x86/ioperm: Remove bitmap if all permissions dropped

If ioperm() results in a bitmap with all bits set (no permissions to any
I/O port), then handling that bitmap on context switch and exit to user
mode is pointless. Drop it.

Move the bitmap exit handling to the ioport code and reuse it for both the
thread exit path and dropping it. This allows to reuse this code for the
upcoming iopl() emulation.

Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Andy Lutomirski <[e-mail]>

arch/x86/include/asm/io_bitmap.h

···
 	unsigned long	bitmap[IO_BITMAP_LONGS];
 };
 
+void io_bitmap_exit(void);
+
 void tss_update_io_bitmap(void);
 
 #endif

arch/x86/kernel/ioport.c

···
 
 static atomic64_t io_bitmap_sequence;
 
+void io_bitmap_exit(void)
+{
+	struct io_bitmap *iobm = current->thread.io_bitmap;
+
+	current->thread.io_bitmap = NULL;
+	clear_thread_flag(TIF_IO_BITMAP);
+	preempt_disable();
+	tss_update_io_bitmap();
+	preempt_enable();
+	kfree(iobm);
+}
+
 /*
  * this changes the io permissions bitmap in the current task.
  */
···
 	 * Search for a (possibly new) maximum. This is simple and stupid,
 	 * to keep it obviously correct:
 	 */
-	max_long = 0;
+	max_long = UINT_MAX;
 	for (i = 0; i < IO_BITMAP_LONGS; i++) {
 		if (iobm->bitmap[i] != ~0UL)
 			max_long = i;
+	}
+	/* All permissions dropped? */
+	if (max_long == UINT_MAX) {
+		io_bitmap_exit();
+		return 0;
 	}
 
 	iobm->max = (max_long + 1) * sizeof(unsigned long);

arch/x86/kernel/process.c

···
 void exit_thread(struct task_struct *tsk)
 {
 	struct thread_struct *t = &tsk->thread;
-	struct io_bitmap *iobm = t->io_bitmap;
 	struct fpu *fpu = &t->fpu;
-	struct tss_struct *tss;
 
-	if (iobm) {
-		preempt_disable();
-		tss = this_cpu_ptr(&cpu_tss_rw);
-
-		t->io_bitmap = NULL;
-		clear_thread_flag(TIF_IO_BITMAP);
-		/* Invalidate the io bitmap base in the TSS */
-		tss->x86_tss.io_bitmap_base = IO_BITMAP_OFFSET_INVALID;
-		preempt_enable();
-		kfree(iobm);
-	}
+	if (test_thread_flag(TIF_IO_BITMAP))
+		io_bitmap_exit();
 
 	free_vm86(t);