[PATCH] serial: imx: fix a race condition in receive path

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

The main irq handler function starts by first masking disabled
interrupts in the status register values to ensure to only handle
enabled interrupts. This is important as when the RX path in the
hardware is disabled reading the RX fifo results in an external abort.

This checking must be done under the port lock, otherwise the following
can happen:

     CPU1                            | CPU2
                                     |
     irq triggers as there are chars |
     in the RX fifo                  |
				     | grab port lock
     imx_uart_int finds RRDY enabled |
     and calls imx_uart_rxint which  |
     has to wait for port lock       |
                                     | disable RX (e.g. because we're
                                     | using RS485 with !RX_DURING_TX)
                                     |
                                     | release port lock
     read from RX fifo with RX       |
     disabled => exception           |

So take the port lock only once in imx_uart_int() instead of in the
functions called from there.

Reported-by: Andre Renaud <arenaud@designa-electronics.com>
Cc: stable@vger.kernel.org
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
Hello,

this problem type was addressed already in commits

	437768962f75 ("serial: imx: Only handle irqs that are actually enabled")
	76821e222c18 ("serial: imx: ensure that RX irqs are off if RX is off")

that entered 4.17-rc1. Backporting to older versions would require to
backport these two, too. I didn't try that, but I think this gets messy,
so I'd recommend to only backport to 4.19.x and 5.4.x (and 5.5.x
assuming this patch won't make it into 5.5).

Andre Renaud tested this patch and confirmed it to fix the problem, he
didn't provide a Tested-by tag, so I didn't add that here.

Best regards
Uwe

 drivers/tty/serial/imx.c | 52 ++++++++++++++++++++++++++++++----------
 1 file changed, 39 insertions(+), 13 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index a9e20e6c63ad..679b2de27c4d 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -700,22 +700,33 @@ static void imx_uart_start_tx(struct uart_port *port)
 	}
 }
 
-static irqreturn_t imx_uart_rtsint(int irq, void *dev_id)
+static irqreturn_t __imx_uart_rtsint(int irq, void *dev_id)
 {
 	struct imx_port *sport = dev_id;
 	u32 usr1;
 
-	spin_lock(&sport->port.lock);
-
 	imx_uart_writel(sport, USR1_RTSD, USR1);
 	usr1 = imx_uart_readl(sport, USR1) & USR1_RTSS;
 	uart_handle_cts_change(&sport->port, !!usr1);
 	wake_up_interruptible(&sport->port.state->port.delta_msr_wait);
 
-	spin_unlock(&sport->port.lock);
 	return IRQ_HANDLED;
 }
 
+static irqreturn_t imx_uart_rtsint(int irq, void *dev_id)
+{
+	struct imx_port *sport = dev_id;
+	irqreturn_t ret;
+
+	spin_lock(&sport->port.lock);
+
+	ret = __imx_uart_rtsint(irq, dev_id);
+
+	spin_unlock(&sport->port.lock);
+
+	return ret;
+}
+
 static irqreturn_t imx_uart_txint(int irq, void *dev_id)
 {
 	struct imx_port *sport = dev_id;
@@ -726,14 +737,12 @@ static irqreturn_t imx_uart_txint(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
-static irqreturn_t imx_uart_rxint(int irq, void *dev_id)
+static irqreturn_t __imx_uart_rxint(int irq, void *dev_id)
 {
 	struct imx_port *sport = dev_id;
 	unsigned int rx, flg, ignored = 0;
 	struct tty_port *port = &sport->port.state->port;
 
-	spin_lock(&sport->port.lock);
-
 	while (imx_uart_readl(sport, USR2) & USR2_RDR) {
 		u32 usr2;
 
@@ -792,11 +801,26 @@ static irqreturn_t imx_uart_rxint(int irq, void *dev_id)
 	}
 
 out:
-	spin_unlock(&sport->port.lock);
 	tty_flip_buffer_push(port);
+
 	return IRQ_HANDLED;
 }
 
+static irqreturn_t imx_uart_rxint(int irq, void *dev_id)
+{
+	struct imx_port *sport = dev_id;
+	struct tty_port *port = &sport->port.state->port;
+	irqreturn_t ret;
+
+	spin_lock(&sport->port.lock);
+
+	ret = __imx_uart_rxint(irq, dev_id);
+
+	spin_unlock(&sport->port.lock);
+
+	return ret;
+}
+
 static void imx_uart_clear_rx_errors(struct imx_port *sport);
 
 /*
@@ -855,6 +879,8 @@ static irqreturn_t imx_uart_int(int irq, void *dev_id)
 	unsigned int usr1, usr2, ucr1, ucr2, ucr3, ucr4;
 	irqreturn_t ret = IRQ_NONE;
 
+	spin_lock(&sport->port.lock);
+
 	usr1 = imx_uart_readl(sport, USR1);
 	usr2 = imx_uart_readl(sport, USR2);
 	ucr1 = imx_uart_readl(sport, UCR1);
@@ -888,27 +914,25 @@ static irqreturn_t imx_uart_int(int irq, void *dev_id)
 		usr2 &= ~USR2_ORE;
 
 	if (usr1 & (USR1_RRDY | USR1_AGTIM)) {
-		imx_uart_rxint(irq, dev_id);
+		__imx_uart_rxint(irq, dev_id);
 		ret = IRQ_HANDLED;
 	}
 
 	if ((usr1 & USR1_TRDY) || (usr2 & USR2_TXDC)) {
-		imx_uart_txint(irq, dev_id);
+		imx_uart_transmit_buffer(sport);
 		ret = IRQ_HANDLED;
 	}
 
 	if (usr1 & USR1_DTRD) {
 		imx_uart_writel(sport, USR1_DTRD, USR1);
 
-		spin_lock(&sport->port.lock);
 		imx_uart_mctrl_check(sport);
-		spin_unlock(&sport->port.lock);
 
 		ret = IRQ_HANDLED;
 	}
 
 	if (usr1 & USR1_RTSD) {
-		imx_uart_rtsint(irq, dev_id);
+		__imx_uart_rtsint(irq, dev_id);
 		ret = IRQ_HANDLED;
 	}
 
@@ -923,6 +947,8 @@ static irqreturn_t imx_uart_int(int irq, void *dev_id)
 		ret = IRQ_HANDLED;
 	}
 
+	spin_unlock(&sport->port.lock);
+
 	return ret;
 }
 
-- 
2.24.0