Re: KVM: x86: Prevent deadlock against tk_core.seq

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On 06/05/21 15:21, Thomas Gleixner wrote:
syzbot reported a possible deadlock in pvclock_gtod_notify():

CPU 0  		  	   	    	    CPU 1
write_seqcount_begin(&tk_core.seq);
   pvclock_gtod_notify()			    spin_lock(&pool->lock);
     queue_work(..., &pvclock_gtod_work)	    ktime_get()
      spin_lock(&pool->lock);		      do {
      						seq = read_seqcount_begin(tk_core.seq)
						...
				              } while (read_seqcount_retry(&tk_core.seq, seq);

While this is unlikely to happen, it's possible.

Delegate queue_work() to irq_work() which postpones it until the
tk_core.seq write held region is left and interrupts are reenabled.

Fixes: 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes")
Reported-by: syzbot+6beae4000559d41d80f8@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
Link: https://lore.kernel.org/r/0000000000001d43ac05c0f5c6a0@xxxxxxxxxx
---
  arch/x86/kvm/x86.c |   22 ++++++++++++++++++----
  1 file changed, 18 insertions(+), 4 deletions(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8040,6 +8040,18 @@ static void pvclock_gtod_update_fn(struc
  static DECLARE_WORK(pvclock_gtod_work, pvclock_gtod_update_fn);
/*
+ * Indirection to move queue_work() out of the tk_core.seq write held
+ * region to prevent possible deadlocks against time accessors which
+ * are invoked with work related locks held.
+ */
+static void pvclock_irq_work_fn(struct irq_work *w)
+{
+	queue_work(system_long_wq, &pvclock_gtod_work);
+}
+
+static DEFINE_IRQ_WORK(pvclock_irq_work, pvclock_irq_work_fn);
+
+/*
   * Notification about pvclock gtod data update.
   */
  static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
@@ -8050,13 +8062,14 @@ static int pvclock_gtod_notify(struct no
update_pvclock_gtod(tk); - /* disable master clock if host does not trust, or does not
-	 * use, TSC based clocksource.
+	/*
+	 * Disable master clock if host does not trust, or does not use,
+	 * TSC based clocksource. Delegate queue_work() to irq_work as
+	 * this is invoked with tk_core.seq write held.
  	 */
  	if (!gtod_is_based_on_tsc(gtod->clock.vclock_mode) &&
  	    atomic_read(&kvm_guest_has_master_clock) != 0)
-		queue_work(system_long_wq, &pvclock_gtod_work);
-
+		irq_work_queue(&pvclock_irq_work);
  	return 0;
  }
@@ -8168,6 +8181,7 @@ void kvm_arch_exit(void)
  	cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
  #ifdef CONFIG_X86_64
  	pvclock_gtod_unregister_notifier(&pvclock_gtod_notifier);
+	irq_work_sync(&pvclock_irq_work);
  	cancel_work_sync(&pvclock_gtod_work);
  #endif
  	kvm_x86_ops.hardware_enable = NULL;


Queued, thanks.

Paolo




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