[PATCH AUTOSEL 5.14 041/252] usb: gadget: u_ether: fix a potential null pointer dereference

[Sasha Levin]

From: Maciej Żenczykowski <maze@xxxxxxxxxx>

[ Upstream commit 8ae01239609b29ec2eff55967c8e0fe3650cfa09 ]

f_ncm tx timeout can call us with null skb to flush
a pending frame.  In this case skb is NULL to begin
with but ceases to be null after dev->wrap() completes.

In such a case in->maxpacket will be read, even though
we've failed to check that 'in' is not NULL.

Though I've never observed this fail in practice,
however the 'flush operation' simply does not make sense with
a null usb IN endpoint - there's nowhere to flush to...
(note that we're the gadget/device, and IN is from the point
 of view of the host, so here IN actually means outbound...)

Cc: Brooke Basile <brookebasile@xxxxxxxxx>
Cc: "Bryan O'Donoghue" <bryan.odonoghue@xxxxxxxxxx>
Cc: Felipe Balbi <balbi@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Lorenzo Colitti <lorenzo@xxxxxxxxxx>
Signed-off-by: Maciej Żenczykowski <maze@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210701114834.884597-6-zenczykowski@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/usb/gadget/function/u_ether.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index d1d044d9f859..85a3f6d4b5af 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -492,8 +492,9 @@ static netdev_tx_t eth_start_xmit(struct sk_buff *skb,
 	}
 	spin_unlock_irqrestore(&dev->lock, flags);
 
-	if (skb && !in) {
-		dev_kfree_skb_any(skb);
+	if (!in) {
+		if (skb)
+			dev_kfree_skb_any(skb);
 		return NETDEV_TX_OK;
 	}
 
-- 
2.30.2