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

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Date:	Thu, 12 Feb 2015 11:45:09 +0100 (CET)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Seth Jennings <sjenning@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

On Wed, 11 Feb 2015, Josh Poimboeuf wrote:

> On Wed, Feb 11, 2015 at 11:21:51AM +0100, Miroslav Benes wrote:
> > 
> > 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.
> 
> No, actually foo_1()'s func->transition will be 0.  Only foo_2()'s
> func->transition will be 1.

Ah, you're right in both cases. Sorry for the noise.

Miroslav

> 
> > 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
> 
> -- 
> Josh
> 
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