KVM: protect concurrent make_all_cpus_request

make_all_cpus_request contains a race condition which can
trigger false request completed status, as follows:

CPU0                                              CPU1

if (test_and_set_bit(req,&vcpu->requests))
   ....                                        	   if (test_and_set_bit(req,&vcpu->requests))
   ..                                                  return
proceed to smp_call_function_many(wait=1)

Use a spinlock to serialize concurrent CPUs.

Cc: stable@kernel.org
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

2 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index aacc5449f586..16713dc672e4 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -125,6 +125,7 @@ struct kvm_kernel_irq_routing_entry {
 struct kvm {
         struct mutex lock; /* protects the vcpus array and APIC accesses */
         spinlock_t mmu_lock;
+        spinlock_t requests_lock;
         struct rw_semaphore slots_lock;
         struct mm_struct *mm; /* userspace tied to this vm */
         int nmemslots;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 013a5b3e9f75..2884baf1d5f9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -746,6 +746,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
                 cpumask_clear(cpus);
 
         me = get_cpu();
+        spin_lock(&kvm->requests_lock);
         for (i = 0; i < KVM_MAX_VCPUS; ++i) {
                 vcpu = kvm->vcpus[i];
                 if (!vcpu)
@@ -762,6 +763,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
                 smp_call_function_many(cpus, ack_flush, NULL, 1);
         else
                 called = false;
+        spin_unlock(&kvm->requests_lock);
         put_cpu();
         free_cpumask_var(cpus);
         return called;
@@ -982,6 +984,7 @@ static struct kvm *kvm_create_vm(void)
         kvm->mm = current->mm;
         atomic_inc(&kvm->mm->mm_count);
         spin_lock_init(&kvm->mmu_lock);
+        spin_lock_init(&kvm->requests_lock);
         kvm_io_bus_init(&kvm->pio_bus);
         mutex_init(&kvm->lock);
         kvm_io_bus_init(&kvm->mmio_bus);