On Wed 2021-10-06 11:04:26, Peter Zijlstra wrote:
On Wed, Oct 06, 2021 at 10:12:17AM +0200, Petr Mladek wrote:
quoted
IMHO, the original solution from v1 was better. We only needed to
It was also terribly broken in other 'fun' ways. See below.
quoted
be careful when updating task->patch_state and clearing
TIF_PATCH_PENDING to avoid the race.
The following might work:
static int klp_check_and_switch_task(struct task_struct *task, void *arg)
{
int ret;
/*
* Stack is reliable only when the task is not running on any CPU,
* except for the task running this code.
*/
if (task_curr(task) && task != current) {
/*
* This only succeeds when the task is in NOHZ_FULL user
* mode. Such a task might be migrated immediately. We
* only need to be careful to set task->patch_state before
* clearing TIF_PATCH_PENDING so that the task migrates
* itself when entring kernel in the meatime.
*/
if (is_ct_user(task)) {
klp_update_patch_state(task);
return 0;
}
return -EBUSY;
}
ret = klp_check_stack(task, arg);
if (ret)
return ret;
/*
* The task neither is running on any CPU and nor it can get
* running. As a result, the ordering is not important and
* barrier is not needed.
*/
task->patch_state = klp_target_state;
clear_tsk_thread_flag(task, TIF_PATCH_PENDING);
return 0;
}
, where is_ct_user(task) would return true when task is running in
CONTEXT_USER. If I get the context_tracking API correctly then
it might be implemeted the following way:
That's not sufficient, you need to tag the remote task with a ct_work
item to also runs klp_update_patch_state(), otherwise the remote CPU can
enter kernel space between checking is_ct_user() and doing
klp_update_patch_state():
CPU0 CPU1
<user>
if (is_ct_user()) // true
<kernel-entry>
// run some kernel code
klp_update_patch_state()
*WHOOPSIE*
So it needs to be something like:
CPU0 CPU1
<user>
if (context_tracking_set_cpu_work(task_cpu(), CT_WORK_KLP))
<kernel-entry>
klp_update_patch_state klp_update_patch_state()
So that CPU0 and CPU1 race to complete klp_update_patch_state() *before*
any regular (!noinstr) code gets run.
Grr, you are right. I thought that we migrated the task when entering
kernel even before. But it seems that we do it only when leaving
the kernel in exit_to_user_mode_loop().
Which then means it needs to look something like:
noinstr void klp_update_patch_state(struct task_struct *task)
{
struct thread_info *ti = task_thread_info(task);
preempt_disable_notrace();
if (arch_test_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags)) {
/*
* Order loads of TIF_PATCH_PENDING vs klp_target_state.
* See klp_init_transition().
*/
smp_rmb();
task->patch_state = __READ_ONCE(klp_target_state);
/*
* Concurrent against self; must observe updated
* task->patch_state if !TIF_PATCH_PENDING.
*/
smp_mb__before_atomic();
IMHO, smp_wmb() should be enough. We are here only when this
CPU set task->patch_state right above. So that CPU running
this code should see the correct task->patch_state.
The read barrier is needed only when @task is entering kernel and
does not see TIF_PATCH_PENDING. It is handled by smp_rmb() in
the "else" branch below.
It is possible that both CPUs see TIF_PATCH_PENDING and both
set task->patch_state. But it should not cause any harm
because they set the same value. Unless something really
crazy happens with the internal CPU busses and caches.
arch_clear_bit(TIF_PATCH_PENDING, (unsigned long *)&ti->flags);
} else {
/*
* Concurrent against self, see smp_mb__before_atomic()
* above.
*/
smp_rmb();
Yeah, this is the counter part against the above smp_wmb().
}
preempt_enable_notrace();
}
Now, I am scared to increase my paranoia level and search for even more
possible races. I feel overwhelmed at the moment ;-)
Best Regards,
Petr