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

[Greg KH <gregkh@linuxfoundation.org>]

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.

thanks,

greg k-h