linux-kernel - Re: [PATCH] kdb: prevent possible null deref in kdb_msg

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Message-ID: <20200629145350.GC6173@alley>
Date:   Mon, 29 Jun 2020 16:53:51 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Cengiz Can <cengiz@...nel.wtf>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Sumit Garg <sumit.garg@...aro.org>
Cc:     Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH] kdb: prevent possible null deref in kdb_msg_write

On Mon 2020-06-29 16:50:20, Petr Mladek wrote:
> On Mon 2020-06-29 16:59:24, Cengiz Can wrote:
> > `kdb_msg_write` operates on a global `struct kgdb_io *` called
> > `dbg_io_ops`.
> > 
> > Although it is initialized in `debug_core.c`, there's a null check in
> > `kdb_msg_write` which implies that it can be null whenever we dereference
> > it in this function call.
> > 
> > Coverity scanner caught this as CID 1465042.
> > 
> > I have modified the function to bail out if `dbg_io_ops` is not properly
> > initialized.
> > 
> > Signed-off-by: Cengiz Can <cengiz@...nel.wtf>
> > ---
> >  kernel/debug/kdb/kdb_io.c | 15 ++++++++-------
> >  1 file changed, 8 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > index 683a799618ad..85e579812458 100644
> > --- a/kernel/debug/kdb/kdb_io.c
> > +++ b/kernel/debug/kdb/kdb_io.c
> > @@ -549,14 +549,15 @@ static void kdb_msg_write(const char *msg, int msg_len)
> >  	if (msg_len == 0)
> >  		return;
> >  
> > -	if (dbg_io_ops) {
> > -		const char *cp = msg;
> > -		int len = msg_len;
> > +	if (!dbg_io_ops)
> > +		return;
> 
> This looks wrong. The message should be printed to the consoles
> even when dbg_io_ops is NULL. I mean that the for_each_console(c)
> cycle should always get called.

Please, forget this mail. Daniel explained that dbg_io_ops must have
been set when this function gets called.

I am sorry for the noise.

Best Regards,
Petr