linux-kernel - Re: [PATCH v2 1/2] livepatch: Allow user to specify functions to search for on a stack

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Date:   Tue, 14 Dec 2021 16:40:11 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, jikos@...nel.org,
        joe.lawrence@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        shuah@...nel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 1/2] livepatch: Allow user to specify functions to
 search for on a stack

On Tue 2021-12-14 13:27:33, Petr Mladek wrote:
> On Tue 2021-12-14 09:47:59, Miroslav Benes wrote:
> > On Mon, 13 Dec 2021, Josh Poimboeuf wrote:
> > > On Fri, Dec 10, 2021 at 01:44:48PM +0100, Miroslav Benes wrote:
> > > > --- a/kernel/livepatch/transition.c
> > > > +++ b/kernel/livepatch/transition.c
> > > > @@ -200,7 +200,10 @@ static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
> > > >  	for (i = 0; i < nr_entries; i++) {
> > > >  		address = entries[i];
> > > >  
> > > > -		if (klp_target_state == KLP_UNPATCHED) {
> > > > +		if (func->stack_only) {
> > > > +			func_addr = (unsigned long)func->old_func;
> > > > +			func_size = func->old_size;
> > > > +		} else if (klp_target_state == KLP_UNPATCHED) {
> > > 
> > > Hm, what does this mean for the unpatching case?  What if the new
> > > function's .cold child is on the stack when we're trying to unpatch?
> > 
> > Good question. I did not realize it worked both ways. Of course it does.
> > 
> > > Would it make sense to allow the user specify a 'new_func' for
> > > stack_only, which is a func to check on the stack when unpatching?  Then
> > > new_func could point to the new .cold child.  And then
> > > klp_check_stack_func() wouldn't need a special case.
> 
> I am confused. My understanding is that .cold child is explicitly
> livepatched to the new .cold child like it is done in the selftest:
> 
> static struct klp_func funcs_stack_only[] = {
> 	{
> 		.old_name = "child_function",
> 		.new_func = livepatch_child_function,
> 	}, {
> 
> We should not need anything special to check it on stack.
> We only need to make sure that we check all .stack_only functions of
> the to-be-disabled livepatch.

We have discussed this with Miroslav and it seems to be even more
complicated. My current understanding is that we actually have
three functions involved:

  parent_func()
    call child_func()
      jmp child_func.cold

We livepatch child_func() that uses jmp and need not be on stack.
This is why we want to check parent_func() on stack.
For this, we define something like:

static struct klp_func funcs[] = {
	{
		.old_name = "child_func",
		.new_func = livepatch_child_func,   // livepatched func
	},
	{
		.old_name = "parent_func",
		.stack_only = true,		    // stack only
	},


Now, there might be the same problem with livepatch_child_func.
The call chain would be:

  parent_func()
    call child_func() ---> livepatch_child_func()
      jmp livepatch_child_func.cold


=> We need to check the very same parent_func() also when unpatching.


Note that already do the same for nops:

static struct klp_func *klp_alloc_func_nop(struct klp_func *old_func,
					   struct klp_object *obj)
{
[...]
	klp_init_func_early(obj, func);
	/*
	 * func->new_func is same as func->old_func. These addresses are
	 * set when the object is loaded, see klp_init_object_loaded().
	 */
	func->old_sympos = old_func->old_sympos;
	func->nop = true;
[...]
}

where

static int klp_init_object_loaded(struct klp_patch *patch,
				  struct klp_object *obj)
{
[...]
	if (func->nop)
			func->new_func = func->old_func;
[...]


This is another argument that we should somehow reuse the nops code
also for stack_only checks.

Does it make sense, please? ;-)

Best Regards,
Petr