In extended addressing mode, function mctp_local_output() fetch netdev
through dev_get_by_index_rcu, which won't increase netdev's reference
counter. Hence, the reference may underflow when mctp_local_output calls
dev_put(), results in possible use after free.

This patch adds dev_hold() to fix the reference bug.

Fixes: 99ce45d5e7db ("mctp: Implement extended addressing")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
---
 net/mctp/route.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/mctp/route.c b/net/mctp/route.c
index e52cef750500..a9e5d6c40c65 100644
--- a/net/mctp/route.c
+++ b/net/mctp/route.c
@@ -817,6 +817,7 @@ int mctp_local_output(struct sock *sk, struct mctp_route *rt,
 			return rc;
 		}
 
+		dev_hold(dev);
 		rt->dev = __mctp_dev_get(dev);
 		rcu_read_unlock();
 
-- 
2.35.1

Hi Lin Ma,

On Thu, 2022-03-24 at 10:39 +0800, Lin Ma wrote:
> In extended addressing mode, function mctp_local_output() fetch netdev
> through dev_get_by_index_rcu, which won't increase netdev's reference
> counter. Hence, the reference may underflow when mctp_local_output calls
> dev_put(), results in possible use after free.
> 
> This patch adds dev_hold() to fix the reference bug.

This was already fixed in net-next to increment the refcount in
__mctp_dev_get() and use mctp_dev_put().

dc121c008491 ("mctp: make __mctp_dev_get() take a refcount hold")
e297db3eadd7 ("mctp: Fix incorrect netdev unref for extended addr")

Thanks,
Matt