KVM: x86: Prevent deadlock against tk_core.seq
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@syzkaller.appspotmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Message-Id: <87h7jgm1zy.ffs@nanos.tec.linutronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 259139a..5bd550e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8095,6 +8095,18 @@ static void pvclock_gtod_update_fn(struct work_struct *work)
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,
@@ -8105,13 +8117,14 @@ static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
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;
}
@@ -8224,6 +8237,7 @@ 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;