[PATCH] net/802/garp: fix potential deadlock on &app->lock

[Chengfeng Ye]

As &app->lock is also acquired by the timer garp_join_timer() which
which executes under soft-irq context, code executing under process
context should disable irq before acquiring the lock, otherwise
deadlock could happen if the process context hold the lock then
preempt by the interruption.

garp_pdu_rcv() is one such function that acquires &app->lock, but I
am not sure whether it is called with irq disable outside thus the
patch could be false.

Possible deadlock scenario:
garp_pdu_rcv()
    -> spin_lock(&app->lock)
        <timer interrupt>
        -> garp_join_timer()
        -> spin_lock(&app->lock)

This flaw was found using an experimental static analysis tool we are
developing for irq-related deadlock.

The tentative patch fix the potential deadlock by spin_lock_irqsave(),
or it should be fixed with spin_lock_bh() if it is a real bug? I am
not very sure.

Signed-off-by: Chengfeng Ye <dg573847474@xxxxxxxxx>
---
 net/802/garp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/802/garp.c b/net/802/garp.c
index ab24b21fbb49..acc6f2f847a6 100644
--- a/net/802/garp.c
+++ b/net/802/garp.c
@@ -515,6 +515,7 @@ static void garp_pdu_rcv(const struct stp_proto *proto, struct sk_buff *skb,
 	struct garp_port *port;
 	struct garp_applicant *app;
 	const struct garp_pdu_hdr *gp;
+	unsigned long flags;
 
 	port = rcu_dereference(dev->garp_port);
 	if (!port)
@@ -530,14 +531,14 @@ static void garp_pdu_rcv(const struct stp_proto *proto, struct sk_buff *skb,
 		goto err;
 	skb_pull(skb, sizeof(*gp));
 
-	spin_lock(&app->lock);
+	spin_lock_irqsave(&app->lock, flags);
 	while (skb->len > 0) {
 		if (garp_pdu_parse_msg(app, skb) < 0)
 			break;
 		if (garp_pdu_parse_end_mark(skb) < 0)
 			break;
 	}
-	spin_unlock(&app->lock);
+	spin_unlock_irqrestore(&app->lock, flags);
 err:
 	kfree_skb(skb);
 }
-- 
2.17.1