From: Al Viro <viro@zeniv.linux.org.uk> mainline inclusion from mainline-v6.16-rc1 commit 5f31c549382bcddbbd754c72c5433b19420d485d category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/ICGF26 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i... -------------------------------- Holding namespace_sem is enough to make sure that result remains valid. It is *not* enough to avoid false negatives from __lookup_mnt(). Mounts can be unhashed outside of namespace_sem (stuck children getting detached on final mntput() of lazy-umounted mount) and having an unrelated mount removed from the hash chain while we traverse it may end up with false negative from __lookup_mnt(). We need to sample and recheck the seqlock component of mount_lock... Bug predates the introduction of path_overmount() - it had come from the code in finish_automount() that got abstracted into that helper. Reviewed-by: Christian Brauner <brauner@kernel.org> Fixes: 26df6034fdb2 ("fix automount/automount race properly") Fixes: 6ac392815628 ("fs: allow to mount beneath top mount") Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Wang Zhaolong <wangzhaolong@huaweicloud.com> --- fs/namespace.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index dd2313f41034..8977588be826 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3012,22 +3012,29 @@ static int do_set_group(struct path *from_path, struct path *to_path) * @path: path to check * * Check if path is overmounted, i.e., if there's a mount on top of * @path->mnt with @path->dentry as mountpoint. * - * Context: This function expects namespace_lock() to be held. + * Context: namespace_sem must be held at least shared. + * MUST NOT be called under lock_mount_hash() (there one should just + * call __lookup_mnt() and check if it returns NULL). * Return: If path is overmounted true is returned, false if not. */ static inline bool path_overmounted(const struct path *path) { + unsigned seq = read_seqbegin(&mount_lock); + bool no_child; + rcu_read_lock(); - if (unlikely(__lookup_mnt(path->mnt, path->dentry))) { - rcu_read_unlock(); - return true; - } + no_child = !__lookup_mnt(path->mnt, path->dentry); rcu_read_unlock(); - return false; + if (need_seqretry(&mount_lock, seq)) { + read_seqlock_excl(&mount_lock); + no_child = !__lookup_mnt(path->mnt, path->dentry); + read_sequnlock_excl(&mount_lock); + } + return unlikely(!no_child); } /** * can_move_mount_beneath - check that we can mount beneath the top mount * @from: mount to mount beneath -- 2.34.3