Re: [PATCH v1 1/1] KVM: s390x: fix SCK locking

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

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@linux.ibm.com>
>> ---
>>   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, &gtod);
>> +    /*
>> +     * 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, &gtod)) {
>> +        kvm_s390_retry_instr(vcpu);
>> +        return -EAGAIN;
>> +    }
>>         kvm_s390_set_psw_cc(vcpu, 0);
>>       return 0;