| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <6801686d-a0b3-7093-1293-cdac6ad055e4@linux.intel.com>
Date: Tue, 6 May 2025 15:00:25 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Yunhui Cui <cuiyunhui@...edance.com>
cc: arnd@...db.de, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
benjamin.larsson@...exis.eu,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
heikki.krogerus@...ux.intel.com, Jiri Slaby <jirislaby@...nel.org>,
jkeeping@...usicbrands.com, john.ogness@...utronix.de,
LKML <linux-kernel@...r.kernel.org>,
linux-serial <linux-serial@...r.kernel.org>, markus.mayer@...aro.org,
matt.porter@...aro.org, namcao@...utronix.de, paulmck@...nel.org,
pmladek@...e.com, schnelle@...ux.ibm.com, sunilvl@...tanamicro.com,
tim.kryger@...aro.org
Subject: Re: [PATCH v5 2/4] serial: 8250: avoid potential PSLVERR issue
On Tue, 6 May 2025, Yunhui Cui wrote:
> 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.
Don't expect reader to know the condition how PSLVERR is triggered. I know
it's worded out in the other patch but also explain it here.
You're only explaining problem and missing what this patch does to solve
the problem.
> Signed-off-by: Yunhui Cui <cuiyunhui@...edance.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;
Not a problem that originates from you, but IMO calling this variable
"status" is quite misleading when it is the character (or NO_POLL_CHAR
is no character is present).
> }
>
> -
> 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.
I don't understand what the last two lines mean and I don't see the
connection to the code that is below the comment either, could you try to
rephrase the comment.
> */
> 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);
>
--
i.
Powered by blists - more mailing lists