linux-kernel - Re: [patch 7/9] x86/iopl: Restrict iopl() permission scope

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Message-ID: <CALCETrXLNub_pTMKvVQiq26=8VQPnx865+w0e-ZnHEg9a397_A@mail.gmail.com>
Date:   Sun, 10 Nov 2019 13:05:05 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 7/9] x86/iopl: Restrict iopl() permission scope

On Sun, Nov 10, 2019 at 12:31 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Sun, 10 Nov 2019, Andy Lutomirski wrote:
> > On 11/6/19 11:35 AM, Thomas Gleixner wrote:
> > > +
> > > +   if (IS_ENABLED(CONFIG_X86_IOPL_EMULATION)) {
> > > +           struct tss_struct *tss;
> > > +           unsigned int tss_base;
> > > +
> > > +           /* Prevent racing against a task switch */
> > > +           preempt_disable();
> > > +           tss = this_cpu_ptr(&cpu_tss_rw);
> > > +           if (level == 3) {
> > > +                   /* Grant access to all I/O ports */
> > > +                   set_thread_flag(TIF_IO_BITMAP);
> > > +                   tss_base = IO_BITMAP_OFFSET_VALID_ALL;
> >
> > Where is the actual TSS updated?
>
> Here. It sets the offset to the all zero bitmap. That's all it needs.

Ah, I see.

> > I think what you need to do is have a single function, called by
> > exit_thread(), switch_to(), and here, that updates the TSS to match a
> > given task's IO bitmap state.  This is probably switch_to_bitmap() or
> > similar.
>
> Well, no. exit_thread() and this here actually fiddle with the TIF bit
> which is not what the switch to case does. There is some stuff which can be
> shared.

I was thinking that the code that read iopl_emul and t->io_bitmap_ptr
and updated x86_tss.io_bitmap_base could be factored out of
switch_to().  Suppose you call that tss_update_io_bitmap().  And you
could have another tiny helper:

static void update_tif_io_bitmap(void)
{
  if (...->iopl_emul || ...->io_bitmap_ptr)
    set_thread_flag(TIF_IO_BITMAP);
  else
    clear_thread_flag(TIF_IO_BITMAP);
}

Then the whole iopl emulation path becomes:

preempt_disable();
...->iopl_emul = iopl;
update_tif_io_bitmap();
tss_update_io_bitmap();

and switch_to() does

if (new or prev has TIF_IO_BITMAP)
  tss_update_io_bitmap();

and exit_thread() does:

...->iopl_emul = 0;
kfree(...);
...->io_bitmap_ptr = 0;
clear_thread_flag(TIF_IO_BITMAP) or update_tif_io_bitmap();
tss_update_io_bitmap();

and ioperm() does:

kmalloc (if needed);
update the thread_struct copy of the bitmap.
set_thread_flag(TIF_IO_BITMAP) or update_tif_io_bitmap();
tss_update_io_bitmap();

Does that make sense?

>
> > (Maybe it already is, but I swear I checked all the patches in the
> > series and I can't find the body of tss_update_io_bitmap().  But you
> > should call it in all branches of this if-else thing.)
>
> It's in that very same patch:
>
> > -static void tss_update_io_bitmap(struct tss_struct *tss,
> > -                                struct thread_struct *thread)

But where did the line you just deleted come from?  I'm obviously just
bad at reading emails somewhere.

> > +void tss_update_io_bitmap(struct tss_struct *tss, struct thread_struct *thread)
> >  {
>
> Let me try to get a bit more reuse. Which still leaves the TIF bit fiddling
> in this code path.
>
> Thanks,
>
>         tglx