linux-kernel - Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

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Message-ID: <20190522081029.GB5109@zhanggen-UX430UQ>
Date:   Wed, 22 May 2019 16:10:29 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

On Tue, May 21, 2019 at 10:43:11PM -0400, Nicolas Pitre wrote:
> On Tue, 21 May 2019, Gen Zhang wrote:
> 
> > On Tue, May 21, 2019 at 12:30:38AM -0400, Nicolas Pitre wrote:
> > > Now imagine that MIN_NR_CONSOLES is defined to 10 instead of 1.
> > > 
> > > What happens with allocated memory if the err_vc condition is met on the 
> > > 5th loop?
> > Yes, vc->vc_screenbuf from the last loop is still not freed, right? I
> > don't have idea to solve this one. Could please give some advice? Since
> > we have to consider the err_vc condition.
> > 
> > > If err_vc_screenbuf condition is encountered on the 5th loop (curcons = 
> > > 4), what is the value of vc_cons[4].d? Isn't it the same as vc that you 
> > > just freed?
> > > 
> > > 
> > > Nicolas
> > Thanks for your explaination! You mean a double free situation may 
> > happen, right? But in vc_allocate() there is also such a kfree(vc) and 
> > vc_cons[currcons].d = NULL operation. This situation is really confusing
> > me.
> 
> What you could do is something that looks like:
> 
> 	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
> 		vc_cons[currcons].d = vc = kzalloc(...);
> 		if (!vc)
> 			goto fail1;
> 		...
> 		vc->vc_screenbuf = kzalloc(...);
> 		if (!vc->vc_screenbuf)
> 			goto fail2;
> 		...
> 
> 	return 0;
> 
> 	/* free already allocated memory on error */
> fail1:
> 	while (curcons > 0) {
> 		curcons--;
> 		kfree(vc_cons[currcons].d->vc_screenbuf);
> fail2:
> 		kfree(vc_cons[currcons].d);
> 		vc_cons[currcons].d = NULL;
> 	}
> 	console_unlock();
> 	return -ENOMEM;
> 
> 
> Nicolas
Thanks for your patient explaination, Nicolas!
I will work on it and resubmit it.
Thanks
Gen