On Fri, Dec 13, 2024 at 09:11:41AM -0500, Jeff Layton wrote:
> On Fri, 2024-12-13 at 00:03 +0100, Christian Brauner wrote:
> > Currently we use a read-write lock but for the simple search case we can
> > make this lockless. Creating a new mount namespace is a rather rare
> > event compared with querying mounts in a foreign mount namespace. Once
> > this is picked up by e.g., systemd to list mounts in another mount in
> > it's isolated services or in containers this will be used a lot so this
> > seems worthwhile doing.
> >
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> > fs/mount.h | 5 ++-
> > fs/namespace.c | 119 +++++++++++++++++++++++++++++++++++----------------------
> > 2 files changed, 77 insertions(+), 47 deletions(-)
> >
> > diff --git a/fs/mount.h b/fs/mount.h
> > index 185fc56afc13338f8185fe818051444d540cbd5b..36ead0e45e8aa7614c00001102563a711d9dae6e 100644
> > --- a/fs/mount.h
> > +++ b/fs/mount.h
> > @@ -12,7 +12,10 @@ struct mnt_namespace {
> > struct user_namespace *user_ns;
> > struct ucounts *ucounts;
> > u64 seq; /* Sequence number to prevent loops */
> > - wait_queue_head_t poll;
> > + union {
> > + wait_queue_head_t poll;
> > + struct rcu_head mnt_ns_rcu;
> > + };
> > u64 event;
> > unsigned int nr_mounts; /* # of mounts in the namespace */
> > unsigned int pending_mounts;
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 10fa18dd66018fadfdc9d18c59a851eed7bd55ad..52adee787eb1b6ee8831705b2b121854c3370fb3 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -79,6 +79,8 @@ static DECLARE_RWSEM(namespace_sem);
> > static HLIST_HEAD(unmounted); /* protected by namespace_sem */
> > static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
> > static DEFINE_RWLOCK(mnt_ns_tree_lock);
> > +static seqcount_rwlock_t mnt_ns_tree_seqcount = SEQCNT_RWLOCK_ZERO(mnt_ns_tree_seqcount, &mnt_ns_tree_lock);
> > +
> > static struct rb_root mnt_ns_tree = RB_ROOT; /* protected by mnt_ns_tree_lock */
> >
> > struct mount_kattr {
> > @@ -105,17 +107,6 @@ EXPORT_SYMBOL_GPL(fs_kobj);
> > */
> > __cacheline_aligned_in_smp DEFINE_SEQLOCK(mount_lock);
> >
> > -static int mnt_ns_cmp(u64 seq, const struct mnt_namespace *ns)
> > -{
> > - u64 seq_b = ns->seq;
> > -
> > - if (seq < seq_b)
> > - return -1;
> > - if (seq > seq_b)
> > - return 1;
> > - return 0;
> > -}
> > -
> > static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
> > {
> > if (!node)
> > @@ -123,19 +114,41 @@ static inline struct mnt_namespace *node_to_mnt_ns(const struct rb_node *node)
> > return rb_entry(node, struct mnt_namespace, mnt_ns_tree_node);
> > }
> >
> > -static bool mnt_ns_less(struct rb_node *a, const struct rb_node *b)
> > +static int mnt_ns_cmp(struct rb_node *a, const struct rb_node *b)
> > {
> > struct mnt_namespace *ns_a = node_to_mnt_ns(a);
> > struct mnt_namespace *ns_b = node_to_mnt_ns(b);
> > u64 seq_a = ns_a->seq;
> > + u64 seq_b = ns_b->seq;
> > +
> > + if (seq_a < seq_b)
> > + return -1;
> > + if (seq_a > seq_b)
> > + return 1;
> > + return 0;
> > +}
> >
> > - return mnt_ns_cmp(seq_a, ns_b) < 0;
> > +static inline void mnt_ns_tree_write_lock(void)
> > +{
> > + write_lock(&mnt_ns_tree_lock);
> > + write_seqcount_begin(&mnt_ns_tree_seqcount);
> > +}
> > +
> > +static inline void mnt_ns_tree_write_unlock(void)
> > +{
> > + write_seqcount_end(&mnt_ns_tree_seqcount);
> > + write_unlock(&mnt_ns_tree_lock);
> > }
> >
> > static void mnt_ns_tree_add(struct mnt_namespace *ns)
> > {
> > - guard(write_lock)(&mnt_ns_tree_lock);
> > - rb_add(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_less);
> > + struct rb_node *node;
> > +
> > + mnt_ns_tree_write_lock();
> > + node = rb_find_add_rcu(&ns->mnt_ns_tree_node, &mnt_ns_tree, mnt_ns_cmp);
> > + mnt_ns_tree_write_unlock();
> > +
> > + WARN_ON_ONCE(node);
> > }
> >
> > static void mnt_ns_release(struct mnt_namespace *ns)
> > @@ -150,41 +163,36 @@ static void mnt_ns_release(struct mnt_namespace *ns)
> > }
> > DEFINE_FREE(mnt_ns_release, struct mnt_namespace *, if (_T) mnt_ns_release(_T))
> >
> > +static void mnt_ns_release_rcu(struct rcu_head *rcu)
> > +{
> > + struct mnt_namespace *mnt_ns;
> > +
> > + mnt_ns = container_of(rcu, struct mnt_namespace, mnt_ns_rcu);
> > + mnt_ns_release(mnt_ns);
> > +}
> > +
> > static void mnt_ns_tree_remove(struct mnt_namespace *ns)
> > {
> > /* remove from global mount namespace list */
> > if (!is_anon_ns(ns)) {
> > - guard(write_lock)(&mnt_ns_tree_lock);
> > + mnt_ns_tree_write_lock();
> > rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
> > + mnt_ns_tree_write_unlock();
> > }
> >
> > - mnt_ns_release(ns);
> > + call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu);
> > }
> >
> > -/*
> > - * Returns the mount namespace which either has the specified id, or has the
> > - * next smallest id afer the specified one.
