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Date:   Wed, 20 May 2020 20:45:29 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Jon Maloy <jmaloy@...hat.com>, Ying Xue <ying.xue@...driver.com>
Cc:     netdev@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH] tipc: Disable preemption when calling send_msg method.

syzbot is reporting that tipc_udp_send_msg() is calling dst_cache_get()
without preemption disabled [1]. Since b->media->send_msg() is called
with RCU lock already held, we can also disable preemption at that point.

[1] https://syzkaller.appspot.com/bug?id=dc6352b92862eb79373fe03fdf9af5928753e057

Reported-by: syzbot+1a68504d96cd17b33a05@...kaller.appspotmail.com
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
 net/tipc/bearer.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/tipc/bearer.c b/net/tipc/bearer.c
index 34ca7b789eba..e5cf91665881 100644
--- a/net/tipc/bearer.c
+++ b/net/tipc/bearer.c
@@ -516,6 +516,7 @@ void tipc_bearer_xmit_skb(struct net *net, u32 bearer_id,
 	struct tipc_msg *hdr = buf_msg(skb);
 	struct tipc_bearer *b;
 
+	preempt_disable();
 	rcu_read_lock();
 	b = bearer_get(net, bearer_id);
 	if (likely(b && (test_bit(0, &b->up) || msg_is_reset(hdr)))) {
@@ -528,6 +529,7 @@ void tipc_bearer_xmit_skb(struct net *net, u32 bearer_id,
 		kfree_skb(skb);
 	}
 	rcu_read_unlock();
+	preempt_enable();
 }
 
 /* tipc_bearer_xmit() -send buffer to destination over bearer
@@ -543,6 +545,7 @@ void tipc_bearer_xmit(struct net *net, u32 bearer_id,
 	if (skb_queue_empty(xmitq))
 		return;
 
+	preempt_disable();
 	rcu_read_lock();
 	b = bearer_get(net, bearer_id);
 	if (unlikely(!b))
@@ -560,6 +563,7 @@ void tipc_bearer_xmit(struct net *net, u32 bearer_id,
 		}
 	}
 	rcu_read_unlock();
+	preempt_enable();
 }
 
 /* tipc_bearer_bc_xmit() - broadcast buffers to all destinations
@@ -574,6 +578,7 @@ void tipc_bearer_bc_xmit(struct net *net, u32 bearer_id,
 	struct sk_buff *skb, *tmp;
 	struct tipc_msg *hdr;
 
+	preempt_disable();
 	rcu_read_lock();
 	b = bearer_get(net, bearer_id);
 	if (unlikely(!b || !test_bit(0, &b->up)))
@@ -591,6 +596,7 @@ void tipc_bearer_bc_xmit(struct net *net, u32 bearer_id,
 			b->media->send_msg(net, skb, b, dst);
 	}
 	rcu_read_unlock();
+	preempt_enable();
 }
 
 /**
-- 
2.18.2

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