linux-kernel - Re: [PATCH] vt: Reject zero-sized screen buffer size.

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Message-ID: <7adf8aee-9bdd-8184-6cbe-291357677edd@i-love.sakura.ne.jp>
Date:   Fri, 10 Jul 2020 20:31:42 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>, Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org,
        syzbot <syzbot+017265e8553724e514e8@...kaller.appspotmail.com>
Subject: Re: [PATCH] vt: Reject zero-sized screen buffer size.

On 2020/07/10 19:55, Greg Kroah-Hartman wrote:
>> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
>> index 48a8199f7845..8497e9206607 100644
>> --- a/drivers/tty/vt/vt.c
>> +++ b/drivers/tty/vt/vt.c
>> @@ -1126,7 +1126,7 @@ int vc_allocate(unsigned int currcons)	/* return 0 on success */
>>  		con_set_default_unimap(vc);
>>  
>>  	vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_KERNEL);
>> -	if (!vc->vc_screenbuf)
>> +	if (ZERO_OR_NULL_PTR(vc->vc_screenbuf))
> 
> No, let's check this before we do kzalloc() please, that's just an odd
> way of doing an allocation we shouldn't have had to do.

OK. I can change to

+	if (vc->vc_screenbuf_size > KMALLOC_MAX_SIZE || !vc->vc_screenbuf_size)
+		goto err_free;
 	vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_KERNEL);
 	if (!vc->vc_screenbuf)
 		goto err_free;

like vc_do_resize() does. But I'm currently waiting for syzbot to test this patch, for
I don't have an environment for reproducing this problem.

> 
>>  		goto err_free;
>>  
>>  	/* If no drivers have overridden us and the user didn't pass a
>> @@ -1212,7 +1212,7 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
>>  	if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
>>  		return 0;
>>  
>> -	if (new_screen_size > KMALLOC_MAX_SIZE)
>> +	if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
>>  		return -EINVAL;
>>  	newscreen = kzalloc(new_screen_size, GFP_USER);
>>  	if (!newscreen)
>> @@ -3393,6 +3393,7 @@ static int __init con_init(void)
>>  		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
>>  		tty_port_init(&vc->port);
>>  		visual_init(vc, currcons, 1);
>> +		/* Assuming vc->vc_screenbuf_size is sane here, for this is __init code. */
> 
> Shouldn't we also check this here, or before we get here, too?

This is an __init function. Can we somehow pass column=0 or row=0 ?

> 
> Just checking the values and rejecting that as a valid screen size
> should be sufficient.

Hmm, where are we checking that column * row does not exceed UINT_MAX, given that
"struct vc_data"->vc_{cols,rows,screenbuf_size} are "unsigned int" and we do

  vc->vc_size_row = vc->vc_cols << 1;
  vc->vc_screenbuf_size = vc->vc_rows * vc->vc_size_row;

in visual_init() ? Don't we need to reject earlier?