Re: [PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads

[Petr Mladek]

On Wed 2023-01-25 10:57:30, Seth Forshee wrote:
> On Wed, Jan 25, 2023 at 12:34:26PM +0100, Petr Mladek wrote:
> > On Tue 2023-01-24 11:21:39, Seth Forshee wrote:
> > > On Tue, Jan 24, 2023 at 03:17:43PM +0100, Petr Mladek wrote:
> > > > On Fri 2023-01-20 16:12:22, Seth Forshee (DigitalOcean) wrote:
> > > > > Livepatch relies on stack checking of sleeping tasks to switch kthreads,
> > > > > so a busy kthread can block a livepatch transition indefinitely. We've
> > > > > seen this happen fairly often with busy vhost kthreads.
> > > > 
> > > > > --- a/drivers/vhost/vhost.c
> > > > > +++ b/drivers/vhost/vhost.c
> > > > > @@ -366,6 +367,9 @@ static int vhost_worker(void *data)
> > > > >  			if (need_resched())
> > > > >  				schedule();
> > > > >  		}
> > > > > +
> > > > > +		if (unlikely(klp_patch_pending(current)))
> > > > > +			klp_switch_current();
> > > > 
> > > > I suggest to use the following intead:
> > > > 
> > > > 		if (unlikely(klp_patch_pending(current)))
> > > > 			klp_update_patch_state(current);
> > > > 
> > > > We already use this in do_idle(). The reason is basically the same.
> > > > It is almost impossible to livepatch the idle task when a CPU is
> > > > very idle.
> > > > 
> > > > klp_update_patch_state(current) does not check the stack.
> > > > It switches the task immediately.
> > > > 
> > > > It should be safe because the kthread never leaves vhost_worker().
> > > > It means that the same kthread could never re-enter this function
> > > > and use the new code.
> > > 
> > > My knowledge of livepatching internals is fairly limited, so I'll accept
> > > it if you say that it's safe to do it this way. But let me ask about one
> > > scenario.
> > > 
> > > Let's say that a livepatch is loaded which replaces vhost_worker(). New
> > > vhost worker threads are started which use the replacement function. Now
> > > if the patch is disabled, these new worker threads would be switched
> > > despite still running the code from the patch module, correct? Could the
> > > module then be unloaded, freeing the memory containing the code these
> > > kthreads are executing?
> > 
> > The above scenario would require calling klp_update_patch_state() from
> > the code in the livepatch module. It is not possible at the moment because
> > this function is not exported for modules.
> 
> vhost can be built as a module, so in order to call
> klp_update_patch_state() from vhost_worker() it would have to be
> exported to modules.

I see.

> > Hmm, the same problem might be when we livepatch a function that calls
> > another function that calls klp_update_patch_state(). But in this case
> > it would be kthread() from kernel/kthread.c. It would affect any
> > running kthread. I doubt that anyone would seriously think about
> > livepatching this function.
> 
> Yes, there are clearly certain functions that are not safe/practical to
> patch, and authors need to know what they are doing. Most kthread main()
> functions probably qualify as impractical at best, at least without a
> strategy to restart relevant kthreads.
> 
> But a livepatch transition will normally stall if patching these
> functions when a relevant kthread is running (unless the patch is
> forced), so a patch author who made a mistake should quickly notice.
> vhost_worker() would behave differently.

Another crazy idea:

/**
 * klp_update_patch_state_safe() - do not update the path state when
 *	called from a livepatch.
 * @task: task_struct to be updated
 * @calller_addr: address of the function which  calls this one
 *
 * Do not update the patch set when called from a livepatch.
 * It would allow to remove the livepatch module even when
 * the code still might be in use.
 */
void klp_update_patch_state_safe(struct task_struct *task, void *caller_addr)
{
	static bool checked;
	static bool safe;

	if (unlikely(!checked)) {
		struct module *mod;

		preempt_disable();
		mod = __module_address(caller_addr);
		if (!mod || !is_livepatch_module(mod))
			safe = true;
		checked = true;
		preempt_enable();
	}

	if (safe)
		klp_update_patch_state(task);
}

and use in vhost_worker()

		if (unlikely(klp_patch_pending(current)))
			klp_update_patch_state_safe(current, vhost_worker);

Even better might be to get the caller address using some compiler
macro. I guess that it should be possible.

And even better would be to detect this at the compile time. But
I do not know how to do so.

> > A good enough solution might be to document this. Livepatches could
> > not be created blindly. There are more situations where the
> > livepatch is tricky or not possible at all.
> 
> I can add this if you like. Is Documentation/livepatch/livepatch.rst the
> right place for this?

Yes, the best place probably would be "7. Limitations" section in
Documentation/livepatch/livepatch.rst.

Even better would be to add a document about the best practices.
We have dreamed about it for years ;-)

> > Crazy idea. We could prevent this problem even technically. A solution
> > would be to increment a per-process counter in klp_ftrace_handler() when a
> > function is redirected(). And klp_update_patch_state() might refuse
> > the migration when this counter is not zero. But it would require
> > to use a trampoline on return that would decrement the counter.
> > I am not sure if this is worth the complexity.
> > 
> > One the other hand, this counter might actually remove the need
> > of the reliable backtrace. It is possible that I miss something
> > or that it is not easy/possible to implement the return trampoline.
> 
> I agree this should work for unpatching, and even for patching a
> function which is already patched.
> 
> Maybe I'm misunderstanding, but this would only work for unpatching or
> patching an already-patched function, wouldn't it? Because the original
> functions would not increment the counter so you would not know if tasks
> still had those on their call stacks.

Right. I knew that it could not be that easy. Otherwise, we would have
used it. I just did not spent enough cycles on the idea yesterday.

> > Back to the original problem. I still consider calling
> > klp_update_patch_state(current) in vhost_worker() safe.
> 
> Okay, I can send a v2 which does this, so long as it's okay to export
> klp_update_patch_state() to modules.

It would be acceptable for me if we added a warning above the function
definition and into the livepatch documentation.

But I would prefer klp_update_patch_state_safe() if it worked. It is
possible that I have missed something.

Best Regards,
Petr