linux-kernel - Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop

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Message-ID: <44618f05-9aee-5aa5-b036-dd838285b26f@linux.ibm.com>
Date:   Wed, 23 Mar 2022 08:58:56 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Sven Schnelle <svens@...ux.ibm.com>, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: s390: Fix lockdep issue in vm memop

On 3/22/22 16:32, Janis Schoetterl-Glausch wrote:
> Issuing a memop on a protected vm does not make sense,

Issuing a vm memop on a protected vm...

The cpu memop still makes sense, no?

> neither is the memory readable/writable, nor does it make sense to check
> storage keys. This is why the ioctl will return -EINVAL when it detects
> the vm to be protected. However, in order to ensure that the vm cannot
> become protected during the memop, the kvm->lock would need to be taken
> for the duration of the ioctl. This is also required because
> kvm_s390_pv_is_protected asserts that the lock must be held.
> Instead, don't try to prevent this. If user space enables secure
> execution concurrently with a memop it must accecpt the possibility of
> the memop failing.
> Still check if the vm is currently protected, but without locking and
> consider it a heuristic.
> 
> Fixes: ef11c9463ae0 ("KVM: s390: Add vm IOCTL for key checked guest absolute memory access")
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>

Makes sense to me.

Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>

> ---
>   arch/s390/kvm/kvm-s390.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index ca96f84db2cc..53adbe86a68f 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2385,7 +2385,16 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
>   		return -EINVAL;
>   	if (mop->size > MEM_OP_MAX_SIZE)
>   		return -E2BIG;
> -	if (kvm_s390_pv_is_protected(kvm))
> +	/*
> +	 * This is technically a heuristic only, if the kvm->lock is not
> +	 * taken, it is not guaranteed that the vm is/remains non-protected.
> +	 * This is ok from a kernel perspective, wrongdoing is detected
> +	 * on the access, -EFAULT is returned and the vm may crash the
> +	 * next time it accesses the memory in question.
> +	 * There is no sane usecase to do switching and a memop on two
> +	 * different CPUs at the same time.
> +	 */
> +	if (kvm_s390_pv_get_handle(kvm))
>   		return -EINVAL;
>   	if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
>   		if (access_key_invalid(mop->key))
> 
> base-commit: c9b8fecddb5bb4b67e351bbaeaa648a6f7456912