Subject: [PATCH 1/1: 5.10/5.15] net: bridge: xmit: make sure we have at least eth header len bytes

From: <[email-masked]>
Date: Fri, 1 Nov 2024 11:07:45 -0400

From: Randy MacLeod <[email-masked]>

[ Upstream commit 8bd67ebb50c0145fd2ca8681ab65eb7e8cde1afc ]

Based on above commit but simplified since pskb_may_pull_reason()
does not exist until 6.1.

syzbot triggered an uninit value[1] error in bridge device's xmit path
by sending a short (less than ETH_HLEN bytes) skb. To fix it check if
we can actually pull that amount instead of assuming.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: [email-masked]
Closes: https://syzkaller.appspot.com/bug?extid=a63a1f6a062033cf0f40
Signed-off-by: Randy MacLeod <[email-masked]>
---
 net/bridge/br_device.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index d3ea9d0779fb..84e37108c6b5 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -36,6 +36,11 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
 	const unsigned char *dest;
 	u16 vid = 0;
 
+	if (unlikely(!pskb_may_pull(skb, ETH_HLEN))) {
+		kfree_skb(skb);
+		return NETDEV_TX_OK;
+	}
+
 	memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
 
 	rcu_read_lock();
-- 
2.34.1