The patch below does not apply to the 4.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 3f804f6d201ca93adf4c3df04d1bfd152c1129d6 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner tglx@linutronix.de Date: Thu, 6 May 2021 15:21:37 +0200 Subject: [PATCH] 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 259139a145cb..5bd550eaf683 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8094,6 +8094,18 @@ 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. */ @@ -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;