[SRU X/B/D] propagate_one(): mnt_set_mountpoint() needs mount_lock

[Thadeu Lima de Souza Cascardo]

From: Al Viro <viro at zeniv.linux.org.uk>

... 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 at 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 at kernel.org
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
(backported from commit b0d3869ce9eeacbb1bbd541909beeef4126426d5)
CVE-2020-12114
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/pnode.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/pnode.c b/fs/pnode.c
index 1100e810d855..29e2dc6979d2 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -267,14 +267,13 @@ 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);
 }
-- 
2.20.1