From bb46094057520304073130920c223451c2a694a7 Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Sat, 26 Sep 2026 15:44:25 +0200 Subject: [PATCH] statmount: read the parent of an unmounted mount under mount_lock statmount_mnt_basic() reads the ids of the parent through m->mnt_parent with namespace_sem held for reading. That's fine for a mount that is in a namespace. Everything above it is pinned by that namespace. STATMOUNT_BY_FD also accepts a file descriptor on a mount that has been unmounted on purpose and that only the file descriptor keeps alive. If that mount stayed attached to its parent when it was unmounted the parent is unmounted as well and nothing pins it for the child. The final mntput() unmounts the children under mount_lock alone and frees the parent afterwards. Concurrently, statmount_mnt_basic() reads m->mnt_parent twice while that can happen and can easily dereference a mount that's already gone. Read the parent of an unmounted mount under mount_lock. The parent can't be put while it is held and both ids come from the same mount. A mounted mount's parent is pinned by the namespace and a mount can't leave its namespace while namespace_sem is held, so the common case takes no global lock. Fixes: 0e5032237ee5 ("statmount: accept fd as a parameter") Cc: stable@vger.kernel.org # v7.0+ Link: https://patch.msgid.link/20260926-work-mount-fixes-2-v1-2-f357abf3d17b@kernel.org Signed-off-by: Christian Brauner (Amutable) --- fs/namespace.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 230f95675a93..d1e8dd2e1e8e 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -5335,15 +5335,25 @@ static void statmount_sb_basic(struct kstatmount *s) s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME); } +static void statmount_mnt_parent(struct kstatmount *s, const struct mount *m) +{ + s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; + s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; +} + static void statmount_mnt_basic(struct kstatmount *s) { struct mount *m = real_mount(s->mnt); s->sm.mask |= STATMOUNT_MNT_BASIC; s->sm.mnt_id = m->mnt_id_unique; - s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; s->sm.mnt_id_old = m->mnt_id; - s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; + /* An unmounted mount is cut loose from its parent under mount_lock alone. */ + if (likely(is_mounted(s->mnt))) + statmount_mnt_parent(s, m); + else + scoped_guard(mount_locked_reader) + statmount_mnt_parent(s, m); s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt); s->sm.mnt_propagation = mnt_to_propagation_flags(m); s->sm.mnt_peer_group = m->mnt_group_id;