[SRU][F][PATCH 1/2] KVM: s390x: fix SCK locking

Stefan Bader stefan.bader at canonical.com
Fri Feb 17 09:02:19 UTC 2023 

On 16.02.23 10:54, Roxana Nicolescu wrote:
> From: Claudio Imbrenda <imbrenda at linux.ibm.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1999882
> 
> [ Upstream commit c0573ba5c5a2244dc02060b1f374d4593c1d20b7 ]
> 
> 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 at linux.ibm.com>
> Reviewed-by: Christian Borntraeger <borntraeger at linux.ibm.com>
> Reviewed-by: Janis Schoetterl-Glausch <scgl at linux.ibm.com>
> Link: https://lore.kernel.org/r/20220301143340.111129-1-imbrenda@linux.ibm.com
> Cc: stable at vger.kernel.org
> Signed-off-by: Christian Borntraeger <borntraeger at linux.ibm.com>
> Stable-dep-of: 6973091d1b50 ("KVM: s390: pv: don't allow userspace to set the clock under PV")
> Signed-off-by: Sasha Levin <sashal at kernel.org>
> (backported from commit c0573ba5c5a2244dc02060b1f374d4593c1d20b7)

It would be helpful if you described here what had to be adjusted. That 
helps reviewers to know what to check. Something like

[roxanan: context adjustments ... or ... adjusted arguments for...]

> Signed-off-by: Roxana Nicolescu <roxana.nicolescu at canonical.com>
> ---
>   arch/s390/kvm/kvm-s390.c | 19 ++++++++++++++++---
>   arch/s390/kvm/kvm-s390.h |  4 ++--
>   arch/s390/kvm/priv.c     | 15 ++++++++++++++-
>   3 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 1fb8b8a9fa0f..0f1b0dde0de3 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -3940,14 +3940,12 @@ 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;
>   	struct kvm_s390_tod_clock_ext htod;
>   	int i;
>   
> -	mutex_lock(&kvm->lock);
>   	preempt_disable();
>   
>   	get_tod_clock_ext((char *)&htod);
> @@ -3968,7 +3966,22 @@ 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 817d213693e0..c94c1e29eeca 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -335,8 +335,8 @@ 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 840b383ba756..8f03992d25cc 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -103,7 +103,20 @@ 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 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, for example when the multiple epoch facility or TOD clock
> +	 * steering facility is installed (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.
> +	 */
> +	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;

-- 
- Stefan

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