Re: [PATCH v3 04/13] uprobes: travers uprobe's consumer list locklessly under SRCU protection

Jiri Olsa
2024-08-22T14:22:55Z

On Mon, Aug 12, 2024 at 09:29:08PM -0700, Andrii Nakryiko wrote:

SNIP

> @@ -1125,18 +1103,31 @@ void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
> int err;
>
> down_write(&uprobe->register_rwsem);
> - if (WARN_ON(!consumer_del(uprobe, uc))) {
> - err = -ENOENT;
> - } else {
> - err = register_for_each_vma(uprobe, NULL);
> - /* TODO : cant unregister? schedule a worker thread */
> - if (unlikely(err))
> - uprobe_warn(current, "unregister, leaking uprobe");
> - }
> +
> + list_del_rcu(&uc->cons_node);

hi,
I'm using this patchset as base for my changes and stumbled on this today,
I'm probably missing something, but should we keep the 'uprobe->consumer_rwsem'
lock around the list_del_rcu?

jirka

> + err = register_for_each_vma(uprobe, NULL);
> +
> up_write(&uprobe->register_rwsem);
>
> - if (!err)
> - put_uprobe(uprobe);
> + /* TODO : cant unregister? schedule a worker thread */
> + if (unlikely(err)) {
> + uprobe_warn(current, "unregister, leaking uprobe");
> + goto out_sync;
> + }
> +
> + put_uprobe(uprobe);
> +
> +out_sync:
> + /*
> + * Now that handler_chain() and handle_uretprobe_chain() iterate over
> + * uprobe->consumers list under RCU protection without holding
> + * uprobe->register_rwsem, we need to wait for RCU grace period to
> + * make sure that we can't call into just unregistered
> + * uprobe_consumer's callbacks anymore. If we don't do that, fast and
> + * unlucky enough caller can free consumer's memory and cause
> + * handler_chain() or handle_uretprobe_chain() to do an use-after-free.
> + */
> + synchronize_srcu(&uprobes_srcu);
> }
> EXPORT_SYMBOL_GPL(uprobe_unregister);
>
> @@ -1214,13 +1205,20 @@ EXPORT_SYMBOL_GPL(uprobe_register);
> int uprobe_apply(struct uprobe *uprobe, struct uprobe_consumer *uc, bool add)
> {
> struct uprobe_consumer *con;
> - int ret = -ENOENT;
> + int ret = -ENOENT, srcu_idx;
>
> down_write(&uprobe->register_rwsem);
> - for (con = uprobe->consumers; con && con != uc ; con = con->next)
> - ;
> - if (con)
> - ret = register_for_each_vma(uprobe, add ? uc : NULL);
> +
> + srcu_idx = srcu_read_lock(&uprobes_srcu);
> + list_for_each_entry_srcu(con, &uprobe->consumers, cons_node,
> + srcu_read_lock_held(&uprobes_srcu)) {
> + if (con == uc) {
> + ret = register_for_each_vma(uprobe, add ? uc : NULL);
> + break;
> + }
> + }
> + srcu_read_unlock(&uprobes_srcu, srcu_idx);
> +
> up_write(&uprobe->register_rwsem);
>
> return ret;
> @@ -2085,10 +2083,12 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> struct uprobe_consumer *uc;
> int remove = UPROBE_HANDLER_REMOVE;
> bool need_prep = false; /* prepare return uprobe, when needed */
> + bool has_consumers = false;
>
> - down_read(&uprobe->register_rwsem);
> current->utask->auprobe = &uprobe->arch;
> - for (uc = uprobe->consumers; uc; uc = uc->next) {
> +
> + list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> + srcu_read_lock_held(&uprobes_srcu)) {
> int rc = 0;
>
> if (uc->handler) {
> @@ -2101,17 +2101,24 @@ static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> need_prep = true;
>
> remove &= rc;
> + has_consumers = true;
> }
> current->utask->auprobe = NULL;
>
> if (need_prep && !remove)
> prepare_uretprobe(uprobe, regs); /* put bp at return */
>
> - if (remove && uprobe->consumers) {
> - WARN_ON(!uprobe_is_active(uprobe));
> - unapply_uprobe(uprobe, current->mm);
> + if (remove && has_consumers) {
> + down_read(&uprobe->register_rwsem);
> +
> + /* re-check that removal is still required, this time under lock */
> + if (!filter_chain(uprobe, current->mm)) {
> + WARN_ON(!uprobe_is_active(uprobe));
> + unapply_uprobe(uprobe, current->mm);
> + }
> +
> + up_read(&uprobe->register_rwsem);
> }
> - up_read(&uprobe->register_rwsem);
> }
>
> static void
> @@ -2119,13 +2126,15 @@ handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
> {
> struct uprobe *uprobe = ri->uprobe;
> struct uprobe_consumer *uc;
> + int srcu_idx;
>
> - down_read(&uprobe->register_rwsem);
> - for (uc = uprobe->consumers; uc; uc = uc->next) {
> + srcu_idx = srcu_read_lock(&uprobes_srcu);
> + list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> + srcu_read_lock_held(&uprobes_srcu)) {
> if (uc->ret_handler)
> uc->ret_handler(uc, ri->func, regs);
> }
> - up_read(&uprobe->register_rwsem);
> + srcu_read_unlock(&uprobes_srcu, srcu_idx);
> }
>
> static struct return_instance *find_next_ret_chain(struct return_instance *ri)
> --
> 2.43.5
>