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

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