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:

quoted

On Mon, 13 Dec 2021, Josh Poimboeuf wrote:

quoted

On Fri, Dec 10, 2021 at 01:44:48PM +0100, Miroslav Benes wrote:

quoted

--- 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.

quoted

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