linux-kernel - Re: [PATCH v2] tty: ttynull: implement console write

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Date:   Wed, 15 Feb 2023 15:33:41 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     Michael Thalmeier <michael.thalmeier@...e.at>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        john.ogness@...utronix.de
Subject: Re: [PATCH v2] tty: ttynull: implement console write

On Wed 2023-02-15 12:37:36, Vincent Whitchurch wrote:
> + Cc: John, Petr
> 
> On Tue, Feb 14, 2023 at 12:59:21PM +0100, Michael Thalmeier wrote:
> > Since commit a699449bb13b ("printk: refactor and rework printing logic"),
> > con->write is called without checking if it is implemented in the console
> > driver. This does make some sense, because for all "normal" consoles it
> > is vital to have a write function.
> > As the ttynull console driver does not need any console output the write
> > function was not implemented. This caused a "unable to handle kernel NULL
> > pointer dereference" when the write function is called now.
> > 
> > To fix this issue, implement an empty write function.
> > 
> > Fixes: a699449bb13b ("printk: refactor and rework printing logic")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Michael Thalmeier <michael.thalmeier@...e.at>
> 
> Looking at the referenced commit, the only place I see it calling
> con->write() is from call_console_driver(), which is in turn only called
> from console_emit_next_record().  And console_flush_all(), the only
> caller of console_emit_next_record(), checks that con->write is not NULL
> using console_is_usable() before calling console_emit_next_record().

I see the same. The refactoring moved the check of con->write from
call_console_driver() to console_is_usable(). It detects the NULL
pointer earlier in console_flush_all()...

> What am I missing?  Which code path in the referenced commit calls
> con->write without checking for NULL?

Vincent, could you please provide log with the backtrace?

I wonder if the problem is in the RT-patchset where
console_emit_next_record() is called also from the printk kthread.

That said, the current code is error-prone. The check for non-NULL
con->write is too far from the caller.

I would prefer to make it more safe. For example, I would prevent
registration of consoles without con->write callback in the first
place. It would require, to implement the empty write() callback
for ttynull console as done by this patch.

Anyway, I would like to understand if the "unable to handle kernel NULL
pointer dereference" is a real problem in the current implementation.

Best Regards,
Petr