On 01/24/2018 01:51 PM, David Hildenbrand wrote:
> On 24.01.2018 13:41, Christian Borntraeger wrote:
>> Some parts of the cmma migration bitmap is already protected
>> with the kvm->lock (e.g. the migration start). On the other
>> hand the read of the cmma bits is not protected against a
>> concurrent free, neither is the emulation of the ESSA instruction.
>> Let's extend the locking to all related ioctls by using
>> the slots lock and wait for the freeing until all unlocked
>> users have finished (those hold kvm->srcu for read).
>>
>> Reported-by: David Hildenbrand <david@xxxxxxxxxx>
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx # 4.13+
>> Fixes: 190df4a212a7 (KVM: s390: CMMA tracking, ESSA emulation, migration mode)
>> Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx>
>> ---
>> v1->v2: fix comments in kvm_s390_vm_[start|stop]_migration
>> arch/s390/kvm/kvm-s390.c | 18 +++++++++++-------
>> 1 file changed, 11 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 2a5a9f0617f88..e81b748af4553 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -769,7 +769,7 @@ static void kvm_s390_sync_request_broadcast(struct kvm *kvm, int req)
>>
>> /*
>> * Must be called with kvm->srcu held to avoid races on memslots, and with
>> - * kvm->lock to avoid races with ourselves and kvm_s390_vm_stop_migration.
>> + * kvm->slots_lock to avoid races with ourselves and kvm_s390_vm_stop_migration.
>> */
>> static int kvm_s390_vm_start_migration(struct kvm *kvm)
>> {
>> @@ -824,7 +824,7 @@ static int kvm_s390_vm_start_migration(struct kvm *kvm)
>> }
>>
>> /*
>> - * Must be called with kvm->lock to avoid races with ourselves and
>> + * Must be called with kvm->slots_lock to avoid races with ourselves and
>> * kvm_s390_vm_start_migration.
>> */
>> static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>> @@ -839,6 +839,8 @@ static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>>
>> if (kvm->arch.use_cmma) {
>> kvm_s390_sync_request_broadcast(kvm, KVM_REQ_STOP_MIGRATION);
>> + /* We have to wait for the essa emulation to finish */
>> + synchronize_srcu(&kvm->srcu);
>> vfree(mgs->pgste_bitmap);
>> }
>> kfree(mgs);
>> @@ -848,14 +850,12 @@ static int kvm_s390_vm_stop_migration(struct kvm *kvm)
>> static int kvm_s390_vm_set_migration(struct kvm *kvm,
>> struct kvm_device_attr *attr)
>> {
>> - int idx, res = -ENXIO;
>> + int res = -ENXIO;
>>
>> - mutex_lock(&kvm->lock);
>> + mutex_lock(&kvm->slots_lock);
>
> Why were we holding the kvm lock here? Is it okay to drop this lock now?
>
> (I assume it only existed to avoid having multiple concurrent users
> trying to set the migration state - in that case this is fine)
Yes, it was used to prevent multiple users.
>
>> switch (attr->attr) {
>> case KVM_S390_VM_MIGRATION_START:
>> - idx = srcu_read_lock(&kvm->srcu);> res = kvm_s390_vm_start_migration(kvm);
>> - srcu_read_unlock(&kvm->srcu, idx);
>> break;
>> case KVM_S390_VM_MIGRATION_STOP:
>> res = kvm_s390_vm_stop_migration(kvm);
>> @@ -863,7 +863,7 @@ static int kvm_s390_vm_set_migration(struct kvm *kvm,
>> default:
>> break;
>> }
>> - mutex_unlock(&kvm->lock);
>> + mutex_unlock(&kvm->slots_lock);
>>
>> return res;
>> }
>> @@ -1765,7 +1765,9 @@ long kvm_arch_vm_ioctl(struct file *filp,
>> r = -EFAULT;
>> if (copy_from_user(&args, argp, sizeof(args)))
>> break;
>> + mutex_lock(&kvm->slots_lock);
>> r = kvm_s390_get_cmma_bits(kvm, &args);
>> + mutex_unlock(&kvm->slots_lock);
>> if (!r) {
>> r = copy_to_user(argp, &args, sizeof(args));
>> if (r)
>> @@ -1779,7 +1781,9 @@ long kvm_arch_vm_ioctl(struct file *filp,
>> r = -EFAULT;
>> if (copy_from_user(&args, argp, sizeof(args)))
>> break;
>> + mutex_lock(&kvm->slots_lock);
>> r = kvm_s390_set_cmma_bits(kvm, &args);
>> + mutex_unlock(&kvm->slots_lock);
>> break;
>> }
>> default:
>>
>
>
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