tty: xilinx_uartps: split sysrq handling

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/e22a97700901ba5e8bf8db68056a0d50f9440cae
• https://git.kernel.org/stable/c/de5bd24197bd9ee37ec1e379a3d882bbd15c5065
• https://git.kernel.org/stable/c/8ea0e7b3d7b8f2f0fc9db491ff22a0abe120801c
• https://git.kernel.org/stable/c/9b88a7c4584ba67267a051069b8abe44fc9595b2
• https://git.kernel.org/stable/c/4410dba9807a17a93f649a9f5870ceaf30a675a3
• https://git.kernel.org/stable/c/b06f388994500297bb91be60ffaf6825ecfd2afe