propagate_one(): mnt_set_mountpoint() needs mount_lock

BugLink: https://bugs.launchpad.net/bugs/1877461
commit b0d3869ce9eeacbb1bbd541909beeef4126426d5 upstream.

... 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>
Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/fs/pnode.c b/fs/pnode.c
index 53d411a371ce8d7efd060ee3e9ad37053a54e4ab..7910ae91f17e9dbeda817c22f8eda0409c3f4904 100644 (file)
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -266,14 +266,13 @@ static int propagate_one(struct mount *m)
        if (IS_ERR(child))
                return PTR_ERR(child);
        child->mnt.mnt_flags &= ~MNT_LOCKED;
+       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);
 }