On 3/14/22 14:33, Christian Borntraeger wrote:
> Am 01.03.22 um 15:33 schrieb Claudio Imbrenda:
>> When handling the SCK instruction, the kvm lock is taken, even though
>> the vcpu lock is already being held. The normal locking order is kvm
>> lock first and then vcpu lock. This is can (and in some circumstances
>> does) lead to deadlocks.
>>
>> The function kvm_s390_set_tod_clock is called both by the SCK handler
>> and by some IOCTLs to set the clock. The IOCTLs will not hold the vcpu
>> lock, so they can safely take the kvm lock. The SCK handler holds the
>> vcpu lock, but will also somehow need to acquire the kvm lock without
>> relinquishing the vcpu lock.
>>
>> The solution is to factor out the code to set the clock, and provide
>> two wrappers. One is called like the original function and does the
>> locking, the other is called kvm_s390_try_set_tod_clock and uses
>> trylock to try to acquire the kvm lock. This new wrapper is then used
>> in the SCK handler. If locking fails, -EAGAIN is returned, which is
>> eventually propagated to userspace, thus also freeing the vcpu lock and
>> allowing for forward progress.
>>
>> This is not the most efficient or elegant way to solve this issue, but
>> the SCK instruction is deprecated and its performance is not critical.
>>
>> The goal of this patch is just to provide a simple but correct way to
>> fix the bug.
>>
>> Fixes: 6a3f95a6b04c ("KVM: s390: Intercept SCK instruction")
>> Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
>> ---
>> arch/s390/kvm/kvm-s390.c | 19 ++++++++++++++++---
>> arch/s390/kvm/kvm-s390.h | 4 ++--
>> arch/s390/kvm/priv.c | 14 +++++++++++++-
>> 3 files changed, 31 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>> index 2296b1ff1e02..4e3db4004bfd 100644
>> --- a/arch/s390/kvm/kvm-s390.c
>> +++ b/arch/s390/kvm/kvm-s390.c
>> @@ -3869,14 +3869,12 @@ static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
>> return 0;
>> }
>> -void kvm_s390_set_tod_clock(struct kvm *kvm,
>> - const struct kvm_s390_vm_tod_clock *gtod)
>> +static void __kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)
>> {
>> struct kvm_vcpu *vcpu;
>> union tod_clock clk;
>> unsigned long i;
>> - mutex_lock(&kvm->lock);
>> preempt_disable();
>> store_tod_clock_ext(&clk);
>> @@ -3897,7 +3895,22 @@ void kvm_s390_set_tod_clock(struct kvm *kvm,
>> kvm_s390_vcpu_unblock_all(kvm);
>> preempt_enable();
>> +}
>> +
>> +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)
>> +{
>> + mutex_lock(&kvm->lock);
>> + __kvm_s390_set_tod_clock(kvm, gtod);
>> + mutex_unlock(&kvm->lock);
>> +}
>> +
>> +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod)
>> +{
>> + if (!mutex_trylock(&kvm->lock))
>> + return 0;
>> + __kvm_s390_set_tod_clock(kvm, gtod);
>> mutex_unlock(&kvm->lock);
>> + return 1;
>> }
>> /**
>> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
>> index 098831e815e6..f2c910763d7f 100644
>> --- a/arch/s390/kvm/kvm-s390.h
>> +++ b/arch/s390/kvm/kvm-s390.h
>> @@ -349,8 +349,8 @@ int kvm_s390_handle_sigp(struct kvm_vcpu *vcpu);
>> int kvm_s390_handle_sigp_pei(struct kvm_vcpu *vcpu);
>> /* implemented in kvm-s390.c */
>> -void kvm_s390_set_tod_clock(struct kvm *kvm,
>> - const struct kvm_s390_vm_tod_clock *gtod);
>> +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod);
>> +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod);
>> long kvm_arch_fault_in_page(struct kvm_vcpu *vcpu, gpa_t gpa, int writable);
>> int kvm_s390_store_status_unloaded(struct kvm_vcpu *vcpu, unsigned long addr);
>> int kvm_s390_vcpu_store_status(struct kvm_vcpu *vcpu, unsigned long addr);
>> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
>> index 417154b314a6..7f3e7990ef82 100644
>> --- a/arch/s390/kvm/priv.c
>> +++ b/arch/s390/kvm/priv.c
>> @@ -102,7 +102,19 @@ static int handle_set_clock(struct kvm_vcpu *vcpu)
>> return kvm_s390_inject_prog_cond(vcpu, rc);
>> VCPU_EVENT(vcpu, 3, "SCK: setting guest TOD to 0x%llx", gtod.tod);
>> - kvm_s390_set_tod_clock(vcpu->kvm, >od);
>> + /*
>> + * To set the TOD clock we need to take the kvm lock, but we are
>> + * already holding the vcpu lock, and the usual lock order is the
>> + * opposite. Therefore we use trylock instead of lock, and if the
>> + * kvm lock cannot be taken, we retry the instruction and return
>> + * -EAGAIN to userspace, thus freeing the vcpu lock.
>> + * The SCK instruction is considered legacy and at this point it's
>> + * not worth the effort to find a nicer solution.
>> + */
>
> To comply more with usual comment style (no we, us) and to give more context
> on the legacy I will slightly modify the comment before sending out.
>
> /*
> * To set the TOD clock the kvm lock must be taken, but the vcpu
> * lock is already held in handle_set_clock. The usual lock order
> * is the opposite.
> * As SCK is deprecated and should not be used in several cases
I think you'd want commas around that clause, i.e.
* As SCK is deprecated and should not be used in several cases,
* for example when the multiple-opoch or the TOD-clock-steering
* facility is installed (see Principles of Operation),
* a slow path can be used.
> * like the existence of the multiple epoch facility or TOD clock> * steering (see Principles of Operation) a slow path can be used.
> * If the lock can not be taken via try_lock, the instruction will
> * be retried via -EAGAIN at a later point in time.
> */
>
> Ok with everybody?
>
>
>
>> + if (!kvm_s390_try_set_tod_clock(vcpu->kvm, >od)) {
>> + kvm_s390_retry_instr(vcpu);
>> + return -EAGAIN;
>> + }
>> kvm_s390_set_psw_cc(vcpu, 0);
>> return 0;
