Re: [PATCH v2 2/2] kernel/trace: Remove function callback casts

[Oscar Carter <oscar.carter@gmx.com>]

Hi Steven,

On Tue, Jul 21, 2020 at 02:05:45PM -0400, Steven Rostedt wrote:
> On Sun, 19 Jul 2020 17:50:33 +0200
> Oscar Carter <oscar.carter@gmx.com> wrote:
>
> > In an effort to enable -Wcast-function-type in the top-level Makefile to
> > support Control Flow Integrity builds, there are the need to remove all
> > the function callback casts.
> >
> > ftrace_ops_list_func() can no longer be defined as ftrace_ops_no_ops().
> > The reason for ftrace_ops_no_ops() is to use that when an architecture
> > calls ftrace_ops_list_func() with only two parameters (called from
> > assembly). And to make sure there's no C side-effects, those archs call
> > ftrace_ops_no_ops() which only has two parameters, as the function
> > ftrace_ops_list_func() has four parameters.
> >
> > This patch removes the no longer needed function ftrace_ops_no_ops() and
> > all the function callback casts using the previous defined ftrace_func
> > union and the two function helpers called ftrace_set_ufunc() and
> > ftrace_same_address_ufunc().
>
> Ug, I think I prefer the linker trick better.
>
> >
> > Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
> > ---
> >  kernel/trace/ftrace.c | 48 ++++++++++++++++++++++++++-----------------
> >  1 file changed, 29 insertions(+), 19 deletions(-)
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index fd8fbb422860..124ccf914657 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -143,9 +143,7 @@ static inline bool ftrace_same_address_ufunc(union ftrace_func *ufunc,
> >  	return (ufunc->ops == func);
> >  }
> >  #else
> > -/* See comment below, where ftrace_ops_list_func is defined */
> > -static void ftrace_ops_no_ops(unsigned long ip, unsigned long parent_ip);
> > -#define ftrace_ops_list_func ((ftrace_func_t)ftrace_ops_no_ops)
> > +static void ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip);
> >
> >  static inline void ftrace_set_ufunc(union ftrace_func *ufunc,
> >  				    ftrace_func_no_ops_t func)
> > @@ -198,22 +196,29 @@ static void ftrace_sync_ipi(void *data)
> >  	smp_rmb();
> >  }
> >
> > -static ftrace_func_t ftrace_ops_get_list_func(struct ftrace_ops *ops)
> > +static union ftrace_func ftrace_ops_get_list_func(struct ftrace_ops *ops)
> >  {
> > +	union ftrace_func list_func;
> > +
> >  	/*
> >  	 * If this is a dynamic, RCU, or per CPU ops, or we force list func,
> >  	 * then it needs to call the list anyway.
> >  	 */
> >  	if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_RCU) ||
> >  	    FTRACE_FORCE_LIST_FUNC)
> > -		return ftrace_ops_list_func;
> > +		ftrace_set_ufunc(&list_func, ftrace_ops_list_func);
> > +	else
> > +		list_func.ops = ftrace_ops_get_func(ops);
> >
> > -	return ftrace_ops_get_func(ops);
> > +	return list_func;
>
> Is this the same as returning a pointer? It makes me very nervous about
> returning a union. Can a compiler return something allocated on the stack?

Yes, it can. The union is returned by value (copied). So, there is no
problem.

If you don't like to return a union by value, I can modify the prototype
of the ftrace_ops_get_list_func() to the following one:

static void ftrace_ops_get_list_func(struct ftrace_ops *ops,
				     union ftrace_func *list_func)

This way the old local variable "list_func" is removed and we need to pass
the address of a union to this function. Then the function modify the union.

If you prefer this option I have no problem with the change.

> Also, don't use "ufunc" as that makes me think its a user space variable.

Ok, would "bfunc" or "jfunc" be better?
bfunc => blend - function
jfunc => join - function

