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

[Igor Torrente]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@xxxxxxxxx>
> > ---
> >   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