linux-kernel - Re: [PATCH v3] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

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Message-ID: <20190521064605.ho7tdwaagt4wow4v@butterfly.localdomain>
Date:   Tue, 21 May 2019 08:46:07 +0200
From:   Oleksandr Natalenko <oleksandr@...hat.com>
To:     Gen Zhang <blackgod016574@...il.com>
Cc:     Nicolas Pitre <nico@...xnic.net>, linux-kernel@...r.kernel.org,
        Grzegorz Halat <ghalat@...hat.com>
Subject: Re: [PATCH v3] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

Cc'ing Grzegorz.

On Tue, May 21, 2019 at 11:21:18AM +0800, Gen Zhang wrote:
> On Mon, May 20, 2019 at 10:55:40PM -0400, Nicolas Pitre wrote:
> > As soon as you release the lock, another thread could come along and 
> > start using the memory pointed by vc_cons[currcons].d you're about to 
> > free here. This is unlikely for an initcall, but still.
> > 
> > You should consider this ordering instead:
> > 
> > err_vc_screenbuf:
> > 	kfree(vc);
> > 	vc_cons[currcons].d = NULL;
> > err_vc:
> > 	console_unlock();
> > 	return -ENOMEM;
> In function con_init(), the pointer variable vc_cons[currcons].d, vc and
> vc->vc_screenbuf is allocated a memory space via kzalloc(). And they are
> used in the following codes.
> However, when there is a memory allocation error, kzalloc() can fail.
> Thus null pointer (vc_cons[currcons].d, vc and vc->vc_screenbuf)
> dereference may happen. And it will cause the kernel to crash. Therefore,
> we should check return value and handle the error.
> Further,the loop condition MIN_NR_CONSOLES is defined as 1 in
> include/uapi/linux/vt.h and it is not changed. So there is no need to
> unwind the loop.
> 
> Signed-off-by: Gen Zhang <blackgod016574@...il.com>
> 
> ---
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index fdd12f8..ea47eb3 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3350,10 +3350,14 @@ static int __init con_init(void)
>  
>  	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
>  		vc_cons[currcons].d = vc = kzalloc(sizeof(struct vc_data), GFP_NOWAIT);
> +		if (!vc)
> +			goto err_vc;
>  		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>  		tty_port_init(&vc->port);
>  		visual_init(vc, currcons, 1);
>  		vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
> +		if (!vc->vc_screenbuf)
> +			goto err_vc_screenbuf;
>  		vc_init(vc, vc->vc_rows, vc->vc_cols,
>  			currcons || !vc->vc_sw->con_save_screen);
>  	}
> @@ -3375,6 +3379,13 @@ static int __init con_init(void)
>  	register_console(&vt_console_driver);
>  #endif
>  	return 0;
> +err_vc_screenbuf:
> +	kfree(vc);
> +	vc_cons[currcons].d = NULL;
> +err_vc:
> +	console_unlock();
> +	return -ENOMEM;
> +
>  }
>  console_initcall(con_init);
> ---

-- 
  Best regards,
    Oleksandr Natalenko (post-factum)
    Senior Software Maintenance Engineer