> >  }
> >
> >  static void update_ftrace_function(void)
> >  {
> > -	ftrace_func_t func;
> > +	union ftrace_func func;
> > +#ifndef CONFIG_DYNAMIC_FTRACE
> > +	union ftrace_func tmp;
> > +#endif
> >
> >  	/*
> >  	 * Prepare the ftrace_ops that the arch callback will use.
> > @@ -225,7 +230,7 @@ static void update_ftrace_function(void)
> >
> >  	/* If there's no ftrace_ops registered, just call the stub function */
> >  	if (set_function_trace_op == &ftrace_list_end) {
> > -		func = ftrace_stub;
> > +		func.ops = ftrace_stub;
> >
> >  	/*
> >  	 * If we are at the end of the list and this ops is
> > @@ -239,21 +244,21 @@ static void update_ftrace_function(void)
> >  	} else {
> >  		/* Just use the default ftrace_ops */
> >  		set_function_trace_op = &ftrace_list_end;
> > -		func = ftrace_ops_list_func;
> > +		ftrace_set_ufunc(&func, ftrace_ops_list_func);
> >  	}
> >
> >  	update_function_graph_func();
> >
> >  	/* If there's no change, then do nothing more here */
> > -	if (ftrace_trace_function == func)
> > +	if (ftrace_trace_function == func.ops)
> >  		return;
> >
> >  	/*
> >  	 * If we are using the list function, it doesn't care
> >  	 * about the function_trace_ops.
> >  	 */
> > -	if (func == ftrace_ops_list_func) {
> > -		ftrace_trace_function = func;
> > +	if (ftrace_same_address_ufunc(&func, ftrace_ops_list_func)) {
> > +		ftrace_trace_function = func.ops;
> >  		/*
> >  		 * Don't even bother setting function_trace_ops,
> >  		 * it would be racy to do so anyway.
> > @@ -272,7 +277,9 @@ static void update_ftrace_function(void)
> >  	 * function we want, albeit indirectly, but it handles the
> >  	 * ftrace_ops and doesn't depend on function_trace_op.
> >  	 */
> > -	ftrace_trace_function = ftrace_ops_list_func;
> > +	ftrace_set_ufunc(&tmp, ftrace_ops_list_func);
> > +	ftrace_trace_function = tmp.ops;
> > +
> >  	/*
> >  	 * Make sure all CPUs see this. Yes this is slow, but static
> >  	 * tracing is slow and nasty to have enabled.
> > @@ -287,7 +294,7 @@ static void update_ftrace_function(void)
> >  	/* OK, we are all set to update the ftrace_trace_function now! */
> >  #endif /* !CONFIG_DYNAMIC_FTRACE */
> >
> > -	ftrace_trace_function = func;
> > +	ftrace_trace_function = func.ops;
> >  }
>
> This looks really intrusive for what it's trying to accomplish.

What this patch is trying to accomplish is to remove the warning
-Wcast-function-type in a way that allows the CFI build to get the
necessary info about the prototypes (4 parameters or 2 parameters) of
the functions used for archs that support ftrace ops and for the archs
that don't support ftrace ops. This info is necessary to allow the
compiler to insert a check to guarantee that an indirect call can only
jump to functions that match the prototype. This way, the attack surface
is reduced if an attacker can get some control over the system.

> The linker trick is far less intrusive, and I believe less error prone.

If we use the linker trick, the warning -Wcast-function-type dissapears,
but in a way that makes impossible to the compiler to get the necessary
info about function prototypes to insert the commented check. As far I
know, this linker trick (redirection of a function) is hidden for the
CFI build.

So, in my opinion, the linker trick is not suitable if we want to protect
the function pointers of the ftrace subsystem against an attack that
modifiy the normal flow of the kernel.

> -- Steve

Thanks,
Oscar Carter

> >
> >  static void add_ftrace_ops(struct ftrace_ops __rcu **list,
> > @@ -2680,6 +2687,7 @@ void ftrace_modify_all_code(int command)
> >  	int update = command & FTRACE_UPDATE_TRACE_FUNC;
> >  	int mod_flags = 0;
> >  	int err = 0;
> > +	union ftrace_func func;
> >
> >  	if (command & FTRACE_MAY_SLEEP)
> >  		mod_flags = FTRACE_MODIFY_MAY_SLEEP_FL;
> > @@ -2695,7 +2703,8 @@ void ftrace_modify_all_code(int command)
> >  	 * traced.
> >  	 */
> >  	if (update) {
> > -		err = ftrace_update_ftrace_func(ftrace_ops_list_func);
> > +		ftrace_set_ufunc(&func, ftrace_ops_list_func);
> > +		err = ftrace_update_ftrace_func(func.ops);
> >  		if (FTRACE_WARN_ON(err))
> >  			return;
> >  	}
> > @@ -2705,7 +2714,9 @@ void ftrace_modify_all_code(int command)
> >  	else if (command & FTRACE_DISABLE_CALLS)
> >  		ftrace_replace_code(mod_flags);
> >
> > -	if (update && ftrace_trace_function != ftrace_ops_list_func) {
> > +	ftrace_set_ufunc(&func, ftrace_ops_list_func);
> > +
> > +	if (update && ftrace_trace_function != func.ops) {
> >  		function_trace_op = set_function_trace_op;
> >  		smp_wmb();
> >  		/* If irqs are disabled, we are in stop machine */
> > @@ -6890,14 +6901,13 @@ static void ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
> >  {
> >  	__ftrace_ops_list_func(ip, parent_ip, NULL, regs);
> >  }
> > -NOKPROBE_SYMBOL(ftrace_ops_list_func);
> >  #else
> > -static void ftrace_ops_no_ops(unsigned long ip, unsigned long parent_ip)
> > +static void ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip)
> >  {
> >  	__ftrace_ops_list_func(ip, parent_ip, NULL, NULL);
> >  }
> > -NOKPROBE_SYMBOL(ftrace_ops_no_ops);
> >  #endif
> > +NOKPROBE_SYMBOL(ftrace_ops_list_func);
> >
> >  /*
> >   * If there's only one function registered but it does not support
> > --
> > 2.20.1
>