Re: [PATCH] tty: vt: add some NULL checks for vc_data

[Hang Zhang]

On Tue, Jan 3, 2023 at 4:24 AM Jiri Slaby <jirislaby@xxxxxxxxxx> wrote:
>
> On 29. 12. 22, 7:41, Hang Zhang wrote:
> > vc_selection(), do_blank_screen() and scrollfront() all access "vc_data"
> > structures obtained from the global "vc_cons[fg_console].d", which can
> > be freed and nullified (e.g., in the error path of vc_allocate()). But
> > these functions don't have any NULL checks against the pointers before
> > dereferencing them, causing potentially use-after-free or null pointer
> > dereference.
>
> Could you elaborate under what circumstances is fg_console set to a
> non-allocated console?

Hi, Jiri, thank you for your reply! I am not a developer for tty
subsystem, so the reasoning here is based on my best-effort code
reading. Please correct me if I am wrong.

This patch is based on several observations:

(1) at the beginning of vc_selection() (where one NULL check is
inserted in this patch), poke_blanked_console() is invoked, which
explicitly checks whether "vc_cons[fg_console].d" is NULL, suggesting
the possibility of "fg_console" associated with an unallocated console
at this point. However, poke_blanked_console() returns "void", so
even if "fg_console" is NULL, after returning to vc_selection(),
it will just keep executing, resulting in the possible NULL pointer
dereference later ("vc" in vc_selection() can be "vc_cons[fg_console].d"
if called from set_selection_kernel()). So this patch actually tries
to make the already existing NULL check take effect on the control
flow (e.g., early return if NULL).

(2) a similar NULL check for "vc_cons[fg_console].d" can also be found
in do_unblank_screen() ("if (!vc_cons_allocated(fg_console))") before
accessing the corresponding "vc_data". I do notice that the NULL check
has a comment "/* impossible */", but the check has not been removed so
far. My guess is that there might still be a chance that it can be
unallocated at that point.

(3) regarding how "fg_console" can be unallocated, one place I noticed
is vc_allocate() (invoked from vt_ioctl$VT_ACTIVATE), where the user
can specify the index of "vc_cons" to be activated (can be "fg_console"
in theory). If any error happens and the code under the label "err_free"
is executed, then "vc_cons[fg_console].d" can be freed and nullified.
But I am not sure about the exact scenario/flow to make "fg_console"
unallocated, as this seems to need a thorough understanding of all places
manipulating "fg_console" across the whole subsystem (including the
synchronization between them).

So, in conclusion, I cannot exclude the possibility of potential
NULL dereference/use-after-free issues with my limited domain
knowledge. Inspired by existing checks in the code as aforementioned,
I think it's safer to add some additional NULL checks. If the developers
finally decide that "fg_console" cannot be unallocated in reality,
then maybe we should also consider removing some unnecessary checks.

>
> > Prevent these potential issues by placing NULL checks in these functions
> > before accessing "vc_data" structures. Similar checks can be found in
> > other functions like vt_console_print() and poke_blanked_console().
> >
> > Signed-off-by: Hang Zhang <zh.nvgt@xxxxxxxxx>
> > ---
> >   drivers/tty/vt/selection.c | 3 +++
> >   drivers/tty/vt/vt.c        | 5 +++++
> >   2 files changed, 8 insertions(+)
> >
> > diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
> > index 6ef22f01cc51..c727fd947683 100644
> > --- a/drivers/tty/vt/selection.c
> > +++ b/drivers/tty/vt/selection.c
> > @@ -319,6 +319,9 @@ static int vc_selection(struct vc_data *vc, struct tiocl_selection *v,
> >   {
> >       int ps, pe;
> >
> > +     if (!vc)
> > +             return 0;
> > +
> >       poke_blanked_console();
> >
> >       if (v->sel_mode == TIOCL_SELCLEAR) {
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index 981d2bfcf9a5..00f8fdc61e9f 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -1493,6 +1493,8 @@ void scrollback(struct vc_data *vc)
> >
> >   void scrollfront(struct vc_data *vc, int lines)
> >   {
> > +     if (!vc)
> > +             return;
> >       if (!lines)
> >               lines = vc->vc_rows / 2;
> >       scrolldelta(lines);
> > @@ -4346,6 +4348,9 @@ void do_blank_screen(int entering_gfx)
> >       struct vc_data *vc = vc_cons[fg_console].d;
> >       int i;
> >
> > +     if (!vc)
> > +             return;
> > +
> >       might_sleep();
> >
> >       WARN_CONSOLE_UNLOCKED();
>
> thanks,
> --
> js
> suse labs
>