Re: [PATCH v5 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] An unprivileged host-userspace process can trigger a kernel WARN_ON_ONCE by invoking the KVM_PPC_GET_COMPAT_CAPS ioctl when the L0 hypervisor reports zero nested capabilities.
--

commit 6f57063ca19824c71191d87b7d5e423ad8ddc900
Author: Amit Machhiwal <amachhiw@linux.ibm.com>

KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

This commit adds support for retrieving host CPU compatibility capabilities
for nested guests on PowerVM. It exposes these capabilities to userspace
via the KVM_PPC_GET_COMPAT_CAPS ioctl, leveraging a cached value populated
during module initialization to avoid repeated hcalls.

> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 61dbeea317f3b..8799124b80960 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
[ ... ]
> @@ -6510,6 +6510,25 @@ static bool kvmppc_hash_v3_possible(void)
>  	return true;
>  }
>  
> +
> +static int kvmppc_get_compat_caps(struct kvm_ppc_compat_caps *host_caps)
> +{
> +	unsigned long capabilities = 0;
> +	long rc = -EINVAL;
> +
> +	if (kvmhv_on_pseries()) {
> +		if (kvmhv_is_nestedv2()) {
> +			WARN_ON_ONCE(!nested_capabilities);

[Severity: High]
Could this WARN_ON_ONCE be triggered by an unprivileged host-userspace
process?

If the L0 hypervisor reports no recognized capabilities during module
initialization, nested_capabilities is set to 0. A user process with access
to /dev/kvm could then create a VM and invoke the KVM_PPC_GET_COMPAT_CAPS
ioctl, directly evaluating this condition.

Since a userspace-reachable WARN_ON_ONCE can act as a local denial of
service by flooding dmesg or crashing the host if panic_on_warn is enabled,
would it be safer to return an error to userspace or log via pr_warn_once()
instead of asserting?

> +			capabilities = nested_capabilities;
> +			rc = 0;
> +		}
> +	}
> +
> +	host_caps->compat_capabilities = capabilities & KVM_PPC_COMPAT_BITMASK;
> +
> +	return rc;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260701051409.51820-1-amachhiw@linux.ibm.com?part=2