Re: [PATCH v7 1/6] KVM: x86: Fix potential race in KVM_GET_CLOCK
From: Oliver Upton <hidden>
Date: 2021-08-20 18:26:01
Also in:
kvm, kvmarm
On Thu, Aug 19, 2021 at 11:43 AM Marcelo Tosatti [off-list ref] wrote:
On Mon, Aug 16, 2021 at 12:11:25AM +0000, Oliver Upton wrote:quoted
Sean noticed that KVM_GET_CLOCK was checking kvm_arch.use_master_clock outside of the pvclock sync lock. This is problematic, as the clock value written to the user may or may not actually correspond to a stable TSC. Fix the race by populating the entire kvm_clock_data structure behind the pvclock_gtod_sync_lock.Oliver, Can you please describe the race in more detail? Is it about host TSC going unstable VS parallel KVM_GET_CLOCK ?
Yeah, pretty much any event that causes us to set use_master_clock = false could interleave with the KVM_GET_CLOCK ioctl. A guest could kick its TSCs out of sync, for example, to cause this too. AFAICT, KVM serializes the write side (pvclock_update_vm_gtod_copy()) with pvclock_gtod_sync_lock, as it should. -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel