CVE-2025-39724

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

serial: 8250: fix panic due to PSLVERR

When the PSLVERR_RESP_EN parameter is set to 1, the device generates

an error response if an attempt is made to read an empty RBR (Receive

Buffer Register) while the FIFO is enabled.

In serial8250_do_startup(), calling serial_port_out(port, UART_LCR,

UART_LCR_WLEN8) triggers dw8250_check_lcr(), which invokes

dw8250_force_idle() and serial8250_clear_and_reinit_fifos(). The latter

function enables the FIFO via serial_out(p, UART_FCR, p->fcr).

Execution proceeds to the serial_port_in(port, UART_RX).

This satisfies the PSLVERR trigger condition.

When another CPU (e.g., using printk()) is accessing the UART (UART

is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==

(lcr & ~UART_LCR_SPAR) in dw8250_check_lcr(), causing it to enter

dw8250_force_idle().

Put serial_port_out(port, UART_LCR, UART_LCR_WLEN8) under the port->lock

to fix this issue.

Panic backtrace:

[ 0.442336] Oops - unknown exception [#1]

[ 0.442343] epc : dw8250_serial_in32+0x1e/0x4a

[ 0.442351] ra : serial8250_do_startup+0x2c8/0x88e

...

[ 0.442416] console_on_rootfs+0x26/0x70