lguest: fix race in halt code

When the Guest does the LHCALL_HALT hypercall, we go to sleep, expecting
that a timer or the Waker will wake_up_process() us.

But we do it in a stupid way, leaving a classic missing wakeup race.

So split maybe_do_interrupt() into interrupt_pending() and
try_deliver_interrupt(), and check maybe_do_interrupt() and the
"break_out" flag before calling schedule.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/core.c

@@ -188,6 +188,8 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 {
 	/* We stop running once the Guest is dead. */
 	while (!cpu->lg->dead) {
+		unsigned int irq;
+
 		/* First we run any hypercalls the Guest wants done. */
 		if (cpu->hcall)
 			do_hypercalls(cpu);
@@ -211,7 +213,9 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		/* Check if there are any interrupts which can be delivered now:
 		 * if so, this sets up the hander to be executed when we next
 		 * run the Guest. */
-		maybe_do_interrupt(cpu);
+		irq = interrupt_pending(cpu);
+		if (irq < LGUEST_IRQS)
+			try_deliver_interrupt(cpu, irq);
 
 		/* All long-lived kernel loops need to check with this horrible
 		 * thing called the freezer.  If the Host is trying to suspend,
@@ -227,6 +231,12 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		 * clock timer or LHREQ_BREAK from the Waker will wake us. */
 		if (cpu->halted) {
 			set_current_state(TASK_INTERRUPTIBLE);
+			/* Just before we sleep, make sure nothing snuck in
+			 * which we should be doing. */
+			if (interrupt_pending(cpu) < LGUEST_IRQS
+			    || cpu->break_out)
+				set_current_state(TASK_RUNNING);
+			else
 				schedule();
 			continue;
 		}

drivers/lguest/interrupts_and_traps.c

@@ -128,30 +128,38 @@ static void set_guest_interrupt(struct lg_cpu *cpu, u32 lo, u32 hi,
 /*H:205
  * Virtual Interrupts.
  *
- * maybe_do_interrupt() gets called before every entry to the Guest, to see if
+ * interrupt_pending() returns the first pending interrupt which isn't blocked
- * we should divert the Guest to running an interrupt handler. */
+ * by the Guest.  It is called before every entry to the Guest, and just before
-void maybe_do_interrupt(struct lg_cpu *cpu)
+ * we go to sleep when the Guest has halted itself. */
+unsigned int interrupt_pending(struct lg_cpu *cpu)
 {
 	unsigned int irq;
 	DECLARE_BITMAP(blk, LGUEST_IRQS);
-	struct desc_struct *idt;
 
 	/* If the Guest hasn't even initialized yet, we can do nothing. */
 	if (!cpu->lg->lguest_data)
-		return;
+		return LGUEST_IRQS;
 
 	/* Take our "irqs_pending" array and remove any interrupts the Guest
 	 * wants blocked: the result ends up in "blk". */
 	if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
 			   sizeof(blk)))
-		return;
+		return LGUEST_IRQS;
 	bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
 
 	/* Find the first interrupt. */
 	irq = find_first_bit(blk, LGUEST_IRQS);
-	/* None?  Nothing to do */
+
-	if (irq >= LGUEST_IRQS)
+	return irq;
-		return;
+}
+
+/* This actually diverts the Guest to running an interrupt handler, once an
+ * interrupt has been identified by interrupt_pending(). */
+void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq)
+{
+	struct desc_struct *idt;
+
+	BUG_ON(irq >= LGUEST_IRQS);
 
 	/* They may be in the middle of an iret, where they asked us never to
 	 * deliver interrupts. */

drivers/lguest/lg.h

@@ -139,7 +139,8 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user);
 #define pgd_pfn(x)	(pgd_val(x) >> PAGE_SHIFT)
 
 /* interrupts_and_traps.c: */
-void maybe_do_interrupt(struct lg_cpu *cpu);
+unsigned int interrupt_pending(struct lg_cpu *cpu);
+void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq);
 bool deliver_trap(struct lg_cpu *cpu, unsigned int num);
 void load_guest_idt_entry(struct lg_cpu *cpu, unsigned int i,
 			  u32 low, u32 hi);