CVE-2021-47185

In the Linux kernel, the following vulnerability has been resolved: tty:
tty_buffer: Fix the softlockup issue in flush_to_ldisc When running ltp
testcase(ltp/testcases/kernel/pty/pty04.c) with arm64, there is a soft
lockup, which look like this one: Workqueue: events_unbound flush_to_ldisc
Call trace: dump_backtrace+0x0/0x1ec show_stack+0x24/0x30
dump_stack+0xd0/0x128 panic+0x15c/0x374 watchdog_timer_fn+0x2b8/0x304
__run_hrtimer+0x88/0x2c0 __hrtimer_run_queues+0xa4/0x120
hrtimer_interrupt+0xfc/0x270 arch_timer_handler_phys+0x40/0x50
handle_percpu_devid_irq+0x94/0x220 __handle_domain_irq+0x88/0xf0
gic_handle_irq+0x84/0xfc el1_irq+0xc8/0x180 slip_unesc+0x80/0x214 [slip]
tty_ldisc_receive_buf+0x64/0x80 tty_port_default_receive_buf+0x50/0x90
flush_to_ldisc+0xbc/0x110 process_one_work+0x1d4/0x4b0
worker_thread+0x180/0x430 kthread+0x11c/0x120 In the testcase pty04, The
first process call the write syscall to send data to the pty master. At the
same time, the workqueue will do the flush_to_ldisc to pop data in a loop
until there is no more data left. When the sender and workqueue running in
different core, the sender sends data fastly in full time which will result
in workqueue doing work in loop for a long time and occuring softlockup in
flush_to_ldisc with kernel configured without preempt. So I add
need_resched check and cond_resched in the flush_to_ldisc loop to avoid it.