Messages in this thread | | | From | Andy Lutomirski <> | Date | Sun, 10 Nov 2019 13:05:05 -0800 | Subject | Re: [patch 7/9] x86/iopl: Restrict iopl() permission scope |
| |
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
| |