[PATCH 2/3] livepatch: send a fake signal to all blocking tasks

[Miroslav Benes]

Live patching consistency model is of LEAVE_PATCHED_SET and
SWITCH_THREAD. This means that all tasks in the system have to be marked
one by one as safe to call a new patched function. Safe means when a
task is not (sleeping) in a set of patched functions. That is, no
patched function is on the task's stack. Another clearly safe place is
the boundary between kernel and userspace. The patching waits for all
tasks to get outside of the patched set or to cross the boundary. The
transition is completed afterwards.

The problem is that a task can block the transition for quite a long
time, if not forever. It could sleep in a set of patched functions, for
example.  Luckily we can force the task to leave the set by sending it a
fake signal, that is a signal with no data in signal pending structures
(no handler, no sign of proper signal delivered). Suspend/freezer use
this to freeze the tasks as well. The task gets TIF_SIGPENDING set and
is woken up (if it has been sleeping in the kernel before) or kicked by
rescheduling IPI (if it was running on other CPU). This causes the task
to go to kernel/userspace boundary where the signal would be handled and
the task would be marked as safe in terms of live patching.

There are tasks which are not affected by this technique though. The
fake signal is not sent to kthreads. They should be handled in a
different way. They can be woken up so they leave the patched set and
their TIF_PATCH_PENDING can be cleared thanks to stack checking.

For the sake of completeness, if the task is in TASK_RUNNING state but
not currently running on some CPU it doesn't get the IPI, but it would
eventually handle the signal anyway. Second, if the task runs in the
kernel (in TASK_RUNNING state) it gets the IPI, but the signal is not
handled on return from the interrupt. It would be handled on return to
the userspace in the future when the fake signal is sent again. Stack
checking deals with these cases in a better way.

If the task was sleeping in a syscall it would be woken by our fake
signal, it would check if TIF_SIGPENDING is set (by calling
signal_pending() predicate) and return ERESTART* or EINTR. Syscalls with
ERESTART* return values are restarted in case of the fake signal (see
do_signal()). EINTR is propagated back to the userspace program. This
could disturb the program, but...

* each process dealing with signals should react accordingly to EINTR
  return values.
* syscalls returning EINTR happen to be quite common situation in the
  system even if no fake signal is sent.
* freezer sends the fake signal and does not deal with EINTR anyhow.
  Thus EINTR values are returned when the system is resumed.

The very safe marking is done in entry.S on syscall and
interrupt/exception exit paths, and in a stack checking functions of
livepatch.  TIF_PATCH_PENDING is cleared and the next
recalc_sigpending() drops TIF_SIGPENDING.

Note that the fake signal is not sent to stopped/traced tasks. Such task
prevents the patching to finish till it continues again (is not traced
anymore).

Last, sending the fake signal is not automatic. It is done only when
admin requests it by writing 1 to force sysfs attribute in livepatch
sysfs directory.

Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Miroslav Benes <mbenes@xxxxxxx>
---
 include/linux/livepatch.h     |  3 +++
 kernel/livepatch/core.c       |  3 +++
 kernel/livepatch/transition.c | 40 ++++++++++++++++++++++++++++++++++++++++
 kernel/livepatch/transition.h |  1 +
 kernel/signal.c               |  4 +++-
 5 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 194991ef9347..43cfeebeb42b 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -29,6 +29,9 @@
 
 #include <asm/livepatch.h>
 
+/* values for sysfs force attribute */
+#define KLP_FORCE_FAKE		1
+
 /* task patch states */
 #define KLP_UNDEFINED	-1
 #define KLP_UNPATCHED	 0
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 84f8944704ad..bb3b78fa7d2b 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -466,6 +466,9 @@ static ssize_t force_store(struct kobject *kobj, struct kobj_attribute *attr,
 	}
 
 	switch (val) {
+	case KLP_FORCE_FAKE:
+		klp_send_fake_signal();
+		break;
 	default:
 		return -EINVAL;
 	}
diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index adc0cc64aa4b..bb61aaa196d3 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -551,3 +551,43 @@ void klp_copy_process(struct task_struct *child)
 
 	/* TIF_PATCH_PENDING gets copied in setup_thread_stack() */
 }
+
+/*
+ * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
+ * Kthreads with TIF_PATCH_PENDING set are woken up. Only admin can request this
+ * action currently.
+ */
+void klp_send_fake_signal(void)
+{
+	struct task_struct *g, *task;
+
+	pr_info("sending a fake signal and waking sleeping kthreads up\n");
+
+	read_lock(&tasklist_lock);
+	for_each_process_thread(g, task) {
+		if (!klp_patch_pending(task))
+			continue;
+
+		/*
+		 * There is a small race here. We could see TIF_PATCH_PENDING
+		 * set and decide to wake up a kthread or send a fake signal.
+		 * Meanwhile the task could migrate itself and the action
+		 * would be meaningless. It is not serious though.
+		 */
+		if (task->flags & PF_KTHREAD) {
+			/*
+			 * Wake up a kthread which still has not been migrated.
+			 */
+			wake_up_process(task);
+		} else {
+			/*
+			 * Send fake signal to all non-kthread tasks which are
+			 * still not migrated.
+			 */
+			spin_lock_irq(&task->sighand->siglock);
+			signal_wake_up(task, 0);
+			spin_unlock_irq(&task->sighand->siglock);
+		}
+	}
+	read_unlock(&tasklist_lock);
+}
diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h
index ce09b326546c..1c7ede6eaa77 100644
--- a/kernel/livepatch/transition.h
+++ b/kernel/livepatch/transition.h
@@ -10,5 +10,6 @@ void klp_cancel_transition(void);
 void klp_start_transition(void);
 void klp_try_complete_transition(void);
 void klp_reverse_transition(void);
+void klp_send_fake_signal(void);
 
 #endif /* _LIVEPATCH_TRANSITION_H */
diff --git a/kernel/signal.c b/kernel/signal.c
index 7e59ebc2c25e..3a25cc06231d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -39,6 +39,7 @@
 #include <linux/compat.h>
 #include <linux/cn_proc.h>
 #include <linux/compiler.h>
+#include <linux/livepatch.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/signal.h>
@@ -162,7 +163,8 @@ void recalc_sigpending_and_wake(struct task_struct *t)
 
 void recalc_sigpending(void)
 {
-	if (!recalc_sigpending_tsk(current) && !freezing(current))
+	if (!recalc_sigpending_tsk(current) && !freezing(current) &&
+	    !klp_patch_pending(current))
 		clear_thread_flag(TIF_SIGPENDING);
 
 }
-- 
2.12.2