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Subject: Re: [PATCH RFC] tty: Fix out-of-bound vmalloc access in imageblit
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     jirislaby@kernel.org, linux-kernel@vger.kernel.org,
        syzbot+858dc7a2f7ef07c2c219@syzkaller.appspotmail.com
References: <20210531124713.9048-1-igormtorrente@gmail.com>
 <YLoob2rm3isTp+ln@kroah.com>
From:   Igor Torrente <igormtorrente@gmail.com>
Message-ID: <fb93d5aa-d678-9de1-a7e0-dfa5594ff68a@gmail.com>
Date:   Tue, 8 Jun 2021 11:30:58 -0300
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On 6/4/21 10:19 AM, Greg KH wrote:
> On Mon, May 31, 2021 at 09:47:13AM -0300, Igor Matheus Andrade Torrente wrote:
>> This issue happens when a userspace program does an ioctl
>> FBIOPUT_VSCREENINFO passing the fb_var_screeninfo struct
>> containing only the fields xres, yres, and bits_per_pixel
>> with values.
>>
>> If this struct is the same as the previous ioctl, the
>> vc_resize() detects it and doesn't call the resize_screen(),
>> leaving the fb_var_screeninfo incomplete. And this leads to
>> the updatescrollmode() calculates a wrong value to
>> fbcon_display->vrows, which makes the real_y() return a
>> wrong value of y, and that value, eventually, causes
>> the imageblit to access an out-of-bound address value.
>>
>> To solve this issue I brougth the resize_screen() the
>> beginning of vc_do_resize(), so it will "fix and fill"
>> the fb_var_screeninfo even if the screen does not need any
>> resizing.
>>
>> Reported-and-tested-by: syzbot+858dc7a2f7ef07c2c219@syzkaller.appspotmail.com
>> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@gmail.com>
>> ---
>>   drivers/tty/vt/vt.c | 12 ++++--------
>>   1 file changed, 4 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
>> index fa1548d4f94b..1b90758d8893 100644
>> --- a/drivers/tty/vt/vt.c
>> +++ b/drivers/tty/vt/vt.c
>> @@ -1219,6 +1219,10 @@ static int vc_do_resize(struct tty_struct *tty, struct vc_data *vc,
>>   	new_row_size = new_cols << 1;
>>   	new_screen_size = new_row_size * new_rows;
>>   
>> +	err = resize_screen(vc, new_cols, new_rows, user);
>> +	if (err)
>> +		return err;
>> +
>>   	if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
>>   		return 0;
>>   
> 
> But now if any of the checks below this call fail, the screen will be
> resized and not "put back" to the original size, right?  That could
> cause a mis-match of what is expected here.

Right, this will probably cause problems. :/
Thanks to point out, I'll work to find another solution.

> 
> thanks,
> 
> greg k-h
> 

---
Igor M. A. Torrente