CVE-2025-37936 Information

Description

In the Linux kernel the following vulnerability has been resolved:

perf/x86/intel: KVM: Mask PEBS_ENABLE loaded for guest with vCPU’s value.

When generating the MSR_IA32_PEBS_ENABLE value that will be loaded on VM-Entry to a KVM guest mask the value with the vCPU’s desired PEBS_ENABLE value. Consulting only the host kernel’s host vs. guest masks results in running the guest with PEBS enabled even when the guest doesn’t want to use PEBS. Because KVM uses perf events to proxy the guest virtual PMU simply looking at exclude_host can’t differentiate between events created by host userspace and events created by KVM on behalf of the guest.

Running the guest with PEBS unexpectedly enabled typically manifests as crashes due to a near-infinite stream of PFs. E.g. if the guest hasn’t written MSR_IA32_DS_AREA the CPU will hit page faults on address ‘0’ when trying to record PEBS events.

The issue is most easily reproduced by running perf kvm top from before commit 7b100989b4f6 (\perf evlist: Remove __evlist__add_default) (after which perf kvm top effectively stopped using PEBS). The userspace side of perf creates a guest-only PEBS event which intel_guest_get_msrs() misconstrues a guest-owned PEBS event.

Arguably this is a userspace bug as enabling PEBS on guest-only events simply cannot work and userspace can kill VMs in many other ways (there is no danger to the host). However even if this is considered to be bad userspace behavior there’s zero downside to perf/KVM restricting PEBS to guest-owned events.

Note commit 854250329c02 (\KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations) fixed the case where host userspace is profiling KVM and userspace but missed the case where userspace is profiling only KVM.

Reference

https://git.kernel.org/stable/c/160153cf9e4aa875ad086cc094ce34aac8e13d63 https://git.kernel.org/stable/c/34b6fa11431aef71045ae5a00d90a7d630597eda https://git.kernel.org/stable/c/44ee0afc9d1e7a7c1932698de01362ed80cfc4b5 https://git.kernel.org/stable/c/58f6217e5d0132a9f14e401e62796916aa055c1b https://git.kernel.org/stable/c/86aa62895fc2fb7ab09d7ca40fae8ad09841f66b