5.5 - 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.