[PATCH 3.14 107/140] net: cdc_ncm: fix buffer overflow

[Greg Kroah-Hartman]

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: =?UTF-8?q?Bj=C3=B8rn=20Mork?= <bjorn@xxxxxxx>

[ Upstream commit 9becd707841207652449a8dfd90fe9c476d88546 ]

Commit 4d619f625a60 ("net: cdc_ncm: no point in filling up the NTBs
if we send ZLPs") changed the padding logic for devices with the ZLP
flag set.  This meant that frames of any size will be sent without
additional padding, except for the single byte added if the size is
a multiple of the USB packet size. But if the unpadded size is
identical to the maximum frame size, and the maximum size is a
multiplum of the USB packet size, then this one-byte padding will
overflow the buffer.

Prevent padding if already at maximum frame size, letting usbnet
transmit a ZLP instead in this case.

Fixes: 4d619f625a60 ("net: cdc_ncm: no point in filling up the NTBs if we send ZLPs")
Reported by: Yu-an Shih <yshih@xxxxxxxxxx>
Signed-off-by: Bjøork <bjorn@xxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/net/usb/cdc_ncm.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -768,7 +768,7 @@ cdc_ncm_fill_tx_frame(struct usbnet *dev
 	    skb_out->len > CDC_NCM_MIN_TX_PKT)
 		memset(skb_put(skb_out, ctx->tx_max - skb_out->len), 0,
 		       ctx->tx_max - skb_out->len);
-	else if ((skb_out->len % dev->maxpacket) == 0)
+	else if (skb_out->len < ctx->tx_max && (skb_out->len % dev->maxpacket) == 0)
 		*skb_put(skb_out, 1) = 0;	/* force short packet */
 
 	/* set final frame length */


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