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

Thomas Gleixner
2019-11-10T20:31:29Z

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.

> > + } else if (t->io_bitmap_ptr) {
> > + /* Thread has a I/O bitmap */
> > + tss_update_io_bitmap(tss, t);
> > + set_thread_flag(TIF_IO_BITMAP);
> > + tss_base = IO_BITMAP_OFFSET_VALID_MAP;
> > + } else {
> > + /* Take it out of the context switch work burden */
> > + clear_thread_flag(TIF_IO_BITMAP);
> > + tss_base = IO_BITMAP_OFFSET_INVALID;
>
> Ditto.
>
> 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.

> (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)
> +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