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

[Christian Borntraeger <borntraeger@de.ibm.com>]

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
	 * 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;