linux-kernel - Re: Re: [PATCH] tty: vt: Add checks after calling kzalloc

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Date:   Wed, 31 Aug 2022 11:32:43 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jiasheng Jiang <jiasheng@...as.ac.cn>
Cc:     jirislaby@...nel.org, ilpo.jarvinen@...ux.intel.com,
        johan@...nel.org, penguin-kernel@...ove.sakura.ne.jp,
        zhangxuezhi1@...lpad.com, xyangxi5@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] tty: vt: Add checks after calling kzalloc

On Wed, Aug 31, 2022 at 05:08:10PM +0800, Jiasheng Jiang wrote:
> On Wed, Aug 31, 2022 at 03:57:42PM +0800, Jiasheng Jiang wrote:
> >>  	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
> >>  		vc_cons[currcons].d = vc = kzalloc(sizeof(struct vc_data), GFP_NOWAIT);
> >> +		if (!vc) {
> >> +			console_unlock();
> >> +			return -ENOMEM;
> >> +		}
> >>  		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> >>  		tty_port_init(&vc->port);
> >>  		visual_init(vc, currcons, 1);
> >>  		/* Assuming vc->vc_{cols,rows,screenbuf_size} are sane here. */
> >>  		vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
> >> +		if (!vc->vc_screenbuf) {
> >> +			console_unlock();
> >> +			return -ENOMEM;
> >> +		}
> > 
> > This has been attempted many times in the past, sorry.  Unless you can
> > prove that this can actually happen in real life, we are going to leave
> > these as-is.
> > 
> > Please do not just do random changes like this without actually testing
> > to see if it is possible to happen.
> 
> As the harm of vulnerabilities is much higher than the cost of fixing them,
> it is acceptable to add harmless security checks that guarantee the
> vulnerabilities will never be triggered.

No, not always, many times you are adding new bugs by doing this type of
"unneeded fixes".  We have had this happen in the vt code many times in
the past, let's learn from our mistakes please.

And where is the "vulnerability" here exactly?

thanks,

greg k-h