Re: [patch V2 14/16] x86/iopl: Restrict iopl() permission scope

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, 11 Nov 2019, Thomas Gleixner wrote:
> --- a/arch/x86/kernel/ioport.c
> +++ b/arch/x86/kernel/ioport.c
> @@ -27,15 +27,28 @@ void io_bitmap_share(struct task_struct
>  	set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
>  }
>  
> +static void task_update_io_bitmap(void)
> +{
> +	struct thread_struct *t = &current->thread;
> +
> +	preempt_disable();
> +	if (t->iopl_emul == 3 || t->io_bitmap) {
> +		/* TSS update is handled on exit to user space */
> +		set_thread_flag(TIF_IO_BITMAP);
> +	} else {
> +		clear_thread_flag(TIF_IO_BITMAP);
> +		/* Invalidate TSS */
> +		tss_update_io_bitmap();
> +	}
> +	preempt_enable();
> +}
> +
>  void io_bitmap_exit(void)
>  {
>  	struct io_bitmap *iobm = current->thread.io_bitmap;
>  
> -	preempt_disable();
>  	current->thread.io_bitmap = NULL;
> -	clear_thread_flag(TIF_IO_BITMAP);
> -	tss_update_io_bitmap();
> -	preempt_enable();
> +	task_update_io_bitmap();
>  	if (iobm && refcount_dec_and_test(&iobm->refcnt))
>  		kfree(iobm);

This obviously needs the following delta to be folded in. Noticed too late
after fiddling with the test case some more. git tree is updated
accordingly.

Thanks,

	tglx
---
--- a/arch/x86/kernel/ioport.c
+++ b/arch/x86/kernel/ioport.c
@@ -18,12 +18,15 @@ static atomic64_t io_bitmap_sequence;
 
 void io_bitmap_share(struct task_struct *tsk)
  {
-	/*
-	 * Take a refcount on current's bitmap. It can be used by
-	 * both tasks as long as none of them changes the bitmap.
-	 */
-	refcount_inc(&current->thread.io_bitmap->refcnt);
-	tsk->thread.io_bitmap = current->thread.io_bitmap;
+	 /* Can be NULL when current->thread.iopl_emul == 3 */
+	 if (current->thread.io_bitmap) {
+		 /*
+		  * Take a refcount on current's bitmap. It can be used by
+		  * both tasks as long as none of them changes the bitmap.
+		  */
+		 refcount_inc(&current->thread.io_bitmap->refcnt);
+		 tsk->thread.io_bitmap = current->thread.io_bitmap;
+	 }
 	set_tsk_thread_flag(tsk, TIF_IO_BITMAP);
 }