Re: [PATCH v3 03/10] fs: lockless mntns rbtree lookup

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2024-12-13 at 19:44 +0100, Christian Brauner wrote:
> 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@kernel.org>
> > > ---
> > >  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.

Ahh ok, I missed the detail that the refcount decrement was being done
in the RCU callback. With that, I guess that can't ever happen. Sorry
for the noise!

> 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.).
> 

I don't feel strongly about it, but a comment explaining why you can
unconditionally bump it there would be helpful.

> > 
> > > +	return ns;
> > >  }
> > >  
> > >  static inline void lock_mount_hash(void)
> > > 



-- 
Jeff Layton <jlayton@kernel.org>