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

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

On 3/1/22 15:33, Claudio Imbrenda wrote:
> 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>

Reviewed-by: Janis Schoetterl-Glausch <scgl@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)

Why int instead of bool?

> +{
> +	if (!mutex_trylock(&kvm->lock))
> +		return 0;
> +	__kvm_s390_set_tod_clock(kvm, gtod);
>  	mutex_unlock(&kvm->lock);
> +	return 1;
>  }
> 
[...]