[PATCH net v3 1/2] net: usb: cdc_ncm: don't enable BHs in tx_fixup under netpoll

[Nicolai Buchwitz <nb@tipi-net.de>]

netpoll holds the txq lock and calls ndo_start_xmit() with IRQs off but
BHs on. The spin_unlock_bh() in cdc_ncm_tx_fixup() then runs pending
softirqs. If one of them transmits, it spins on the txq lock that
netpoll holds. With netconsole on a cdc_ncm device this ends in a soft
or hard lockup.

Every caller of cdc_ncm_tx_fixup() holds the txq lock with BHs or IRQs
off, so the softirq user of ctx->mtx cannot interrupt it. Use plain
spin_lock().

Fixes: c84ff1d6dff3 ("cdc_ncm: reduce driver latency in the data path")
Reported-by: Mikkel Eskildsen <mke@visotia.com>
Closes: https://lore.kernel.org/netdev/PRAP189MB185019A5A8D411482BB6E0CDA9982@PRAP189MB1850.EURP189.PROD.OUTLOOK.COM/
Signed-off-by: Nicolai Buchwitz <nb@tipi-net.de>
---
 drivers/net/usb/cdc_ncm.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index 35db38cb3e4a2..3bd7cb66f3fc3 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -1535,14 +1535,17 @@ cdc_ncm_tx_fixup(struct usbnet *dev, struct sk_buff *skb, gfp_t flags)
 	if (ctx == NULL)
 		goto error;
 
-	spin_lock_bh(&ctx->mtx);
+	/* BHs or IRQs are always off here. netpoll only disables IRQs, and
+	 * spin_unlock_bh() would then run softirqs with the txq lock held.
+	 */
+	spin_lock(&ctx->mtx);
 
 	if (ctx->is_ndp16)
 		skb_out = cdc_ncm_fill_tx_frame(dev, skb, cpu_to_le32(USB_CDC_NCM_NDP16_NOCRC_SIGN));
 	else
 		skb_out = cdc_ncm_fill_tx_frame(dev, skb, cpu_to_le32(USB_CDC_NCM_NDP32_NOCRC_SIGN));
 
-	spin_unlock_bh(&ctx->mtx);
+	spin_unlock(&ctx->mtx);
 	return skb_out;
 
 error:

-- 
2.53.0