[PATCH] livepatch: add locking to force and signal functions

[PATCH] livepatch: add locking to force and signal functions

[Miroslav Benes]

klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
because it seemed to be superfluous. A potential race in
klp_send_signals() was harmless and there was nothing in
klp_force_transition() which needed to be synchronized. That changed
with the addition of klp_forced variable during the review process.

There is a small window now, when klp_complete_transition() does not see
klp_forced set to true while all tasks have been already transitioned to
the target state. module_put() is called and the module can be removed.

Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
to be sure. There is no real downside to that.

Reported-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
---
 kernel/livepatch/transition.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index be5bfa533ee8..3f932ff607cd 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -625,6 +625,8 @@ void klp_send_signals(void)
 
 	pr_notice("signaling remaining tasks\n");
 
+	mutex_lock(&klp_mutex);
+
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task) {
 		if (!klp_patch_pending(task))
@@ -653,6 +655,8 @@ void klp_send_signals(void)
 		}
 	}
 	read_unlock(&tasklist_lock);
+
+	mutex_unlock(&klp_mutex);
 }
 
 /*
@@ -671,6 +675,8 @@ void klp_force_transition(void)
 
 	pr_warn("forcing remaining tasks to the patched state\n");
 
+	mutex_lock(&klp_mutex);
+
 	read_lock(&tasklist_lock);
 	for_each_process_thread(g, task)
 		klp_update_patch_state(task);
@@ -680,4 +686,6 @@ void klp_force_transition(void)
 		klp_update_patch_state(idle_task(cpu));
 
 	klp_forced = true;
+
+	mutex_unlock(&klp_mutex);
 }
-- 
2.15.1

Re: [PATCH] livepatch: add locking to force and signal functions

[Petr Mladek]

On Wed 2017-12-20 10:28:07, Miroslav Benes wrote:
> klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
> because it seemed to be superfluous. A potential race in
> klp_send_signals() was harmless and there was nothing in
> klp_force_transition() which needed to be synchronized. That changed
> with the addition of klp_forced variable during the review process.
> 
> There is a small window now, when klp_complete_transition() does not see
> klp_forced set to true while all tasks have been already transitioned to
> the target state. module_put() is called and the module can be removed.
> 
> Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
> to be sure. There is no real downside to that.
> 
> Reported-by: Jason Baron <jbaron@akamai.com>
> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> ---
>  kernel/livepatch/transition.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> index be5bfa533ee8..3f932ff607cd 100644
> --- a/kernel/livepatch/transition.c
> +++ b/kernel/livepatch/transition.c
> @@ -625,6 +625,8 @@ void klp_send_signals(void)
>  
>  	pr_notice("signaling remaining tasks\n");
>  
> +	mutex_lock(&klp_mutex);
> +
>  	read_lock(&tasklist_lock);
>  	for_each_process_thread(g, task) {
>  		if (!klp_patch_pending(task))
> @@ -653,6 +655,8 @@ void klp_send_signals(void)
>  		}
>  	}
>  	read_unlock(&tasklist_lock);
> +
> +	mutex_unlock(&klp_mutex);

It would be cleaner if the lock guarded also the check:

	if (patch != klp_transition_patch)
		return -EINVAL;

in signal_store(). Then we could remove also the comment
above this check.

Same is true also for the force part stuff.

Best Regards,
Petr

PS: I am sorry that I hand waved the proposed solution when
we spoke about it yeasterday. I should have looked into
the code.

Re: [PATCH] livepatch: add locking to force and signal functions

[Miroslav Benes]

On Wed, 20 Dec 2017, Petr Mladek wrote:

> On Wed 2017-12-20 10:28:07, Miroslav Benes wrote:
> > klp_send_signals() and klp_force_transition() do not acquire klp_mutex,
> > because it seemed to be superfluous. A potential race in
> > klp_send_signals() was harmless and there was nothing in
> > klp_force_transition() which needed to be synchronized. That changed
> > with the addition of klp_forced variable during the review process.
> > 
> > There is a small window now, when klp_complete_transition() does not see
> > klp_forced set to true while all tasks have been already transitioned to
> > the target state. module_put() is called and the module can be removed.
> > 
> > Acquire klp_mutex to prevent it. Do the same in klp_send_signals() just
> > to be sure. There is no real downside to that.
> > 
> > Reported-by: Jason Baron <jbaron@akamai.com>
> > Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> > ---
> >  kernel/livepatch/transition.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index be5bfa533ee8..3f932ff607cd 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -625,6 +625,8 @@ void klp_send_signals(void)
> >  
> >  	pr_notice("signaling remaining tasks\n");
> >  
> > +	mutex_lock(&klp_mutex);
> > +
> >  	read_lock(&tasklist_lock);
> >  	for_each_process_thread(g, task) {
> >  		if (!klp_patch_pending(task))
> > @@ -653,6 +655,8 @@ void klp_send_signals(void)
> >  		}
> >  	}
> >  	read_unlock(&tasklist_lock);
> > +
> > +	mutex_unlock(&klp_mutex);
> 
> It would be cleaner if the lock guarded also the check:
> 
> 	if (patch != klp_transition_patch)
> 		return -EINVAL;
> 
> in signal_store(). Then we could remove also the comment
> above this check.
> 
> Same is true also for the force part stuff.

And I even left obsolete comments in sysfs callbacks. Sigh. v2 is 
inevitable...

Miroslav