linux-kernel - Re: [PATCH] livepatch: RCU protect struct klp_func all the time when used in klp_ftrace

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Message-ID: <alpine.LNX.2.00.1502210028160.19198@pobox.suse.cz>
Date:	Sat, 21 Feb 2015 00:29:30 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Petr Mladek <pmladek@...e.cz>
cc:	jpoimboe@...hat.com, sjenning@...hat.com, vojtech@...e.cz,
	masami.hiramatsu.pt@...achi.com, mbenes@...e.cz,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] livepatch: RCU protect struct klp_func all the time when
 used in klp_ftrace_handler()

On Wed, 18 Feb 2015, Petr Mladek wrote:

> func->new_func has been accessed after rcu_read_unlock() in klp_ftrace_handler()
> and therefore the access was not protected.
> 
> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> ---
>  kernel/livepatch/core.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
> index ff7f47d026ac..cde66192e20e 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -314,12 +314,12 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>  	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;
>  
>  	klp_arch_set_pc(regs, (unsigned long)func->new_func);
> +unlock:
> +	rcu_read_unlock();

This can't hurt, even though the only scenario where this could in theory 
trigger as a bug (module removal) is non-issue now.

But I'd like to take it nevertheless ... Seth, Josh?

Thanks,

-- 
Jiri Kosina
SUSE Labs
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