6.6-stable review patch. If anyone has any objections, please let me know.
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From: Al Viro viro@zeniv.linux.org.uk
[ Upstream commit 5f31c549382bcddbbd754c72c5433b19420d485d ]
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: Sasha Levin sashal@kernel.org --- fs/namespace.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c index ef3b2ae2957ec..7942a33451ba0 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3020,18 +3020,25 @@ static int do_set_group(struct path *from_path, struct path *to_path) * 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); }
/**