netdev - [PATCH -next] bridge: fix potential use-after-free when hook returns QUEUE or STOLEN verdict

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Date:	Sat, 12 Mar 2016 11:14:42 +0100
From:	Florian Westphal <fw@...len.de>
To:	<netdev@...r.kernel.org>
Cc:	stephen@...workplumber.org, Florian Westphal <fw@...len.de>,
	Felix Fietkau <nbd@...nwrt.org>,
	Zefir Kurtisi <zefir.kurtisi@...atec.com>
Subject: [PATCH -next] bridge: fix potential use-after-free when hook returns QUEUE or STOLEN verdict

Zefir Kurtisi reported kernel panic with an openwrt specific patch.
However, it turns out that mainline has a similar bug waiting to happen.

Once NF_HOOK() returns the skb is in undefined state and must not be
used.   Moreover, the okfn must consume the skb to support async
processing (NF_QUEUE).

Current okfn in this spot doesn't consume it and caller assumes that
NF_HOOK return value tells us if skb was freed or not, but thats wrong.

It "works" because no in-tree user registers a NFPROTO_BRIDGE hook at
LOCAL_IN that returns STOLEN or NF_QUEUE verdicts.

Once we add NF_QUEUE support for nftables bridge this will break --
NF_QUEUE holds the skb for async processing, caller will erronoulsy
return RX_HANDLER_PASS and on reinject netfilter will access free'd skb.

Fix this by pushing skb up the stack in the okfn instead.

NB: It also seems dubious to use LOCAL_IN while bypassing PRE_ROUTING
completely in this case but this is how its been forever so it seems
preferable to not change this.

Cc: Felix Fietkau <nbd@...nwrt.org>
Cc: Zefir Kurtisi <zefir.kurtisi@...atec.com>
Signed-off-by: Florian Westphal <fw@...len.de>
---

 net/bridge/br_input.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
index f7fba74..1607977 100644
--- a/net/bridge/br_input.c
+++ b/net/bridge/br_input.c
@@ -222,7 +222,10 @@ static int br_handle_local_finish(struct net *net, struct sock *sk, struct sk_bu
 	/* check if vlan is allowed, to avoid spoofing */
 	if (p->flags & BR_LEARNING && br_should_learn(p, skb, &vid))
 		br_fdb_update(p->br, p, eth_hdr(skb)->h_source, vid, false);
-	return 0;	 /* process further */
+
+	BR_INPUT_SKB_CB(skb)->brdev = p->br->dev;
+	br_pass_frame_up(skb);
+	return 0;
 }

 /*
@@ -284,14 +287,9 @@ rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
 		}

 		/* Deliver packet to local host only */
-		if (NF_HOOK(NFPROTO_BRIDGE, NF_BR_LOCAL_IN,
-			    dev_net(skb->dev), NULL, skb, skb->dev, NULL,
-			    br_handle_local_finish)) {
-			return RX_HANDLER_CONSUMED; /* consumed by filter */
-		} else {
-			*pskb = skb;
-			return RX_HANDLER_PASS;	/* continue processing */
-		}
+		NF_HOOK(NFPROTO_BRIDGE, NF_BR_LOCAL_IN, dev_net(skb->dev),
+			NULL, skb, skb->dev, NULL, br_handle_local_finish);
+		return RX_HANDLER_CONSUMED;
 	}

 forward:
-- 
2.4.10