Re: [PATCH] tty: vt: selection: Add check for valid tiocl_selection values

[Jiri Slaby]

On 04. 08. 22, 10:44, Helge Deller wrote:
> On 8/4/22 09:15, Helge Deller wrote:
> > Hello Jiri,
> >
> > Thanks for looking into this patch!
> >
> > On 8/4/22 07:47, Jiri Slaby wrote:
> > > On 30. 07. 22, 20:49, Helge Deller wrote:
> > > > The line and column numbers for the selection need to start at 1.
> > > > Add the checks to prevent invalid input.
> > > >
> > > > Signed-off-by: Helge Deller <deller@xxxxxx>
> > > > Reported-by: syzbot+14b0e8f3fd1612e35350@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > >
> > > > diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
> > > > index f7755e73696e..58692a9b4097 100644
> > > > --- a/drivers/tty/vt/selection.c
> > > > +++ b/drivers/tty/vt/selection.c
> > > > @@ -326,6 +326,9 @@ static int vc_selection(struct vc_data *vc, struct tiocl_selection *v,
> > > >            return 0;
> > > >        }
> > > >
> > > > +    if (!v->xs || !v->ys || !v->xe || !v->ye)
> > > > +        return -EINVAL;
> > >
> > > Hmm, I'm not sure about this. It potentially breaks userspace (by
> > > returning EINVAL now).
> >
> > Right.
> > According to the code below, my interpretation is that all xs/ys/xe/ye values
> > should be > 0. But of course I might be wrong on this, as I didn't find any
> > documentation for TIOCL_SETSEL.
> >
> > And if userspace tries to set an invalid selection (e.g. by selecting row 0),
> > my patch now returns -EINVAL, while it returned success before.
> >
> > > And the code below should handle this just fine, right:
> > > >        v->xs = min_t(u16, v->xs - 1, vc->vc_cols - 1);
> > > >        v->ys = min_t(u16, v->ys - 1, vc->vc_rows - 1);
> > > >        v->xe = min_t(u16, v->xe - 1, vc->vc_cols - 1);
> >
> > It "handles it fine" in the sense that it can cope with the
> > input and will not crash.
> > But it returns (maybe?) unexpected results...
>
> After some more thinking maybe you are right.
> In case a user provided invalid values in the past, simply an unexpected
> selection was set, but nothing broke.
> Since the patch doesn't fix any critical issue, we could just drop this patch
> and leave it as is.

We can still do a trial and revert it if something breaks... It's just 
that _noone_ knows with all this undocumented stuff ;).

But in fact, 0 currently means full row/column. Isn't it on purpose?

Today, we are out of luck, codesearch.debian.net gives no clue about users:
https://codesearch.debian.net/search?q=%5CbTIOCL_SETSEL%5Cb&literal=0

thanks,
--
js
suse labs