but the check for namespace
being an anon one is broken in that case - is_anon_ns() needs
ns to be non-NULL.  Better fixed here than in is_anon_ns(), since
the rest of the callers is guaranteed to get a non-NULL argument...

Reported-by: syzbot+494c7ddf66acac0ad747@syzkaller.appspotmail.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 3357c3d..ffb13f0 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2599,7 +2599,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
        if (attached && !check_mnt(old))
                goto out;
 
-       if (!attached && !is_anon_ns(ns))
+       if (!attached && !(ns && is_anon_ns(ns)))
                goto out;
 
        if (old->mnt.mnt_flags & MNT_LOCKED)