[PATCH v5 2/4] serial: 8250: avoid potential PSLVERR issue

Yunhui Cui
2025-05-06T11:23:37Z

Failure to check the UART_LSR_DR before reading UART_RX, or the
non-atomic nature of clearing the FIFO and reading UART_RX, poses
potential risks that could lead to PSLVERR.

Signed-off-by: Yunhui Cui <cuiyunhui@bytedance.com>
---
drivers/tty/serial/8250/8250.h | 13 +++++++++
drivers/tty/serial/8250/8250_port.c | 43 +++++++++++++++--------------
2 files changed, 35 insertions(+), 21 deletions(-)

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index b861585ca02a..6f97ff3a197d 100644
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -162,6 +162,19 @@ static inline u16 serial_lsr_in(struct uart_8250_port *up)
return lsr;
}

+/*
+ * To avoid PSLVERR, check UART_LSR_DR in UART_LSR before
+ * reading UART_RX.
+ */
+static inline void serial8250_discard_data(struct uart_8250_port *up)
+{
+ u16 lsr;
+
+ lsr = serial_in(up, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ serial_in(up, UART_RX);
+}
+
/*
* For the 16C950
*/
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index a913135d5217..1666b965f6a0 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1357,9 +1357,8 @@ static void autoconfig_irq(struct uart_8250_port *up)
/* Synchronize UART_IER access against the console. */
uart_port_lock_irq(port);
serial_out(up, UART_IER, UART_IER_ALL_INTR);
+ serial8250_discard_data(up);
uart_port_unlock_irq(port);
- serial_in(up, UART_LSR);
- serial_in(up, UART_RX);
serial_in(up, UART_IIR);
serial_in(up, UART_MSR);
serial_out(up, UART_TX, 0xFF);
@@ -2137,25 +2136,22 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits)
static int serial8250_get_poll_char(struct uart_port *port)
{
struct uart_8250_port *up = up_to_u8250p(port);
- int status;
+ int status = NO_POLL_CHAR;
u16 lsr;
+ unsigned long flags;

serial8250_rpm_get(up);

+ uart_port_lock_irqsave(port, &flags);
lsr = serial_port_in(port, UART_LSR);
+ if (lsr & UART_LSR_DR)
+ status = serial_port_in(port, UART_RX);
+ uart_port_unlock_irqrestore(port, flags);

- if (!(lsr & UART_LSR_DR)) {
- status = NO_POLL_CHAR;
- goto out;
- }
-
- status = serial_port_in(port, UART_RX);
-out:
serial8250_rpm_put(up);
return status;
}

-
static void serial8250_put_poll_char(struct uart_port *port,
unsigned char c)
{
@@ -2264,13 +2260,17 @@ int serial8250_do_startup(struct uart_port *port)
* Clear the FIFO buffers and disable them.
* (they will be reenabled in set_termios())
*/
+ uart_port_lock_irqsave(port, &flags);
serial8250_clear_fifos(up);

/*
- * Clear the interrupt registers.
+ * Read UART_RX to clear interrupts (e.g., Character Timeout).
+ * No data on UART_IIR_RX_TIMEOUT, UART_LSR_DR won't set.
+ * FIFO disabled, read UART_RX without LSR check, no PSLVERR.
*/
serial_port_in(port, UART_LSR);
serial_port_in(port, UART_RX);
+ uart_port_unlock_irqrestore(port, flags);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);

@@ -2429,15 +2429,14 @@ int serial8250_do_startup(struct uart_port *port)
}

dont_test_tx_en:
- uart_port_unlock_irqrestore(port, flags);

/*
* Clear the interrupt registers again for luck, and clear the
* saved flags to avoid getting false values from polling
* routines or the previous session.
*/
- serial_port_in(port, UART_LSR);
- serial_port_in(port, UART_RX);
+ serial8250_discard_data(up);
+ uart_port_unlock_irqrestore(port, flags);
serial_port_in(port, UART_IIR);
serial_port_in(port, UART_MSR);
up->lsr_saved_flags = 0;
@@ -2519,7 +2518,6 @@ void serial8250_do_shutdown(struct uart_port *port)
port->mctrl &= ~TIOCM_OUT2;

serial8250_set_mctrl(port, port->mctrl);
- uart_port_unlock_irqrestore(port, flags);

/*
* Disable break condition and FIFOs
@@ -2527,6 +2525,14 @@ void serial8250_do_shutdown(struct uart_port *port)
serial_port_out(port, UART_LCR,
serial_port_in(port, UART_LCR) & ~UART_LCR_SBC);
serial8250_clear_fifos(up);
+ /*
+ * Read data port to reset things, and then unlink from
+ * the IRQ chain.
+ * Since reading UART_RX clears interrupts, doing so with
+ * FIFO disabled won't trigger PSLVERR.
+ */
+ serial_port_in(port, UART_RX);
+ uart_port_unlock_irqrestore(port, flags);

#ifdef CONFIG_SERIAL_8250_RSA
/*
@@ -2535,11 +2541,6 @@ void serial8250_do_shutdown(struct uart_port *port)
disable_rsa(up);
#endif

- /*
- * Read data port to reset things, and then unlink from
- * the IRQ chain.
- */
- serial_port_in(port, UART_RX);
serial8250_rpm_put(up);

up->ops->release_irq(up);
--
2.39.2