nbd: Fix hung when signal interrupts nbd_start_device_ioctl()

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

nbd: Fix hung when signal interrupts nbd_start_device_ioctl()

syzbot reported hung task [1]. The following program is a simplified

version of the reproducer:

int main(void)

{

int sv[2], fd;

if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0)

return 1;

if ((fd = open("/dev/nbd0", 0)) < 0)

return 1;

if (ioctl(fd, NBD_SET_SIZE_BLOCKS, 0x81) < 0)

return 1;

if (ioctl(fd, NBD_SET_SOCK, sv[0]) < 0)

return 1;

if (ioctl(fd, NBD_DO_IT) < 0)

return 1;

return 0;

}

When signal interrupt nbd_start_device_ioctl() waiting the condition

atomic_read(&config->recv_threads) == 0, the task can hung because it

waits the completion of the inflight IOs.

This patch fixes the issue by clearing queue, not just shutdown, when

signal interrupt nbd_start_device_ioctl().