Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

Miroslav Benes
2015-02-11T10:21:56Z

On Mon, 9 Feb 2015, Josh Poimboeuf wrote:

[...]

> @@ -38,14 +39,34 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> ops = container_of(fops, struct klp_ops, fops);
>
> rcu_read_lock();
> +
> func = list_first_or_null_rcu(&ops->func_stack, struct klp_func,
> stack_node);
> - rcu_read_unlock();
>
> if (WARN_ON_ONCE(!func))
> - return;
> + goto unlock;
> +
> + if (unlikely(func->transition)) {
> + /* corresponding smp_wmb() is in klp_init_transition() */
> + smp_rmb();
> +
> + if (current->klp_universe == KLP_UNIVERSE_OLD) {
> + /*
> + * Use the previously patched version of the function.
> + * If no previous patches exist, use the original
> + * function.
> + */
> + func = list_entry_rcu(func->stack_node.next,
> + struct klp_func, stack_node);
> +
> + if (&func->stack_node == &ops->func_stack)
> + goto unlock;
> + }
> + }
>
> klp_arch_set_pc(regs, (unsigned long)func->new_func);
> +unlock:
> + rcu_read_unlock();
> }

I decided to understand the code more before answering the email about the
race and found another problem. I think.

Imagine we patched some function foo() with foo_1() from patch_1 and now
we'd like to patch it again with foo_2() in patch_2. __klp_enable_patch
calls klp_init_transition which sets klp_universe for all processes to
KLP_UNIVERSE_OLD and marks the foo_2() for transition (it is gonna be 1).
Then __klp_enable_patch adds foo_2() to the RCU-protected list for foo().
BUT what if somebody calls foo() right between klp_init_transition and
the loop in __klp_enable_patch? The ftrace handler first returns the
first entry in the list which is foo_1() (foo_2() is still not present),
then it checks for func->transition. It is 1. It checks for
current->klp_universe which is KLP_UNIVERSE_OLD and so the next entry is
retrieved. There is no such and therefore foo() is called. This is
obviously wrong because foo_1() was expected.

Everything would work fine if one would call foo() before
klp_start_transition and after the loop in __klp_enable_patch. The
solution might be to move the setting of func->transition to
klp_start_transition, but this could break something different. I don't
know yet.

Am I wrong?

Miroslav