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

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 22 Mar 2022 16:32:04 +0100
Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:

> Issuing a memop on a protected vm does not make sense,
> 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@linux.ibm.com>

Reviewed-by: Claudio Imbrenda <imbrenda@linux.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