linux-kernel - RE: [PATCH] tty:vt: Add check the return value of kzalloc to avoid oops

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Date:   Fri, 20 Sep 2019 02:29:49 +0000
From:   Nixiaoming <nixiaoming@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     "penberg@...helsinki.fi" <penberg@...helsinki.fi>,
        "jslaby@...e.com" <jslaby@...e.com>,
        "nico@...xnic.net" <nico@...xnic.net>,
        "textshell@...uujin.de" <textshell@...uujin.de>,
        "sam@...nborg.org" <sam@...nborg.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "mpatocka@...hat.com" <mpatocka@...hat.com>,
        "ghalat@...hat.com" <ghalat@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yangyingliang <yangyingliang@...wei.com>,
        yuehaibing <yuehaibing@...wei.com>,
        Zengweilin <zengweilin@...wei.com>
Subject: RE: [PATCH] tty:vt: Add check the return value of kzalloc to avoid
 oops

On 2019/9/19 17:30, Greg KH wrote:
> On Thu, Sep 19, 2019 at 05:18:15PM +0800, Xiaoming Ni wrote:
>> Using kzalloc() to allocate memory in function con_init(), but not
>> checking the return value, there is a risk of null pointer references
>> oops.
>>
>> Signed-off-by: Xiaoming Ni <nixiaoming@...wei.com>
>
> We keep having this be "reported" 
>
>> ---
>>  drivers/tty/vt/vt.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
>> index 34aa39d..db83e52 100644
>> --- a/drivers/tty/vt/vt.c
>> +++ b/drivers/tty/vt/vt.c
>> @@ -3357,15 +3357,33 @@ 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 (unlikely(!vc)) {
>> +			pr_warn("%s:failed to allocate memory for the %u vc\n",
>> +					__func__, currcons);
>> +			break;
>> +		}
>
> At init, this really can not happen.  Have you see it ever happen?

I did not actually observe the null pointer here.
I am confused when I see the code allocated here without check the return value.
Small memory allocation failures are difficult to occur during system initialization
But is it not safe enough if the code is not judged?
Also if the memory allocation failure is not allowed here, is it better to add the __GFP_NOFAIL flags?

>>  		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 (unlikely(!vc->vc_screenbuf)) {
>
> Never use likely/unlikely unless you can actually measure the speed
> difference.  For something like this, the compiler will always get it
> right without you having to do anything.
>
Thank you for your guidance.
 I misuse it.

> And again, how can this fail?  Have you seen it fail?
>
>> +			pr_warn("%s:failed to allocate memory for the %u vc_screenbuf\n",
>> +					__func__, currcons);
>> +			visual_deinit(vc);
>> +			tty_port_destroy(&vc->port);
>> +			kfree(vc);
>> +			vc_cons[currcons].d = NULL;
>> +			break;
>> +		}
>>  		vc_init(vc, vc->vc_rows, vc->vc_cols,
>>  			currcons || !vc->vc_sw->con_save_screen);
>>  	}
>>  	currcons = fg_console = 0;
>>  	master_display_fg = vc = vc_cons[currcons].d;
>> +	if (unlikely(!vc)) {
>
> Again, never use likely/unlikely unless you can measure it.
>
> thanks,
>
> greg k-h
>
> .
>
thanks,
Xiaoming Ni