linux-kernel - Re: UART/TTY console deadlock

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

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200702082008.GE3703480@smile.fi.intel.com>
Date:   Thu, 2 Jul 2020 11:20:08 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tony Lindgren <tony@...mide.com>, Petr Mladek <pmladek@...e.com>,
        Raul Rangel <rrangel@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kurt@...utronix.de, "S, Shirish" <Shirish.S@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        John Ogness <john.ogness@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: UART/TTY console deadlock

On Thu, Jul 02, 2020 at 02:40:33PM +0900, Sergey Senozhatsky wrote:
> On (20/07/02 14:12), Sergey Senozhatsky wrote:
> > From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > On (20/06/30 11:02), Tony Lindgren wrote:

...

> > I think we can do it. serial8250_do_startup() and irq handler take
> > port->lock, so they should be synchronized.
> 
> Hmm, hold on. Why does it disable IRQ in the first place? IRQ handlers
> should grab the port->lock. So if there is already running IRQ, then
> serial8250_do_startup() will wait until IRQ handler unlocks the port->lock.
> If serial8250_do_startup() grabs the port->lock first, then IRQ will wait
> for serial8250_do_startup() to unlock it. serial8250_do_startup() does
> not release the port->unlock until its done:
> 
> 		spin_lock_irqsave(&port->lock, flags);
> 
> 		wait_for_xmitr(up, UART_LSR_THRE);
> 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> 		udelay(1); /* allow THRE to set */
> 		iir1 = serial_port_in(port, UART_IIR);
> 		serial_port_out(port, UART_IER, 0);
> 		serial_port_out_sync(port, UART_IER, UART_IER_THRI);
> 		udelay(1); /* allow a working UART time to re-assert THRE */
> 		iir = serial_port_in(port, UART_IIR);
> 		serial_port_out(port, UART_IER, 0);
> 
> 		spin_unlock_irqrestore(&port->lock, flags);
> 
> so IRQ will not see the inconsistent device state.
> 
> What exactly is the purpose of disable_irq_nosync()? Can we just remove
> disable_irq_nosync()/enable_irq() instead? Are there any IRQ handlers
> that don't acquire the port->lock?

I didn't look into this deeply, but my understanding that this is something for
special case when you have several UART ports sharing the IRQ (multi-port card)
and IRQ even maybe undesirable b/c it will confuse real IRQ handler. I don't
remember details, but AFAIR IRQ handler does a busyloop to service as much as
possible and in between it may release the lock (again, multi-port UART cards),
that's why we better avoid IRQ event in the first place.
But it's my pure speculation.

-- 
With Best Regards,
Andy Shevchenko