propagate_one(): mnt_set_mountpoint() needs mount_lock

... to protect the modification of mp->m_count done by it.  Most of
the places that modify that thing also have namespace_lock held,
but not all of them can do so, so we really need mount_lock here.
Kudos to Piotr Krysiuk <piotras@gmail.com>, who'd spotted a related
bug in pivot_root(2) (fixed unnoticed in 5.3); search for other
similar turds has caught out this one.

Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/pnode.c b/fs/pnode.c
index 49f6d7f..1106137 100644 (file)
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -261,14 +261,13 @@ static int propagate_one(struct mount *m)
        child = copy_tree(last_source, last_source->mnt.mnt_root, type);
        if (IS_ERR(child))
                return PTR_ERR(child);
+       read_seqlock_excl(&mount_lock);
        mnt_set_mountpoint(m, mp, child);
+       if (m->mnt_master != dest_master)
+               SET_MNT_MARK(m->mnt_master);
+       read_sequnlock_excl(&mount_lock);
        last_dest = m;
        last_source = child;
-       if (m->mnt_master != dest_master) {
-               read_seqlock_excl(&mount_lock);
-               SET_MNT_MARK(m->mnt_master);
-               read_sequnlock_excl(&mount_lock);
-       }
        hlist_add_head(&child->mnt_hash, list);
        return count_mounts(m->mnt_ns, child);
 }