net: ks8851: Reinstate disabling of BHs around IRQ handler

In the Linux kernel, the following vulnerability has been resolved:

net: ks8851: Reinstate disabling of BHs around IRQ handler

If the driver executes ks8851_irq() AND a TX packet has been sent, then

the driver enables TX queue via netif_wake_queue() which schedules TX

softirq to queue packets for this device.

If CONFIG_PREEMPT_RT=y is set AND a packet has also been received by

the MAC, then ks8851_rx_pkts() calls netdev_alloc_skb_ip_align() to

allocate SKBs for the received packets. If netdev_alloc_skb_ip_align()

is called with BH enabled, then local_bh_enable() at the end of

netdev_alloc_skb_ip_align() will trigger the pending softirq processing,

which may ultimately call the .xmit callback ks8851_start_xmit_par().

The ks8851_start_xmit_par() will try to lock struct ks8851_net_par

.lock spinlock, which is already locked by ks8851_irq() from which

ks8851_start_xmit_par() was called. This leads to a deadlock, which

is reported by the kernel, including a trace listed below.

If CONFIG_PREEMPT_RT is not set, then since commit 0913ec336a6c0

("net: ks8851: Fix deadlock with the SPI chip variant") the deadlock

can also be triggered without received packet in the RX FIFO. The

pending softirqs will be processed on return from

spin_unlock_bh(&ks->statelock) in ks8851_irq(), which triggers the

deadlock as well.

Fix the problem by disabling BH around critical sections, including the

IRQ handler, thus preventing the net_tx_action() softirq from triggering

during these critical sections. The net_tx_action() softirq is triggered

once BH are re-enabled and at the end of the IRQ handler, once all the

other IRQ handler actions have been completed.

__schedule from schedule_rtlock+0x1c/0x34

schedule_rtlock from rtlock_slowlock_locked+0x548/0x904

rtlock_slowlock_locked from rt_spin_lock+0x60/0x9c

rt_spin_lock from ks8851_start_xmit_par+0x74/0x1a8

ks8851_start_xmit_par from netdev_start_xmit+0x20/0x44

netdev_start_xmit from dev_hard_start_xmit+0xd0/0x188

dev_hard_start_xmit from sch_direct_xmit+0xb8/0x25c

sch_direct_xmit from __qdisc_run+0x1f8/0x4ec

__qdisc_run from qdisc_run+0x1c/0x28

qdisc_run from net_tx_action+0x1f0/0x268

net_tx_action from handle_softirqs+0x1a4/0x270

handle_softirqs from __local_bh_enable_ip+0xcc/0xe0

__local_bh_enable_ip from __alloc_skb+0xd8/0x128

__alloc_skb from __netdev_alloc_skb+0x3c/0x19c

__netdev_alloc_skb from ks8851_irq+0x388/0x4d4

ks8851_irq from irq_thread_fn+0x24/0x64

irq_thread_fn from irq_thread+0x178/0x28c

irq_thread from kthread+0x12c/0x138

kthread from ret_from_fork+0x14/0x28