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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Xiyu Yang <xiyuyang19@fudan.edu.cn>,
        Xin Tan <tanxin.ctf@gmail.com>,
        "David S. Miller" <davem@davemloft.net>
Subject: [PATCH 5.6 057/167] net/x25: Fix x25_neigh refcnt leak when receiving frame
Date:   Tue, 28 Apr 2020 20:23:53 +0200
Message-Id: <20200428182232.201265461@linuxfoundation.org>
In-Reply-To: <20200428182225.451225420@linuxfoundation.org>
References: <20200428182225.451225420@linuxfoundation.org>
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From: Xiyu Yang <xiyuyang19@fudan.edu.cn>

[ Upstream commit f35d12971b4d814cdb2f659d76b42f0c545270b6 ]

x25_lapb_receive_frame() invokes x25_get_neigh(), which returns a
reference of the specified x25_neigh object to "nb" with increased
refcnt.

When x25_lapb_receive_frame() returns, local variable "nb" becomes
invalid, so the refcount should be decreased to keep refcount balanced.

The reference counting issue happens in one path of
x25_lapb_receive_frame(). When pskb_may_pull() returns false, the
function forgets to decrease the refcnt increased by x25_get_neigh(),
causing a refcnt leak.

Fix this issue by calling x25_neigh_put() when pskb_may_pull() returns
false.

Fixes: cb101ed2c3c7 ("x25: Handle undersized/fragmented skbs")
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/x25/x25_dev.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/x25/x25_dev.c
+++ b/net/x25/x25_dev.c
@@ -115,8 +115,10 @@ int x25_lapb_receive_frame(struct sk_buf
 		goto drop;
 	}
 
-	if (!pskb_may_pull(skb, 1))
+	if (!pskb_may_pull(skb, 1)) {
+		x25_neigh_put(nb);
 		return 0;
+	}
 
 	switch (skb->data[0]) {