[PATCH -tip v2] [BUGFIX] kprobes: Skip kretprobe hit in NMI context to avoid deadlock

[Masami Hiramatsu]

Skip kretprobe hit in NMI context, because if an NMI happens
inside the critical section protected by kretprobe_table.lock
and another(or same) kretprobe hit, pre_kretprobe_handler
tries to lock kretprobe_table.lock again.
Normal interrupts have no problem because they are disabled
with the lock.

Changes in v2:
 - Add unlikely for in_nmi.
 - Add a comment for nmissed counter.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
Acked-by: Ananth N Mavinakayanahalli <ananth@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
---
 kernel/kprobes.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 734e9a7..138219b 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1778,6 +1778,16 @@ static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 	unsigned long hash, flags = 0;
 	struct kretprobe_instance *ri;
 
+	/* To avoid deadlock, prohibit return probing in NMI context */
+	if (unlikely(in_nmi())) {
+		/*
+		 * Do not get a lock. nmissed is not accurate.
+		 * This just informs user some misses are happened.
+		 */
+		rp->nmissed++;
+		return 0;
+	}
+
 	/*TODO: consider to only swap the RA after the last pre_handler fired */
 	hash = hash_ptr(current, KPROBE_HASH_BITS);
 	raw_spin_lock_irqsave(&rp->lock, flags);


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