linux-kernel - Re: [PATCH] x86/ftrace: Make sure that ftrace trampolines are not RWX

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Date:   Wed, 24 May 2017 11:04:25 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Kees Cook <keescook@...omium.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/ftrace: Make sure that ftrace trampolines are not
 RWX

On Wed, 24 May 2017 15:47:17 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> ftrace uses module_alloc() to allocate trampoline pages. The mapping
> of module_alloc() is RWX, which makes sense as the memory is written
> to right after allocation. But nothing makes these pages RO after
> writing to them.
> 
> This problem exists since ftrace uses trampolines on x86, but it went
> unnoticed because the W=X sanity check only triggers when the tracer
> builtin selftests are enabled. Though the mappings are also created
> W+X w/o the self tests when the tracer is used after booting.
> 
> Add proper set_memory_rw/ro() calls to [un]protect the trampolines
> before and after modification.
> 
> Fixes: f3bea49115b2 ("ftrace/x86: Add dynamic allocated trampoline
> for ftrace_ops") Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Thanks! I was thinking that this was the issue after your last reply.
I'll send this to my box at home and run my tests against it. I'll let
you know tomorrow the results.

-- Steve


> ---
>  arch/x86/kernel/ftrace.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -839,7 +839,7 @@ void arch_ftrace_update_trampoline(struc
>  	unsigned long offset;
>  	unsigned long ip;
>  	unsigned int size;
> -	int ret;
> +	int ret, npages;
>  
>  	if (ops->trampoline) {
>  		/*
> @@ -848,11 +848,14 @@ void arch_ftrace_update_trampoline(struc
>  		 */
>  		if (!(ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP))
>  			return;
> +		npages = PAGE_ALIGN(ops->trampoline_size) >>
> PAGE_SHIFT;
> +		set_memory_rw(ops->trampoline, npages);
>  	} else {
>  		ops->trampoline = create_trampoline(ops, &size);
>  		if (!ops->trampoline)
>  			return;
>  		ops->trampoline_size = size;
> +		npages = PAGE_ALIGN(size) >> PAGE_SHIFT;
>  	}
>  
>  	offset = calc_trampoline_call_offset(ops->flags &
> FTRACE_OPS_FL_SAVE_REGS); @@ -863,6 +866,7 @@ void
> arch_ftrace_update_trampoline(struc /* Do a safe modify in case the
> trampoline is executing */ new = ftrace_call_replace(ip, (unsigned
> long)func); ret = update_ftrace_func(ip, new);
> +	set_memory_ro(ops->trampoline, npages);
>  
>  	/* The update should never fail */
>  	WARN_ON(ret);