[RFC][PATCH] Lock down ftrace

[RFC][PATCH] Lock down ftrace

[David Howells]

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@redhat.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.
    
    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@gmail.com>
    Signed-off-by: David Howells <dhowells@redhat.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
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[RFC][PATCH] Lock down ftrace

[Steven Rostedt]

On Thu, 09 Nov 2017 16:42:12 +0000
David Howells <dhowells@redhat.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@redhat.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@gmail.com>
>     Signed-off-by: David Howells <dhowells@redhat.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

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[RFC][PATCH] Lock down ftrace

[David Howells]

Steven Rostedt <rostedt@goodmis.org> wrote:

> 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.

Hmmm...  Okay, I'm not sure what the sysctl achieves in non-dynamic mode.
Some of the functions used by ftrace_enable_sysctl() are stubbed out in that
case.  I was thinking that was stubbed out also, but apparently not.

Anyway, ftrace_enable_sysctl() is also prohibited in lockdown mode.

David
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