linux-kernel - Re: [RFC][PATCH] Lock down ftrace

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Date:   Thu, 9 Nov 2017 13:03:29 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Howells <dhowells@...hat.com>
Cc:     mingo@...hat.com, alexei.starovoitov@...il.com,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Lock down ftrace

On Thu, 09 Nov 2017 16:42:12 +0000
David Howells <dhowells@...hat.com> wrote:

> Hi,
> 
> I (may) need to lock down ftrace under secure boot conditions as part of the
> patch series that can be found here:
> 
> 	https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=efi-lock-down
> 
> Can you tell me that if the attached patch is sufficient to the cause?
> 
> Thanks,
> David
> ---
> commit 3fb4590c0ad0a751b2090c489df79193510e6aaa
> Author: David Howells <dhowells@...hat.com>
> Date:   Wed Nov 8 15:41:02 2017 +0000
> 
>     Lock down ftrace
>     
>     Disallow the use of ftrace when the kernel is locked down.  This patch
>     turns off ftrace_enabled late in the kernel boot so that the selftest can
>     still be potentially be run.
>     
>     The sysctl that controls ftrace_enables is also disallowed when the kernel
>     is locked down.  If the lockdown is lifted, then the sysctl can be used to
>     reenable ftrace - if ftrace was compiled with CONFIG_DYNAMIC_FTRACE, that
>     is; if it wasn't then it won't be possible to reenable it.

Actually, I see it being enabled with DYNAMIC_FTRACE not set. Calling
into sysctl and enabling ftrace_enable, will allow the
ftrace_trace_function to be set to something other than ftrace_stub
again, allowing for static function tracing to run too.

>     
>     This prevents crypto data theft by analysis of execution patterns, and, if
>     in future ftrace also logs the register contents at the time, will prevent
>     data theft by that mechanism also.
>     
>     Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
>     Signed-off-by: David Howells <dhowells@...hat.com>
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 6abfafd7f173..9c7135963d80 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -6508,6 +6508,9 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
>  {
>  	int ret = -ENODEV;
>  
> +	if (kernel_is_locked_down("Use of ftrace"))
> +		return -EPERM;
> +
>  	mutex_lock(&ftrace_lock);
>  
>  	if (unlikely(ftrace_disabled))
> @@ -6896,3 +6899,22 @@ void ftrace_graph_exit_task(struct task_struct *t)
>  	kfree(ret_stack);
>  }
>  #endif
> +
> +#ifdef CONFIG_LOCK_DOWN_KERNEL
> +static int __init ftrace_lock_down(void)
> +{
> +	mutex_lock(&ftrace_lock);
> +
> +	if (!ftrace_disabled && ftrace_enabled &&
> +	    kernel_is_locked_down("Use of ftrace")) {
> +		ftrace_enabled = false;
> +		last_ftrace_enabled = false;
> +		ftrace_trace_function = ftrace_stub;
> +		ftrace_shutdown_sysctl();
> +	}
> +
> +	mutex_unlock(&ftrace_lock);
> +	return 0;
> +}
> +late_initcall(ftrace_lock_down);
> +#endif

Looks fine to me. We discussed this offline, and this appears to
implement what we finally decided would be sufficient.

-- Steve