CVE-2022-49882

In the Linux kernel, the following vulnerability has been resolved:

KVM: Reject attempts to consume or refresh inactive gfn_to_pfn_cache

Reject kvm_gpc_check() and kvm_gpc_refresh() if the cache is inactive. Not checking the active flag during refresh is particularly egregious, as KVM can end up with a valid, inactive cache, which can lead to a variety of use-after-free bugs, e.g. consuming a NULL kernel pointer or missing an mmu_notifier invalidation due to the cache not being on the list of gfns to invalidate.

Note, "active" needs to be set if and only if the cache is on the list of caches, i.e. is reachable via mmu_notifier events. If a relevant mmu_notifier event occurs while the cache is "active" but not on the list, KVM will not acquire the cache's lock and so will not serailize the mmu_notifier event with active users and/or kvm_gpc_refresh().

A race between KVM_XEN_ATTR_TYPE_SHARED_INFO and KVM_XEN_HVM_EVTCHN_SEND can be exploited to trigger the bug.

1. Deactivate shinfo cache:

kvm_xen_hvm_set_attr case KVM_XEN_ATTR_TYPE_SHARED_INFO kvm_gpc_deactivate kvm_gpc_unmap gpc->valid = false gpc->khva = NULL gpc->active = false

Result: active = false, valid = false

2. Cause cache refresh:

kvm_arch_vm_ioctl case KVM_XEN_HVM_EVTCHN_SEND kvm_xen_hvm_evtchn_send kvm_xen_set_evtchn kvm_xen_set_evtchn_fast kvm_gpc_check return -EWOULDBLOCK because !gpc->valid kvm_xen_set_evtchn_fast return -EWOULDBLOCK kvm_gpc_refresh hva_to_pfn_retry gpc->valid = true gpc->khva = not NULL

Result: active = false, valid = true

3. Race ioctl KVM_XEN_HVM_EVTCHN_SEND against ioctl KVM_XEN_ATTR_TYPE_SHARED_INFO:

kvm_arch_vm_ioctl case KVM_XEN_HVM_EVTCHN_SEND kvm_xen_hvm_evtchn_send kvm_xen_set_evtchn kvm_xen_set_evtchn_fast read_lock gpc->lock kvm_xen_hvm_set_attr case KVM_XEN_ATTR_TYPE_SHARED_INFO mutex_lock kvm->lock kvm_xen_shared_info_init kvm_gpc_activate gpc->khva = NULL kvm_gpc_check [ Check passes because gpc->valid is still true, even though gpc->khva is already NULL. ] shinfo = gpc->khva pending_bits = shinfo->evtchn_pending CRASH: test_and_set_bit(..., pending_bits)

This vulnerability is rated as Moderate impact for Red Hat Enterprise Linux because exploitation requires local access with permissions to execute specific KVM ioctl operations, restricting the attack surface and blast radius to a host denial of service. Under default deployment models, access to the KVM device node is restricted to privileged processes, preventing unprivileged users from directly issuing virtualization controls. Triggering the flaw relies on racing Xen-emulation attributes and event-channel commands against guest frame caching routines, making remote or unauthorized execution unfeasible.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.