CVE-2025-21820

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

tty: xilinx_uartps: split sysrq handling

lockdep detects the following circular locking dependency:

CPU 0                      CPU 1
========================== ============================
cdns_uart_isr()            printk()
  uart_port_lock(port)       console_lock()
			     cdns_uart_console_write()
                               if (!port->sysrq)
                                 uart_port_lock(port)
  uart_handle_break()
    port->sysrq = ...
  uart_handle_sysrq_char()
    printk()
      console_lock()

The fixed commit attempts to avoid this situation by only taking the
port lock in cdns_uart_console_write if port->sysrq unset. However, if
(as shown above) cdns_uart_console_write runs before port->sysrq is set,
then it will try to take the port lock anyway. This may result in a
deadlock.

Fix this by splitting sysrq handling into two parts. We use the prepare
helper under the port lock and defer handling until we release the lock.