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

Andy Lutomirski
2019-11-10T21:05:21Z

On Sun, Nov 10, 2019 at 12:31 PM Thomas Gleixner <tglx@linutronix.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