linux-kernel - Re: [PATCH v10 07/10] livepatch: Correctly handle atomic replace for not yet loaded modules

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Date:   Tue, 13 Mar 2018 15:55:36 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>
Cc:     Jason Baron <jbaron@...mai.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 07/10] livepatch: Correctly handle atomic replace for
 not yet loaded modules

On Wed 2018-03-07 09:20:36, Petr Mladek wrote:
> The atomic replace feature uses dynamically allocated struct klp_func to
> handle functions that will no longer be patched. These structures are
> of the type KLP_FUNC_NOP. They cause the ftrace handler to jump to
> the original code. But the address of the original code is not known
> until the patched module is loaded.
> 
> This patch allows the late initialization. Also it adds a sanity check
> into the ftrace handler.
> 
> diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> index 54b3c379bb0f..1f5c3eea9ee1 100644
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -118,7 +118,12 @@ static void notrace klp_ftrace_handler(unsigned long ip,
>  		}
>  	}
>  
> +	/* Survive ugly mistakes, for example, when handling NOPs. */
> +	if (WARN_ON_ONCE(!func->new_func))
> +		goto unlock;

JFYI, this is not enough. We really have to skip klp_arch_set_pc()
for NOPs. Otherwise, we end up in an infinite loop. NOPs cause that
we go back to the beginning of the original function, enter
the ftrace handler again, ...

I am going to fix this in v11.

> +
>  	klp_arch_set_pc(regs, (unsigned long)func->new_func);
> +
>  unlock:
>  	preempt_enable_notrace();

Kudos to Mirek for testing and hitting the problem.

Best Regards,
Petr