linux-kernel - [PATCH 3.16 02/99] propagate_one(): mnt_set_mountpoint() needs mount

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Message-ID: <lsq.1589984008.458461925@decadent.org.uk>
Date:   Wed, 20 May 2020 15:13:30 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        stable@...nel.org, "Piotr Krysiuk" <piotras@...il.com>,
        "Al Viro" <viro@...iv.linux.org.uk>
Subject: [PATCH 3.16 02/99] propagate_one(): mnt_set_mountpoint() needs
 mount_lock

3.16.84-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@...iv.linux.org.uk>

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@...il.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@...nel.org
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Piotr Krysiuk <piotras@...il.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/pnode.c |    9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -249,14 +249,13 @@ 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);
 }