Re: [PATCH v4] tty: Fix out-of-bound vmalloc access in imageblit

[Igor Torrente]

On 6/25/21 3:09 AM, Jiri Slaby wrote:
> On 24. 06. 21, 22:27, 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 made the resize_screen() be called
> > even if the screen does not need any resizing, so it will
> > "fix and fill" the fb_var_screeninfo independently.
> >
> > Reported-and-tested-by:
> > syzbot+858dc7a2f7ef07c2c219@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Igor Matheus Andrade Torrente <igormtorrente@xxxxxxxxx>
> > ---
> >   drivers/tty/vt/vt.c | 17 ++++++++++++++++-
> >   1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index fa1548d4f94b..e342f5c905bc 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -1220,7 +1220,22 @@ static int vc_do_resize(struct tty_struct *tty,
> > struct vc_data *vc,
> >       new_screen_size = new_row_size * new_rows;
> >       if (new_cols == vc->vc_cols && new_rows == vc->vc_rows)
> > -        return 0;
> > +        /* This function is being called here to cover the case
>
> This was already commented on v3:
> <quote>
>   Please do not use networking style comments, use normal ones:
>                  /*
>                   * This function...
> </quote>

I don't know how, but I didn't see this comment in the original email. I
will change it.

> > +         * where the userspace calls the FBIOPUT_VSCREENINFO twice,
> > +         * passing the same fb_var_screeninfo containing the fields:
> > +         * yres/xres equal to a number non-multiple of vc_font.height
> > +         * and yres_virtual/xres_virtual equal to number lesser than the
> > +         * vc_font.height and yres/xres.
> > +         * In the second call, the struct fb_var_screeninfo isn't
> > +         * being modified by the underlying driver because of the
> > +         * if above, and this causes the fbcon_display->vrows to become
> > +         * negative and it eventually leads to out-of-bound
> > +         * access by the imageblit function.
> > +         * To give the correct values to the struct and to not have
> > +         * to deal with possible errors from the code below, we call
> > +         * the resize_screen here as well.
> > +         */
> > +        return resize_screen(vc, new_cols, new_rows, user);
>
> I suppose you need to add { } after the if given how its body grew.
>
> With what all console drivers have you tested this with?

fbcon

> >       if (new_screen_size > KMALLOC_MAX_SIZE || !new_screen_size)
> >           return -EINVAL;

Thanks,
Igor M. A. Torrente