linux-kernel - Re: [PATCH RFC] tty: Fix out-of-bound vmalloc access in imageblit

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Message-ID: <29a21488-3d2e-e07b-f9e9-6d5dc1754c1c@gmail.com>
Date:   Mon, 31 May 2021 11:44:43 -0300
From:   Igor Torrente <igormtorrente@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jirislaby@...nel.org, linux-kernel@...r.kernel.org,
        syzbot+858dc7a2f7ef07c2c219@...kaller.appspotmail.com
Subject: Re: [PATCH RFC] tty: Fix out-of-bound vmalloc access in imageblit

Hi Greg,

On 5/31/21 10:18 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@...kaller.appspotmail.com
>> Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@...il.com>
>> ---
>>   drivers/tty/vt/vt.c | 12 ++++--------
>>   1 file changed, 4 insertions(+), 8 deletions(-)
> 
> Why is this "RFC"?
> 

Maybe I'm using it wrongly, but I usually use RFC to indicate that patch 
is probably not ready and needs comments/suggestions to improve it. In 
case, I don't have much experience with vt driver and I'm a little bit 
afraid to break something.