[PATCH -tip v2] kprobes: Use synchronize_rcu_tasks() for optprobe with CONFIG_PREEMPT

[Masami Hiramatsu <mhiramat@kernel.org>]

To enable jump optimized probe with CONFIG_PREEMPT, use
synchronize_rcu_tasks() to wait for all tasks preempted
on trampoline code back on track.

Since the jump optimized kprobes can replace multiple
instructions, there can be tasks which are interrupted
on the 2nd (or 3rd) instructions. If the kprobe
replaces those instructions by a jump instruction,
when those tasks back to the interrupted place, it is
a middle of the jump instruction and causes a kernel
panic.
To avoid such tragedies in advance, kprobe optimizer
prepare a detour route using normal kprobe (e.g.
int3 breakpoint on x86), and wait for the tasks which
is interrrupted on such place by synchronize_sched()
when CONFIG_PREEMPT=n.
If CONFIG_PREEMPT=y, things be more complicated, because
such interrupted thread can be preempted (other thread
can be scheduled in interrupt handler.) So, kprobes
optimizer has to wait for those tasks scheduled normally.
In this case we can use synchronize_rcu_tasks() which
ensures that all preempted tasks back on track and
schedule it.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 arch/Kconfig     |    2 +-
 kernel/kprobes.c |   18 +++++++++++++-----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 2520ca5b42eb..d495c06ae961 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -90,7 +90,7 @@ config STATIC_KEYS_SELFTEST
 config OPTPROBES
 	def_bool y
 	depends on KPROBES && HAVE_OPTPROBES
-	depends on !PREEMPT
+	select TASKS_RCU if PREEMPT
 
 config KPROBES_ON_FTRACE
 	def_bool y
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index a1606a4224e1..6243b8b02511 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -574,12 +574,20 @@ static void kprobe_optimizer(struct work_struct *work)
 
 	/*
 	 * Step 2: Wait for quiesence period to ensure all running interrupts
-	 * are done. Because optprobe may modify multiple instructions
-	 * there is a chance that Nth instruction is interrupted. In that
-	 * case, running interrupt can return to 2nd-Nth byte of jump
-	 * instruction. This wait is for avoiding it.
+	 * are done. Because optprobe may modify multiple instructions,
+	 * there is a chance that the Nth instruction is interrupted. In that
+	 * case, running interrupt can return to the Nth byte of jump
+	 * instruction. This can be avoided by waiting for returning of
+	 * such interrupts, since (until here) the first byte of the optimized
+	 * probe is already replaced with normal kprobe (sw breakpoint) and
+	 * all threads which reach to the probed address will hit it and
+	 * bypass the copied instructions instead of executing the original.
+	 * With CONFIG_PREEMPT, such interrupts can be preepmted. To wait
+	 * for such thread, we will use synchronize_rcu_tasks() which ensures
+	 * all preeempted tasks are scheduled normally (= not preempted.)
+	 * So we can ensure there is no threads preempted at probed address.
 	 */
-	synchronize_sched();
+	synchronize_rcu_tasks();
 
 	/* Step 3: Optimize kprobes after quiesence period */
 	do_optimize_kprobes();