> > - */
> > -static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
> > +static int mnt_ns_find(const void *key, const struct rb_node *node)
> > {
> > - struct rb_node *node = mnt_ns_tree.rb_node;
> > - struct mnt_namespace *ret = NULL;
> > -
> > - lockdep_assert_held(&mnt_ns_tree_lock);
> > -
> > - while (node) {
> > - struct mnt_namespace *n = node_to_mnt_ns(node);
> > + const u64 mnt_ns_id = *(u64 *)key;
> > + const struct mnt_namespace *ns = node_to_mnt_ns(node);
> >
> > - if (mnt_ns_id <= n->seq) {
> > - ret = node_to_mnt_ns(node);
> > - if (mnt_ns_id == n->seq)
> > - break;
> > - node = node->rb_left;
> > - } else {
> > - node = node->rb_right;
> > - }
> > - }
> > - return ret;
> > + if (mnt_ns_id < ns->seq)
> > + return -1;
> > + if (mnt_ns_id > ns->seq)
> > + return 1;
> > + return 0;
> > }
> >
> > /*
> > @@ -194,18 +202,37 @@ static struct mnt_namespace *mnt_ns_find_id_at(u64 mnt_ns_id)
> > * namespace the @namespace_sem must first be acquired. If the namespace has
> > * already shut down before acquiring @namespace_sem, {list,stat}mount() will
> > * see that the mount rbtree of the namespace is empty.
> > + *
> > + * Note the lookup is lockless protected by a sequence counter. We only
> > + * need to guard against false negatives as false positives aren't
> > + * possible. So if we didn't find a mount namespace and the sequence
> > + * counter has changed we need to retry. If the sequence counter is
> > + * still the same we know the search actually failed.
> > */
> > static struct mnt_namespace *lookup_mnt_ns(u64 mnt_ns_id)
> > {
> > - struct mnt_namespace *ns;
> > + struct mnt_namespace *ns;
> > + struct rb_node *node;
> > + unsigned int seq;
> > +
> > + guard(rcu)();
> > + do {
> > + seq = read_seqcount_begin(&mnt_ns_tree_seqcount);
> > + node = rb_find_rcu(&mnt_ns_id, &mnt_ns_tree, mnt_ns_find);
> > + if (node)
> > + break;
> > + } while (read_seqcount_retry(&mnt_ns_tree_seqcount, seq));
> >
> > - guard(read_lock)(&mnt_ns_tree_lock);
> > - ns = mnt_ns_find_id_at(mnt_ns_id);
> > - if (!ns || ns->seq != mnt_ns_id)
> > - return NULL;
> > + if (!node)
> > + return NULL;
> >
> > - refcount_inc(&ns->passive);
> > - return ns;
> > + /*
> > + * The last reference count is put with RCU delay so we can
> > + * unconditonally acquire a reference here.
> > + */
> > + ns = node_to_mnt_ns(node);
> > + refcount_inc(&ns->passive);
>
> I'm a little uneasy with the unconditional refcount_inc() here. It
> seems quite possible that this could to a 0->1 transition here. You may
> be right that that technically won't cause a problem with the rcu lock
> held, but at the very least, that will cause a refcount warning to pop.
>
> Maybe this should be a refcount_inc_not_zero() and then you just return
> NULL if the increment doesn't occur?
So this shouldn't be possible (and Paul is on the thread and can tell me
if I'm wrong) because:
call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu);
-> mnt_ns_release_rcu()
-> mnt_ns_release()
-> refcount_dec_and_test()
Which means that decrements are RCU delayed. Any reader must walk the
list holding the RCU lock. If they find the mount namespace still on the
list then mnt_ns_release() will be deferred until they are done.
In order for what you describe to happen a reader must find that mount
namespace still in the list or rbtree and mnt_ns_release() is called
directly.
But afaict that doesn't happen. mnt_ns_release() is only called directly
when the mount namespace has never been on any of the lists.
refcount_inc() will already WARN() if the previous value was 0 and
splat. If we use refcount_inc_not_zero() we're adding additional memory
barriers when we simply don't need them.
But if that's important to you though than I'd rather switch the passive
count to atomic_t and use atomic_dec_and_test() in mnt_ns_release() and
then use similar logic as I used for file_ref_inc():
unsigned long prior = atomic_fetch_inc_relaxed(&ns->passive);
WARN_ONCE(!prior, "increment from zero UAF condition present");
This would give us the opportunity to catch a "shouldn't happen
condition". But using refcount_inc_not_zero() would e.g., confuse me a
few months down the line (Yes, we could probably add a comment.).
>
> > + return ns;
> > }
> >
> > static inline void lock_mount_hash(void)
> >
>
> --
> Jeff Layton <jlayton@xxxxxxxxxx>
