Re: [PATCH v6 13/14] KVM: s390: Fix kvm_arch_commit_memory_region() when low on memory

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Unprivileged userspace can trigger a host-side WARN_ON_ONCE via KVM_BUG_ON by exhausting memory cgroup limits.
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--- Patch [13]: [PATCH v6 13/14] KVM: s390: Fix kvm_arch_commit_memory_region() when low on memory ---
commit c04a67c8bae206347441b13dff0d06941031c158
Author: Claudio Imbrenda <[email protected]>

KVM: s390: Fix kvm_arch_commit_memory_region() when low on memory

When low on memory, kvm_arch_commit_memory_region() might not perform
the required action completely. The patch attempts to fix this by refilling
the MMU cache and retrying, using KVM_BUG_ON if it still fails.

> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index ba600c9488871..035476ec7fbc3 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
[ ... ]
> @@ -5862,11 +5860,17 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  		case KVM_MR_FLAGS_ONLY:
>  			break;
>  		default:
> +			rc = 0;
>  			WARN(1, "Unknown KVM MR CHANGE: %d\n", change);
>  		}
>  	}
> +	if (rc == -ENOMEM) {
> +		rc = kvm_s390_mmu_cache_topup(mc);
> +		if (!rc)
> +			goto retry;
> +	}
>  out:
> -	if (rc)
> +	if (KVM_BUG_ON(rc, kvm))

[Severity: High]
Could this KVM_BUG_ON() be triggered by an unprivileged host-userspace
process exhausting its memory cgroup limit?

If kvm_s390_new_mmu_cache() fails to allocate memory (for example, due to a
memcg limit restricting its GFP_KERNEL_ACCOUNT allocation), the function jumps
to the out label with rc set to -ENOMEM.

Passing this error to KVM_BUG_ON() executes a WARN_ON_ONCE. If the host has
panic_on_warn enabled, this would crash the system, potentially allowing a
host-side denial of service directly reachable from unprivileged userspace.

Is it possible to use pr_warn_once() or a similar logging mechanism here to
avoid assertions on userspace-driven inputs?

>  		pr_warn("failed to commit memory region\n");
>  	return;
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=13