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SubjectRe: [KVM PATCH v3 1/3] KVM: fix race in irq_routing logic
On Mon, Oct 26, 2009 at 12:21:57PM -0400, Gregory Haskins wrote:
> The current code suffers from the following race condition:
>
> thread-1 thread-2
> -----------------------------------------------------------
>
> kvm_set_irq() {
> rcu_read_lock()
> irq_rt = rcu_dereference(table);
> rcu_read_unlock();
>
> kvm_set_irq_routing() {
> mutex_lock();
> irq_rt = table;
> rcu_assign_pointer();
> mutex_unlock();
> synchronize_rcu();
>
> kfree(irq_rt);
>
> irq_rt->entry->set(); /* bad */
>
> -------------------------------------------------------------
>
> Because the pointer is accessed outside of the read-side critical
> section. There are two basic patterns we can use to fix this bug:
>
> 1) Switch to sleeping-rcu and encompass the ->set() access within the
> read-side critical section,
>
> OR
>
> 2) Add reference counting to the irq_rt structure, and simply acquire
> the reference from within the RSCS.
>
> This patch implements solution (1).

Looks like a good transformation! A few questions interspersed below.

> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> include/linux/kvm_host.h | 6 +++++-
> virt/kvm/irq_comm.c | 50 +++++++++++++++++++++++++++-------------------
> virt/kvm/kvm_main.c | 1 +
> 3 files changed, 35 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index bd5a616..1fe135d 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -185,7 +185,10 @@ struct kvm {
>
> struct mutex irq_lock;
> #ifdef CONFIG_HAVE_KVM_IRQCHIP
> - struct kvm_irq_routing_table *irq_routing;
> + struct {
> + struct srcu_struct srcu;

Each structure has its own SRCU domain. This is OK, but just asking
if that is the intent. It does look like the SRCU primitives are
passed a pointer to the correct structure, and that the return value
from srcu_read_lock() gets passed into the matching srcu_read_unlock()
like it needs to be, so that is good.

> + struct kvm_irq_routing_table *table;
> + } irq_routing;
> struct hlist_head mask_notifier_list;
> struct hlist_head irq_ack_notifier_list;
> #endif

[ . . . ]

> @@ -155,21 +156,19 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level)
> * IOAPIC. So set the bit in both. The guest will ignore
> * writes to the unused one.
> */
> - rcu_read_lock();
> - irq_rt = rcu_dereference(kvm->irq_routing);
> + idx = srcu_read_lock(&kvm->irq_routing.srcu);
> + irq_rt = rcu_dereference(kvm->irq_routing.table);
> if (irq < irq_rt->nr_rt_entries)
> - hlist_for_each_entry(e, n, &irq_rt->map[irq], link)
> - irq_set[i++] = *e;
> - rcu_read_unlock();
> + hlist_for_each_entry(e, n, &irq_rt->map[irq], link) {

What prevents the above list from changing while we are traversing it?
(Yes, presumably whatever was preventing it from changing before this
patch, but what?)

Mostly kvm->lock is held, but not always. And if kvm->lock were held
all the time, there would be no point in using SRCU. ;-)

> + int r;
>
> - while(i--) {
> - int r;
> - r = irq_set[i].set(&irq_set[i], kvm, irq_source_id, level);
> - if (r < 0)
> - continue;
> + r = e->set(e, kvm, irq_source_id, level);
> + if (r < 0)
> + continue;
>
> - ret = r + ((ret < 0) ? 0 : ret);
> - }
> + ret = r + ((ret < 0) ? 0 : ret);
> + }
> + srcu_read_unlock(&kvm->irq_routing.srcu, idx);
>
> return ret;
> }
> @@ -179,17 +178,18 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
> struct kvm_irq_ack_notifier *kian;
> struct hlist_node *n;
> int gsi;
> + int idx;
>
> trace_kvm_ack_irq(irqchip, pin);
>
> - rcu_read_lock();
> - gsi = rcu_dereference(kvm->irq_routing)->chip[irqchip][pin];
> + idx = srcu_read_lock(&kvm->irq_routing.srcu);
> + gsi = rcu_dereference(kvm->irq_routing.table)->chip[irqchip][pin];
> if (gsi != -1)
> hlist_for_each_entry_rcu(kian, n, &kvm->irq_ack_notifier_list,
> link)

And same question here -- what keeps the above list from changing while
we are traversing it?

Thanx, Paul


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