From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C42C46A019; Fri, 12 Jan 2024 13:28:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="B88b5jAL"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="aJ+PHM6K" From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1705066134; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=5PpAnkyBikMPd+fkvnJS1FP8hHu1DaIJO182JY2898Q=; b=B88b5jALuLKzn77McNRQ8kARoRQQ1O+F5/G484snrPAXu2BwC9NiFygIlT80t57Lfb6kuB pB9WL9x7GFEba4aSiW8wxTUjWVc1L7kRn02efCFDMpBndpSxNaRN4xjRzo//Qvak4sSySL Ac9nOdY05N6loI9Zpu6zDtNXjnslPg+tN4W7D0jgZVS/gmvsAVZiKqluCq9enaCCXs3Ymd 0Cxg2Ioc/XyXLJ73qIiEwAON4R6csC+CMhdPLWel6eWdaSMVbrBVdi2QEo1oaIwwxjirsN 9MkkKkwZIfdsf0EnRnyKX6S+vTQ4w2fXkpQ3btUsZiPmWlu418auXh9Pdu+/Bw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1705066134; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=5PpAnkyBikMPd+fkvnJS1FP8hHu1DaIJO182JY2898Q=; b=aJ+PHM6Kk/LrQscByIkEzUBDPXW1pP/K1fg8c11IA7qfem/vbsshb0nwmgT32bitxTrDM5 k2ezn9+xAJu7D1BQ== To: Gui-Dong Han <2045gemini@gmail.com>, gregkh@linuxfoundation.org, jirislaby@kernel.org, ilpo.jarvinen@linux.intel.com, tony@atomide.com, l.sanfilippo@kunbus.com, tglx@linutronix.de, andriy.shevchenko@linux.intel.com Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, baijiaju1990@outlook.com, Gui-Dong Han <2045gemini@gmail.com>, stable@vger.kernel.org Subject: Re: [PATCH] serial: core: Fix double fetch in uart_throttle/uart_unthrottle In-Reply-To: <20240112121844.17580-1-2045gemini@gmail.com> References: <20240112121844.17580-1-2045gemini@gmail.com> Date: Fri, 12 Jan 2024 14:34:52 +0106 Message-ID: <87h6ji7ka3.fsf@jogness.linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain On 2024-01-12, Gui-Dong Han <2045gemini@gmail.com> wrote: > In uart_throttle() and uart_unthrottle(): > if (port->status & mask) { > port->ops->throttle/unthrottle(port); > mask &= ~port->status; > } > // Code segment utilizing the mask value to determine UART behavior > > In uart_change_line_settings(): > uart_port_lock_irq(uport); > // Code segment responsible for updating uport->status > uart_port_unlock_irq(uport); > > In the uart_throttle() and uart_unthrottle() functions, there is a double > fetch issue due to concurrent execution with uart_change_line_settings(). > In uart_throttle() and uart_unthrottle(), the check > if (port->status & mask) is made, followed by mask &= ~port->status, > where the relevant bits are cleared. However, port->status may be modified > in uart_change_line_settings(). The current implementation does not ensure > atomicity in the access and modification of port->status and mask. This > can result in mask being updated based on a modified port->status value, > leading to improper UART actions. > > This possible bug is found by an experimental static analysis tool > developed by our team, BassCheck[1]. This tool analyzes the locking APIs > to extract function pairs that can be concurrently executed, and then > analyzes the instructions in the paired functions to identify possible > concurrency bugs including data races and atomicity violations. The above > possible bug is reported when our tool analyzes the source code of > Linux 5.17. > > To resolve this double fetch, it is suggested to add a uart_port_lock pair > in uart_throttle() and uart_unthrottle(). With this patch applied, our > tool no longer reports the bug, with the kernel configuration allyesconfig > for x86_64. Due to the absence of the requisite hardware, we are unable to > conduct runtime testing of the patch. Therefore, our verification is > solely based on code logic analysis. > > [1] https://sites.google.com/view/basscheck/ > > Fixes: 391f93f2ec9f ("serial: core: Rework hw-assisted flow control support") > Cc: stable@vger.kernel.org > Signed-off-by: Gui-Dong Han <2045gemini@gmail.com> > --- > drivers/tty/serial/serial_core.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c > index 80085b151b34..9d905fdf2843 100644 > --- a/drivers/tty/serial/serial_core.c > +++ b/drivers/tty/serial/serial_core.c > @@ -723,11 +723,13 @@ static void uart_throttle(struct tty_struct *tty) > mask |= UPSTAT_AUTOXOFF; > if (C_CRTSCTS(tty)) > mask |= UPSTAT_AUTORTS; > - > + > + uart_port_lock_irq(port); > if (port->status & mask) { > port->ops->throttle(port); > mask &= ~port->status; > } > + uart_port_unlock_irq(port); You would also need to remove uart_port_lock_irq() out of all the throttle() callbacks. > > if (mask & UPSTAT_AUTORTS) > uart_clear_mctrl(port, TIOCM_RTS); > @@ -753,10 +755,12 @@ static void uart_unthrottle(struct tty_struct *tty) > if (C_CRTSCTS(tty)) > mask |= UPSTAT_AUTORTS; > > + uart_port_lock_irq(port); > if (port->status & mask) { > port->ops->unthrottle(port); > mask &= ~port->status; > } > + uart_port_unlock_irq(port); You would also need to remove uart_port_lock_irq() out of all the unthrottle() callbacks. John Ogness